rails in each case. Messrs. Strachey and Dickens, However, propose to employ a 361b. rail, which, however suitable it may be for the traffic it is designed for, is nevertheless inadmissible in forming comparison of cost.

It will be noticed that the differences of cost per mile between the two estimates framed for the Indian railways, and those tabulated above for a line in this country, are identical. This exact Agreement is, however, merely a coincidence, for the different conditions involved in the construction of various lines would of necessity produce discrep. ancies in the comparison of the proportion of cost to gauge, ranging between the conditions of a light surface line, where the difference would be least, and railways running through such country as that of Norway, where Mr. Pihl has found that the cost of the 3ft. 6in. lines to be only two-thirds of those 4ft. Sin. wide.

We think little more need be added to dispose of the question of economy sought to be effected in the adoption of the extremely narrow railways advocated by Colonels Strachey and Dickens. It may be urged that the partial estimates given are insufficient to allow a practical and valuable opinion to be formed. But as these estimates are framed for a proposed railway 480 miles in length, the basis is sufficiently extended to argue fairly upon, and as the figures show that by adopting a width of 2ft. 9in. instead of 3ft. 6in., a saving sufficient to construct only 11 miles more of line of equal character could be effected, the advocates of this system must abandon the argument of economy and occupy other ground. It is only just to the authors of the report advocating a width of 2ft. 9in., that they do not urge emphatically the argument of economy; indeed, they say that however small the difference of cost may be between a railway of a gauge that is enough and a railway on any broader gauge, it is the duty of the Government to select the one and avoid the other in making their final selection. But it is exactly in that question of what is enough that we consider Messrs. Dickens and Strachey are at fault. The carrying capacity of a railway and the speed at which trains may be taken upon it, are the measures of its usefulness, and in selecting a gauge, the maximum amount of work to be thrown upon it must be considered, and not that which it would have to perform under ordinary conditions. The importance of this consideration cannot be too prominently kept in view, and any error of selection committed now would be irreparable, or remedied at best with an enormous outlay. The time may arrive, and in these days no one can forecast events, even in the immediate future, when the safety of India as a British possession may depend upon her railway system. How shortsighted, then, would be that policy which should advocate the adoption of a gauge "wide enough" for her requirements, that is wide enough for the conveyance of the people and the products of sparsely habited districts through which these lines would pass. Doubtless a 2ft. 9in. railway would be enough for such requirements, not only at present but for many years to come; but how in the exent of poli tical difficulties? what if the troubles of war broke over the country? Could such railways as those proposed convey efficiently an army and its arms, could men, and horses, and guns and all the muni. tions of war be concentrated by the agency of an iron way 30in. wide, or at that season of supreme requirement, would not the means be found inadequate, and the consequence of an unwise policy be keenly felt?-Engineering.

ON THE SEPARATION OF THE RAYS OF HEAT FROM THE RAYS OF LIGHT IN SOLAR AND OXYHYDROGEN GAS MICROSCOPES.

T

BY THE LATE REV. J. B. READE, F.R.S. HE earlier experiments on the rays of light and heat were, for the most part, conducted with thermal rays that emanate from terrestrial sources. Melloni's experiments are entirely of this kind, and he abstains from making any general reflections on the question of the identity of radiant light and heat, because the rays of the sun, in which these two agents are the most intimately united, had not, in his opinion, been subjected to rigorous examination. He asserts, however, that convex lenges cannot be employed for the purpose of ascer taining whether radiant heat be subject to changes of direction analogous to those of light in passing through refracting media, and he denies the possibility of forming points of light, without heat, before the fire, with burning-glasses.

converged to a focus by a convex condensing lens, through a second convex lens placed at a certain distance beyond that focus-that distance being so adjusted as that the calorific rays, which, from their smaller refrangibility, are collected into a focus more remote from the first lens than the colorific rays, and consequently, nearer to the second lens, shall, on emerging from the latter, be either parallel or divergent; while the calorific rays, which, being more refrangible, had been collected into a focus nearer the first lens and more distant from the second, will be rendered convergent by this second lens, so that the second focus into which they are thus collected will exhibit a brilliant light without manifesting any sensible degree of heat. An attempt to improve the defective though usual construction of the solar microscope led me, in the year 1836, to the discovery of this new fact connected with solar heat. (See abstracts of the Philosophical Transactions, December 22, 1836) In the ordinary construction of the instrument the second lens, or, as it is termed, the field glass, is placed within the first converging cone of rays, thus causing a greater convergence and therefore greater condensation of heat, to the manifest injury of cemented object-glasses and objects mounted in balsam. But by placing the second lens beyond the focus of the large condenser, and therefore in the second or opposite cone of rays, the course of the rays of light and heat is just that already de. scribed; for the former may be collected at a second focus, while the latter become parallel and inert. A little within this second focus, the focus of the field-glass, we place the achromatic object-glass of the microscope, and so obtain a large and well-illuminated disc of white light, while the object-glass itself is rescued from all chance of injury. This optical arrangement being completed, microscopic objects are placed in their proper cell between the object-glass and the field-glass, and the relative position of these two glasses is not disturbed when the objects are brought into focus by their own rack-work adjustment. Thus all the necessary conditions for accurate microscopic delineation are fulfilled, and we obtain, at one and the same time, a perfect focal adjustment of the object-a large illuminated disc-and light without heat.

It is evident that by this arrangement of lenses we convert the parallel solar beam first of all into a cone of light-giving rays within a cone of heatgiving rays, and the principal focus of heat is further from the condensing lens than the principal focus of light. But after these rays cross the axis we have, conversely, an equal and opposite cone of heat-giving rays within a cone of light-giving rays, and a plano-convex lens or hemisphere, if placed in this second cone at the distance of its own focal length from the principal focus of heat, will be at a distance greater than its focal length from the principal focus of light; and, consequently, the rays of heat, after passing through this lens, will become parallel while the rays of light converge to a second focus.

I have approximately measured the heating power of the thermal rays of the second cone when rendered parallel by the plano-convex lens, and I found, in the month of December, that the murcury In a sensitive thermometer, when placed in the second focus, did not reach 90° Fahr., while at the same time the heat at the focus of the first cone was sufficient to discharge gunpowder.

A large drawing of the head of a flea by Lens Aldous, with the beautiful apparatus of the mouth, which I had mounted in Canada balsam, was an appropriate illustration of my communication to the Royal Society, and of the practical value of this and it may interest photographers to know that be new arrangement of lenses in the solar microscope; fore Lens Aldous prepared from this drawing the first coloured lithograph of the head only, I took several enlarged negatives of the flea as I stood, as it were, inside the solar camera, and unconsciously developed the latent image by washing the moistexcited paper with a weak solution of Aleppo galls. These negatives, and other solar-microscope enlargements, were fixed, as I have stated in other papers, by hyposulphite of soda.* Dr. Diamond saw one of these negatives of the flea in June, 1837. It was the first negative he ever saw, and, more than this, it was the first link in the chain which has bound him so firmly to our art.

I need only add that, as the oxyhydrogen gas microscope is not free from the great fault of its compeer, inasmuch as its artificial source of light is also a source of intense and destroying heat, the proposed arrangement of lenses is equally applicable to both instruments.

PLAIN CYLINDRICAL BOILERS.

BY MR. JEREMIAH HEAD, MIDDLESBROUGH.

which are, however, counterbalanced by certain serious defects. He proposes to investigate the cause and extent of these defects, and to point out satisfactory remedies. Anticipating the objections of those who are likely to urge that plain cylindrical boilers are wrong in principle, and ought to be entirely superseded, he submits that new enterprises, under the most unexceptional management, still continue to be furnished with these, in preference to other kinds of boilers, and were this not the case, the number in use is sufficient to give importance to any plan whereby greater efficiency and durability are likely to be secured. Out of 17,825 boilers now on the books of the various in surance companies, 4,052, or 22-7 per cent. of the whole, belong to the class referred to. They are, however, differently regarded in different districts, the proportion varying from 121 per cent., as re ported by the Manchester Steam Users' Association, to 69 per cent. as returned by the northern district of the Midland Steam Boiler Inspection and Insurance Company. The writer next de scribes and illustrates the details and arrangements connected with an ordinary plain cylindrical boiler, dwelling upon their simplicity, ease of examination and repair, and small first cost. Excluding brickwork and fittings not riveted to the boiler, a 50-horse power boiler, or one capable of evaporating 50 cubic feet of boiling water per hour, would cost, according to present market price, about £100. To accomplish the same duty, an internally fired boiler would cost nearly three times that amount, and the saving of fuel effected by it would only be about 9 per cent. He, therefore, asks how it is that many are prepared to adopt the more complex appliance, at nearly three times the outlay, unless there is some further reason for such preference? The reply to this question may be expressed in homely language in a very few words. Plain cylindrical boilers are liable to break their backs. The manner in which the serious fractures so described destroy the structure is next illustrated. They do not occur in the region of the most severe heat, but, on the contrary, where the plates are unscaled and retain their original appearance. They can only be due to heavy tensile strains. The writer has often observed that the ends of boilers are apt to lift themselves up clear of their supports, leaving the whole weight-say, including water, 16 tons-upon one bearer alone. This happens when the boiler is at work, and is owing to the superior heat and greater expansion of the bottom of the boiler as compared with the top. On withdrawal of the fire and release of the steam the boiler ends were observed to return to their bearings, and the centre to show an inclination to arch itself up clear of its support. In the former case, the bottom of the boiler must have been subject to severe compression, and in the latter to tension. The ultimate result of such a state of things could not be other than fracture across the weakest seam. The aver age difference of temperature between the top and the bottom of externally fired boilers is next estimated, and the difference in expansion computed. A boiler 45ft. long when at work constantly endea vours to lift its ends something like 3in. By reason of its own weight, that of the water within, and the rigidity of the structure, it is prevented from changing its form to that extent. It is, therefore, only by compression of the bottom plates and exclusion of the top that the forces brought into play can be absorbed. So much for expansion of the bottom when the boiler is at work. But what takes place when it becomes cool? Has it become per manently elongated? Does it return exactly to its original length? Or is it permanently contracted? If the latter that is, iron heated and cooled in operation is repeated, then at the end of every contact with water-contracts every time the week the bottom of a boiler must evidently be come shorter, and the whole structure must ultimately either arch itself up clear of the middle support, or must break across the bottom. To satisfy himself respecting the behaviour of iron under these circumstances, the writer made a series of experiments upon strips of boiler plate, fitted to a gange exactly 1ft. in length. These strips were carefully heated to a temperature of about 890°, and then slacked in boiling water. This was intended to imitate the condition of the bottom of a boiler at work and off work, as at the end of a week. One strip was so heated and cooled 20 times; a second, 40; a third, 60; a fourth, 80; and a fifth 100 times. The result evinced a small but steady permanent contraction, amounting in the last case to 1-14th of an inch. The writer contends that these facts entirely account for the fracture of boilers as described, and that the same fate must eventually befall all which are externally tired, provided they are so set as to be unable readily to

removing the evil, which is to substitute yielding for rigid supports. He recommends that boilers should be hung upon evolute springs, and illustrates how this may most readily be done. The alteration and expenses entailed appear to be but slight. Besides the lengthening of the suspension rods, and the introduction of the evolute springs, the connections between each boiler and the general mains must be made circuitous instead of

AT

SCIENTIFIC SOCIETIES.

ROYAL ASTRONOMICAL SOCIETY. Ta meeting of this society, held on Dec. 9, 1870, W. Lassell, Esq., President in the chair, a paper, by Mr. Joynson, entitled " Observations of Coggia's Comet," was read in which the author described the passage of the comet over a small star, and remarked that the light of the comet was so faint as to be quite overpowered by the light of the star.

Mr. W. T. Lynn, of the Royal Observatory, Greenwich, read a paper on the position of the Star Argelander, 17415-6, as determined by the Greenwich observations. He also read a paper on the proper motions of the binary star 36 Ophiuchi and the star 36 Scorpii. The author examined the proper motions of these stars with a view of ascertaining how far they would support Mr. Procter's theory of "star-drift," and found that they exhibited motions of nearly the same value and in the same direction.

Professor Cayley explained his method for the graphical construction of solar eclipses. In this method the centre of the moon is always the centre of the diagram, the relative orbit of the sun being described on the diagram in its proper position. By the aid of a ruler graduated for the purpose certain are laid down on the diagrams, which being transferred to a suitable projection of the earth the progression of the shadow is shown on

curves

the surface.

Mr. Browning explained to the meeting, by the help of a model, some improvements suggested to him by Mr. Proctor in his automatic spectroscope, a description of which will be found in the Monthly Notices, Vol. XXX., p. 198. The instrument in its improved form was exhibited after the meeting.

the best that has yet been offered to account for the exterior portions of the corona.

event will be the ratio of the first sum to the sum of approximate value for the odds might easily be ob calculation would the coefficients-that is to 10008. Of course the direct tained. I have not myself any leisure (to speak of be wholly impossible; but a nor am I likely to have. Could "E. L. G." or some competent mathematician run through this problem? purpose of a random distribution of points according (2nd edition only), I found peculiarities fully as marked In taking out a number of digits (8,000 in all) for the of 500 digits, out of sixteen such sets. They give for to the plan described at p. 274 of my "Ot er Worlds the nine digits the numbers shown below. as the observel relation. I take at random three set 0 1 2 345

present, said the meeting would be much gratified The President, addressing Dr. Draper, who was if the doctor would kindly communicate anything which might appear to him as interesting. The doctor, in reply, alluded to the large silvered-glass reflector of 28in. aperture, recently constructed and equatorially mounted by his son, a little distance north of New York. Dr. Draper spoke of the performance of this instrument as very satisfactory. Its definition of stars is very good; it gives a 6in. image of the sun or moon, and possesses other ex; cellences. on the subject of repolishing and preserving the A rather animated discussion followed surface, in the course of which Mr. Browning First set 43 47 48 alluded to the importance of leaving the silvered 6 7 surface alone, and not allowing any organic sub89 stance, as black velvet, to come in contact with it. 53 52 49 52 42 60 64 Mr. De La Rue strongly urged the great superiority 39 47 46 47 56 47 49 of metallic over silvered-glass reflectors, and after 58 85 55 60 a few further remarks the meeting adjourned. 49 48 58 40 46 62 54 45 55 43 It will be observed that in the second set there are but 35 sevens, and 60 nines. Increasing these in the proportion of 608 to 500, we get 42 sevens and 78 nise (This increase is not a strictly legitimate mode of comparison; but not far from correctness in this case. This is a more marked peculiarity than the one noted by De Morgan. The 62 fives in the third set are als worthy of notice.

LETTERS TO THE EDITOR.

[We do not hold ourselves responsible for the opinions of our correspondents. The EDITOR respectfully requests that all communications should be drawn up as briefly as possible.] **All communications should be addressed to the EDITOR of the ENGLISH MECHANIC, 81, Tavistockstreet, Covent Garden, W.C.

able to J. PASSMORE EDWARDS. All Cheques and Post Office Orders to be made pay

STAR-DRIFT.

soon throw

RICHARD A. PROCTOR.

[Upon referring to the Monthly Notices, Vol. XXVIII., pp. 6 to 8, the reader will find Mr. Buckingham's observations of the small stars near Alpha Lyra. In the diagram of positions accompanying Mr. Buckingham's paper, the stars B and e occupy the places of Mr. Sims' stars on October 10 and 11 respectively. The star e, north, preceding the larger star was discovered by Mr. Buckingham on August 10, 1867, with his 21 in. object-glass. Angle of position, about 295°; distance, about 34 seconds

of arc.-REPORTER.]

Mr. De La Rue, alluding to the principal object of the Eclipse Expedition-viz., the determination of the nature of the corona, called the attention of the fellows present to a theory suggested by Professor Oudeinans, which recognizes the existence of a large number of minute discrete bodies in interplanetary space; such bodies in the immediate neighbourhood of the moon would, Mr. De La Rue said, reflect a small amount of light into the moon's shadow, producing the corona. illustrated his remarks by a diagram on the blackMr. De La Rue board, which it would be difficult to reproduce, of the exact dimensions as to diameters and distances necessary to show the manner in which the sun's light is reflected from such small bodies, so as to become visible as an appendage to the moon. De La Rue considered the professor's theory to be Mr.

Let any one who has time take out 608 digits at random, according to any plan-say as I did, by opening a table of logarithms, or the like, and bringing the point of s digit on which, or nearest to which it falls. pencil down at random on the open page, taking the he will find in nearly every case that some one digit is I believe this, I should be interested to hear the results. N in excess and some other in defect, as greatly as in the case dwelt on by Professor Smyth. If any one does own 8,000 random digits go far to convince me that the relation is by no means so noteworthy as has been figures 8, 6, 5, and so on would have the advantage over be careful to consider all the digits in the page as insupposed. In taking out digits on this plan one mast the lean ones, 1, 7, &c. cluded within equal quadrangles; otherwise the fet RICHARD A. PROCTOR.

"B. A." ON LIGHT.

[1018] I HAVE looked up my letter about spectral lines (727) and am relieved to find that I have not anywhere One might as well talk of Baily's beads in a beer-barrel. the existence of "spectroscopic lines in a window-slit." talked (as "B. A." has now twice asserted I have) about What I actually wrote was that "in the breadth of a single diffraction band we have a complete spectram, lines and all." This is right, though a diffraction fringe-like a Newtonian ring-shows no lines. Frainhofer himself determined the wave-length of light bands, seen when a single fine slit is viewed with a belonging to different parts of the spectrum by studying telescope. Angström and Von der Willingen studied the places of the Fraunhofer lines in the diffraction of fine, parallel, equidistant slits. Referring to the be called normal spectra. So pure and undiluted inthe bands produced by a diffraction grating-or system seen, Sir John Herschel says, "they may spicuous of these lines ascertained with great pretelescope, the fixed lines' may be seen in them, and deed are their tints, that by the aid of a magnifying thus the wave-lengths corresponding to the most con

spectra thus

cision."

in a window-slit. But if he has made a mistake we I cannot think that "F.R.A.S." has made a mistake in this matter, though I can well believe he has scouted the idea of the existence of spectroscopic lines must remember how many questions he answers, and that no one can avoid an occasional error under such circumstances.

formation, he really wanted either an argument or to asked a question about the parabola as desiring inI am left to suppose that though "B. A." originally discussing the strict fairness of such a course, I may as show his superior knowledge of the subject. Without well, for the benefit of other readers, point out that the proof of the property of a tangent to a parabola. quoted by "B. A.," is by no means new; and that so has been by no means proved when the proposition far from being the proper proof, it has long since been deservedly rejected as imperfect. For it assumes, what points of inflection. For other reasons the proof given by Goodwin, Whewell, and others, is to be comes first to be dealt with, that the parabola has no preferred. The plan of taking a secant through two points close by each other, and conceiving one point to move up to coincidence with the other, supplies the only with curves of the higher orders; and Goodwin and the available conception of a tangent, when we are dealing rest do well to introduce this conception as soon as the student has passed beyond the geometry of the straight "B.A." should not have said that the

the latter question, the following description of one I am now making (nearly completed) will show an easy mode of constructing such, which doubtless may be improved upon after further trials; and as to maintaining the balance, it may be shown that the method of preserving the equilibrium is identical in either case, and perfect case and steadiness in steering being secured, full power of the feet on the treadles without any of the tremulous reaction on the wrists, greater security in maintaining the seat must necessarily follow. The inclosed sketch will show the general construction. I use wire for the brake instead of cord or chain. I shall be happy to add further description if acceptable. The wheels are the same size 34in., but of course this may be varied as in other machines. F. M. CRICHTON.

BICYCLE RIDING.

[1020] BICYCLE riding must certainly be on its last legs, since some writers in your valuable journal are advocating such useless things as steps, and even walking sticks to assist us to mount our iron horse. We are not told how the stick is to be carried, but I suggest it should be in the mouth; it would then be very ornamental, and not interfere with the rider's movements.

I intend carrying out the principle, by constructing a four-wheeler; to be propelled partly by the feet, in the usual manner, and to have a pair of crutches of the old-fashioned sort, to help up hill. I hope then to be able to do the 4 miles per hour which some of your correspondents find a very fatiguing rate of progress. Joking aside, cannot your correspondents suggest something that will enable us so to improve the machine, by reducing weight and friction, as to make it perfect? The machine of the present day is a very different affair from the old clumsy articles with wood wheels which were in vogue two years ago. We thought then we were doing well when we covered a mile in 7 minutes, and a 36in. driving wheel was the maximum size. Now the time for a mile run on a course is 3 minutes, and 7 miles can be done in 30 minutes on a macadamized road, and the driving wheels are from 48 to 50in. diameter, the trailing wheel 28in. (N.B. "Irish Mechanic" says, there should be only 4in. difference).

What has enabled us to obtain such good results as the above, is the introduction of the "spider" wheel. These wheels are made of V iron in the rims, and have a lin. rubber cord tire; the spokes are made of charcoal wire, and are let into the hubs through hollow screws which allow of free play in a vertical direction, thus making the wheel very elastic. A full description of the wheel would require drawings, which I have not ime to make, but with your permission I will return to the subject at an early day.

One word of advice to would-be purchasers of bicycles. Do not expect great speed, with little exertion, to be got out of a low-priced machine. It is the flooding of the market with rubbish which has brought the veloci pede into disrepute, and ruined the trade of

A BICYCLE MAKER.

PNEUMATICS, LOCOMOTIVES, MAGNETIZED NEEDLES, MARINE ENGINEERS, HYDROELECTRIC MACHINE, COILS, BOILER EXPLOSIONS.

[1021] IF R. Rhodes (let. 873) bears in mind the condition of the air when placed in his experimental bottle, he will see that the air was at the time of bottling, reduced in volume by a pressure equal to the whole height of the atmosphere, and by the law of elasticity the force thus stored up by condensation will cause the air to press upon the interior of the bottle with the same force per square inch as the atmosphere does upon the outside; then if the bottle cannot resist this force when the outer pressure is removed it will break, and the confined air expand to the full dimensions of the receiver, losing part of the stored up energy. It will thus be seen that the pressure due to the few grains of confined air has nothing to do with its weight, but the conditions under which it was bottled. The most simple yet decisive experiment that suggests itself to me at this moment is the common air-gun. If "R. R." will borrow one and weigh the air chamber both before and after charging it, he will find that he has not added many grains by weight of air to the receiver, and yet a small fraction of these few grains when released will carry a fair-sized ball some fifty or sixty yards, and kill a hare or rabbit at that distance. Here it will be seen that the effect is not due to the number of grains, as the quantity of air escaping at each discharge of a bullet is probably far less than that which he has stored up in his thin bottle at the mean atmospheric pressure.

But if he bears in mind the muscular energy he expended in filling the condenser, he will find that, with an equal effort (added to that necessary for transferring his body) he could have carried all the balls the charge of air will project to a distance equal to their mean range.

In the experiment suggested in his last paragraph nothing could be shown affecting the question at issue, because if the small metallic vessel can be exhausted in the open air, and then closed air-tight, it is evident such a vessel is capable of resisting an external pressure of 15lb. per square inch, and would, therefore, remain unaffected, whether in the bottle or out of it, since removing the pressure from the outside of the bottle does not increase the pressure within, but merely removes an obstruction which previously kept the internal pressure in equilibrium, so that the bottle would be broken, while the metallic vessel would re

main intact. Now let us reverse the order of things. Instead of a metallic vessel, make one of glass, in the form of a tube closed at the bottom but open at the top, and of the same thickness and cohesive strength as the bottle designed to be broken.

Fill the bottle with air, insert the tube through a cork, with the open end outwards, and make all secure. We have now a philosophical arrangement that will test whether the air in the bottle presses upon the inserted vessel as well as upon the sides of the outer one, and it will also prove something more, viz., the astounding fact given to the engineering world about the year 1860, by W. Fairbairn, Esq., as the result of a great number of most carefully-conducted experiments, when he showed "that a tube subjected to uniform internal pressure was more than seven times as strong as that of a similar tube subject to external pressure."

In the case suggested for the present experiment it will be seen that the bottle arranged and placed under the receiver, the vacuum would take place simultaneously in the receiver and the interior of the tube, so that the pressure in the bottle acting simultaneously upon the interior of the bottle and the exterior of the partially exhausted tube, the latter would give way first, and the bottle be saved.

As "R. R." implies that great advantages can be derived from the results of such an experiment, it must surely be worth his while to try it himself, as it cannot be a very costly one under any circumstances. (874.) I am much obliged to "Sigma" for calling my attention to the word "immersed," in letter 803, which evidently should have been “immersed surface." At the same time allow me to inform him that during | his present busy season I shall do all that lies in my power to relieve him of all the tantalizing little things which come within my scope; and I do this for two reasons-in the first place, I know from experience that the time and patient investigation necessarily occupied in the continuation and completion of the series of papers he has in hand is quite a sufficient tax upon the leisure and even physical strength of any single contributor who has other business matters to contend with in addition to writing; and in the second place, judging from what I have had time to read since those papers (which have appeared) came to my hands, I feel he is infinitely better employed, in the interests of the science itself, as well as those of the readers of the MECHANIC who take a real interest in it, by carrying out the series, than by sending hosts of short letters upon desultory subjects.

can only be restored by rewinding, a severe tax oper an amateur's skill and patience.

(5780.) Magnetized Needles.-All needles must hardened and finished before magnetizing, since a r heat will destroy the magnetism of any needle or othe magnet. The coil described by "Sigma," Vol. XII p. 2, and, in short, all coils intended for producing magnetic effects, must be continuous in the direct started with; you can begin to wind from right to let or vice versa, as you choose, the result will be the sc (5781.) Working Battery.-If I understand the d scription rightly, you want to operate two electr magnets alternately with the current from one o or battery. Any ordinary commutator will ef this. It is difficult to describe without a diagram but I will make the attempt. Having wound magnets, you will have two loose ends to each. Tak one of each of these and couple up to say the pole of your battery. Join the remaining loose cou of wire to one each of two [stont metallic spris fixed on your base board say in. apart from eat other. Upon an axis passing through the mic between these springs place a cam of any shape so that on causing this ax s to revolve the poin the cam shall be in contact with one spring and e of the other alternately. To the pillar (metallic) which supports this axis and cam attach the wire leading and coupled with the negative pole of your battery, every complete revolution of the cam will then send a current alternately through the two magnets. If t is not sufficiently clear, I must in a future paper se a diagram.

(5786.) Marine Engineers.-Without venturing any! dogmatic assertion as to the practice adopted in seed ing engineers for the Royal Navy, to which I pres the query refers, I limit my reply to my own opinio, and leave your querist and other readers to form ther judgment thereon.

Of all other qualifications that may be necessary should place it as indispensable that a candidate shoul not only have served a full apprenticeship to an engin builder (in all branches which this term include but that he should have had at least two or three years experience in a good marine engineer's shop, entirely dependent upon his own application and consequ proficiency as a recommendation.

This I look upon as vital, other matters of as edtcational nature being considered as accessory to able him to uphold the position in society, consider ing the rank he either does or ought to hold in lation to the other officers on the ship's books. Since a young man is rarely if ever appointed to a grade above third class engineer whe first accepted, his having been

(5516.) Locomotives.-Mr. D. K. Clarke, C.E., the most recent and reliable writer upon English locomotives at least, in the contents of his first division, section 1 of his large treatise on railway machinery, &c., gives a very useful epitome of the history of the loco-engine-room motive. It reads thus:

Suggestion of the locomotive, by Watt, 1759.
Murdoch's locomotive, 1784.
Trevithick and Vivian's, 1802.
Blenkinsop's, two cylinders and rack rail, 1810.
Chapman's, with chain gear, 1812.
Brunton's, with leg propellors, 1818.

at sea in the

for any given period becere subsidiary, since there are always two others he have (or should have) passed through the interme grades always at his elbow, who are willing to teach so long as he is willing to learn his duties. In th matter the junior must ever cultivate the humility Mr. Uriah Heep, with at the same time a nobility i soul that would scorn to obtain that worthy gene

Blackett's, with traction by simple adhesion, about man's aim by the moral degeneracy that "not

1813-4.

individual " so willingly stooped to. Every marke engineer while at sea, where there are erecting shops and few sidings to run into in case mechanic, and in making himself such, there of a break-down, should be a thoroughly competent nothing per se which can derogate from his ability for being at the same time an equally accomplished gentleman.

(5801.) Power of Cylinder.-The only power of this unfortunate cylinder is to excite risibility in the ratio of the square of its diameter and length of face it mus produce, upon mature consideration of the possibility of such a question being so asked after all that has been written in ours and other journals upon the steam

Of these the Patent Museum, at Kensington, contains those marked thus*, "and one called the Wylam engine, built about 1813, by Wm. Hedley. The Stock-engine and its adjuncts. ton and Darlington Railway was the first line known to history upon which passenger trains ran, and the engine is now placed on a pedestal at Darlington.

(5648.) Rule Marking. · Steel rules, 12in. long, serving the purpose of a pocket straight-edge also, are marked the first inch with 1-100in., and the next with 1-50in. by a dividing machine. They are made, I believe, in America, and cost here about 12s. 6d. Those which I have seen are very well finished and very accurate.

(5781.) Earth Circuit.-This question has been handled by F. C. Webb, C.E., in a series of papers upon "Some Considerations on the Principles of Electrical Conduction and Accumulation," contributed to the Electrician (see Vol. I., No. 10, et seq.), and afterwards collected and published in a small volume, but now, I fear, out of print. The gist of his reasoning was that the vast size of the earth as a reservoir, was utterly untenable as a reason for its forming part of the voltaic circuit. His own view would require too much transcription and too many diagrams for me to attempt to render it intelligible in a short note.

(5767.) Water Pipes.-The diameters of the pipes given being as 2: 1 the quantities of water delivered as per question would be as 23:2 x 12, or as 4 to 2; hence the large pipe would deliver twice the quantity that the two small ones would in a given time.

(5779.) Coil.-"A Subscriber" gives an incorrect description of his induction coil, since he cannot want a set of binding screws for battery wires and another for the primary wire. This set may be for his condenser (although they are unnecessary), if his coil has one. Judging from the description, two such cells should, if in good order, be ample for such a coil; but his results point definitely to ruined insulation in his secondary, a very common result of over-tasking these very useful but sadly too delicate coils in the hands of inexperienced manipulators. The insulation

(5808.) Hydro-Electric Machine.-Take a friend's advice and leave the "hydro-electric machine" in the caboose to which it has been consigned.

If you merely wish to try an experiment, to prove that electricity can be developed and accumulated from such a source, procure some accommodating owner of a steam engine in your neighbourhood who employs a cheap stoker to waste steam at the safety valve instead of employing it in the cylinder or its plural, and havi obtained his permission for your experiments, provide four dry bottles with which and a deal board you im provise an insulating stool, upon which perch yourse as a dunce is elevated on a form at school (you can omit the cap, as that is too pointed, and may give of the electricity at the wrong place); then, taking a pointe conducting rod in your hand, hold the extremity of i in the escaping steam, without touching any part the valve, &c. Your body will then become charged and sparks can be obtained from it by any of you admirers who have an earth contact, by merely presen ing their knuckles to any part of your person.

Caution! Be sure and keep your temper as th sparks fly from you, for they transcend those named Job, being intimations of trouble, whether their fig is upward or downward.

(5812.) Solution for Cast-Fron Battery.-No sing fluid battery can be used in which nitric acid form a constituent of the electrotype, wherein aine amalgamated zine is used as a positive element. (5881.) Silvering Block-tin-Having carefully clean the surface of your block-tin figure, insert it in a ba of the double cyanide of copper and potassium, ansic of sulphate of copper as for ordinary deposition.

The solution must be worked at a temperature about 180° Fahr., everything elso being the same ordinary copper deposition. This will give a true of copper in a few minutes, when the " figure removed from the bath to a vessel of clean water, a