which the body is moving. If difference of specific gravity be regarded as the impelling cause of any current, the deflecting power of rotation will certainly be infinitesimal.

Difference of specific gravity, resulting from difference of temperature between the ocean in equatorial and polar regions, might, if sufficiently great, produce some such interchange of equatorial and polar water as Dr. Carpenter supposes; but surely the difference of temperature between the equator and the poles could not produce currents like the equatorial current and Gulf Stream in a wide expanse of water. Such a general difference of temperature might tend to produce a general motion of the ocean; but it is inconceivable that it should produce motion in particular parts of the ocean, as Maury, Colding, and others, conclude.

But I think it is by no means difficult to prove that the circulation of the waters of the ocean cannot be due to the difference of temperature between the equatorial and polar regions. And Dr. Carpenter must be mistaken in supposing that it requires great mathematical skill to determine the value of the forces to which he attributes the circulation of the ocean. The whole subject, when properly viewed, resolves itself into a mechanical problem of such extreme simplicity as not to require for its solution the aid of any mathematics whatever in the ordinary sense of the term. Taking Dr. Carpenter's own data as to the difference of temperature between the waters at the equator and the poles, and also his estimate of the rate at which the temperature of the equatorial waters decreases from the surface downwards, I have, in my paper in the Philosophical Magazine for October last, proved that the amount of force which gravity exerts on, say, a pound of water, tending to make it move from the equator to the poles supposing the pound of water to be placed under the most favourable circumstances possible, is only of a grain.

I have shown also that the greatest amount of work that gravity can perform in impelling the waters from the equator to the poles as a surface current, and back from the poles to the equator as an under current (assuming that the waters would actually move under an impulse so infinitesimal) is only nine foot-pounds per pound of water. And in regard to the Gibraltar current, the amount of work which gravity can perform does not exceed one foot-pound per pound.

If these results be anything like correct, and it be admitted that a force so small is insufficient to produce the necessary motion, then it is needless to expect that any future observations in reference to currents of the ocean will in the least degree aid Dr. Carpenter's theory; for, supposing it were found that the waters of the ocean do circulate in some such manner as he concludes a supposition very improbable-still we should be obliged to refer the motion of the water to some other cause than to that of differences of temperature. JAMES CROLL Edinburgh, Dec. 22, 1871

IN a spirited article under this title in the last number of the Medical Times and Gazette, we are accused of "the most dismal want of appreciation of the true scope of the medical art and science." This is hard! The ground for it is to be found in the following sentences in the short notice of the Brown Institution in NATURE of Dec. 21 :—

"The true physician fears to meddle with the processes of which he is the attentive and anxious spectator. Although the more ignorant members of the medical craft-the so-called 'practical' men-may sometimes, with the best intentions, experiment on their patients with harmful drugs, such experimen tation is repudiated by the man of Science."

If objection had been taken by our guarded suggestion that it may happen that practitioners may sometimes use powerful agents by way of remedies without any adequate knowledge of their property, we should not have been surprised, and would have been very willing to apologise had we been assured that the insinuation was unfounded. What our critic finds fault with, however, is the second part of the sentence, viz., our assertion that such experimentation on human beings with harmful drugs is objectionable. If experiments had never been made on human beings, he argues, we should not have learnt to know some of our most useful and valuable drugs. This may be so; but even if it is, it perhaps scarcely affords a sufficient justification for a continuance of the practice.

In another part of the article we are accused of "unconsciously reproducing the superstitious and false philosophy of

2,000 years back," and we are distinguished by the epithet "Nature worshippers." Let us quote the superstitious sentence which has laid us open to so unexpected an imputation:

"The pathologist at the bedside is not in the position of an experimenter, but only in that of a student, who stands by at a greater or less distance; while another over which he has no control performs experiments in his presence without deigning to explain to him their nature or purpose."

By these words we are supposed to imply that while nature works we worship. Does the student who stands by while the professor performs an experiment in his presence, the nature of which he very imperfectly understands, ready to help if need be, but fearing to meddle or even ask a question least he spoil the wished-for result, worship his teacher? Or is it the mere speaking of Nature as a teacher at all that is superstitious and unphilosophical ?

The truth is, that our contemporary has obviously found the sentences quoted from our article a convenient text for a telling homily on a subject with which our remarks had nothing whatever to do. Our object was to point out that for the purposes of pathological investigation, and for trying the action of unknown remedies, a fellow mortal stretched on a sick bed is not a fit subject; that it is better to use dogs, cats, and rabbits. His aim, on the other hand, is to impress upon his readers the important practical lesson, that the doctor when called to see a patient must not stand by inactive, but use every means at his disposal for the relief of suffering and the prolongation of life. If he had found that he could add force to the admonition by clothing it in figurative language, and had said that the physician should grapple with the disease as with a fiend, it would not have occurred to us to call him a "devil worshipper." THE WRITER OF THE NOTICE

Prof. Helmholtz and Prof. Jevons JEALOUS of any and every restriction to that full liberty of scientific thought which cannot be over-advocated, we have recently gone so far as to deny the necessary and universal validity of the old axioms or "self-evident principles," not only in geometry, but in logic. Now I would submit that, if without some elementary or initial certainties all scientific thought is impossible, we must either retract these denials altogether, or so far limit them as to leave the logical certainties intact. But can we do the latter while geometrical axioms are in dispute? Towards answering this question, I propose to consider the hypothesis advanced by Prof. Helmholtz, to be found in NATURE, No. 103, October 19, and ably commented on by Prof. Jevons.

In order to show how geometrical axioms, with conclusions based thereon, may not be necessarily or universally true, Prof. Helmholtz tells us to imagine the existence of creatures whose bodies should have no thickness, and who should live in the mere superficies of an empty globe," and then, as a consequence, to admit that, "while, with us, the three angles of a rectilineal triangle are exactly equal to two right angles, with them, the angles of a triangle would always, more or less, exceed two right angles." I propose to show that this position, so far as it affects the question, contains a logical uncertainty and unsound. ness, which, if admitted, would vitiate all reasonings whatsoever. We should premise that the "imagined creatures" are sup posed to be "in possession of human powers of intellect," however their external conditions differ from ours. This assumed (and conceded), Prof. Helmholtz has to prove that the assumed difference of the external conditions will necessitate the intellectual difference assigned in his hypothesis; but he cannot assume this also without begging the whole question.

Let us first ask, what here is the import of the expression, "with them, the angles of a triangle would always, more or less, exceed two right angles"? To take the term "exceed," do the supposed beings detect the excess, or not? If they do, they find these three angles exceed two of our right angles, and they are acquainted with our right angles, and are consequently capable of conceiving four such rectilinear angles, and, thence, a rectilinear triangle with all its angles together equal to two right angles; and thus the entire supposition is unproductive. If we assert now that they do not detect the excess because they cannot, under their new conditions, conceive a rectilinear figure, we are simply begging the question we proposed to institute, viz., whether we derive our geometrical notions through our con. ditions, or whether these notions are intuitive? And, lastly, if we say that the beings in question take the spherical angles they have for rectilinear angles, and their four equal angles about a

Now, first, the fallacy lies in what the late Professor John Grote called the "pseudo-psychology," the confusion of thought and thing, of the psychical and the physical. For the question is here of geometry, the science which regards (say) all the sup. posed or postulated rectilinear angles about a point as equal to four right angles: the question is not of the physical science which discovers ". 'more or less" exactly what angular or other qualities may belong to any physical object; and so true is this, that geometry is not conversant with right and left hand, nor with above and below. And, secondly, the fallacy is concealed by an ambiguous use of terms in the statement, "with them, the angles of a triangle would always, more or less, exceed two right angles." The "with them may mean with them in imagination, or with them in fact; and, but for this ambiguity, the fallacy must have exposed itself; for, first, it is obvious that two angles which they imagined right ones would, in their imagination, equal, and not be exceeded by," the angles of a triangle they imagined rectilinear; we could not have said otherwise than this, with the case clearly stated. And, secondly, we could never have said (distinctly) that the physical fact being one way or another, could affect the universality of a geometrical position which does not affirm anything of physical facts; but we should have perceived that we were only combating a statement that the angles of a physical triangle supposed to be, though not really, rectilinear, are together really equal to two right angles; a statement obviously not true, and as obviously not geometrical.

In mathematical argument, anything I should bring in aid of Prof. Jevons's able comments would be equally presumptuous and useless; and it is only because I feel that his reasonings are not quite so unassailable on the psychological side that I venture any additional evidence. Prof. Jevons asks (I think needlessly), "Could the dwellers in a spherical world appreciate the truth of the 32nd proposition of Euclid's first book? I feel sure that, if in possession of human powers of intellect, they could. In large angles the proposition would altogether fail to be verified; but they could hardly help perceiving that, as smaller and smaller angles were examined, the spherical excess of the angles decreased, so that the nature of a rectilineal triangle would present itself to them under the form of a limit." Now the terms 66 spherical excess here mean the quantum by which all the angles of their triangle would, to the knowledge of these beings, exceed two bond fide right angles. They therefore know already (by Prof. Jevons's supposition) what a rectilinear angle is, and, thence, what a rectilinear triangle is with all its geometrical properties (as above shown), for it is admitted that we require no objective experience beyond that of a rectilinear angle in order to deduce said properties, and these beings, having our intellectual powers and our data, can deduce the same. I would only suggest here that, after this, to suppose any experimental evidence necessary to "verify' the proposition is very much like conceding the hypothesis that geometrical conclusions are not independent of experience.

"the

Another point not directly met by Prof. Jevons is ingenious, but amounts to the assertion that, if we could not actually draw a straight line, we should not be able to define it as shortest distance between two points;" for these imagined beings, who cannot possess a physical straight line, will have "an infinite number of shortest lines between any two diametrically opposite points in their sphere." An argument, interesting only so far as it illustrates to what lengths of ingenuity a sophism may be carried; for have we not to prove that our geometrical conception or definition depends upon our physical experience, and are we not here advancing for proof, that beings without this experience cannot have the geometrical conception, and that they cannot have it because we cannot have it? If anything could convince us of the inherent impotence of these experimental hypotheses, it should be this inevitable appearance of the "circle" just when proof is called for. And again, "shortest distance" here has two senses. First it means the shortest path available to the imagined beings, and then (in order to invalidate the definition of a straight line) it means the shortest path conceivable.

In this case it appears then (as I proposed to show) that, while

the geometrical certainties have been questioned, the logical code has been violated, and all logical certainty confounded by an ambiguous use of terms. I have here attempted no demonstration of the opposite theory; but I think if the eminent supporters of the hypothesis just examined would be content to affirm roundly that all our notions, conclusions, and beliefs are mere resultants of intellectual action plus given experience, and to forbear any hypothetic deductions till this thesis is made good, they would find that the essence of the question is distinctly psychological, and that any experiments with hypothetical physics are so many attempts to get out of a complex thing that which is simply J. L. TUPPER

not in it.

Meteorological Phenomena

ON the 10th of November, a little after 4 P.M., the sun was behind a bank of thick stratus clouds, on the upper edge of which, attached to it, about 10° above the sun's position, and 15° to 20° to the north of it, I, with two other persons, observed a small irregularly-shaped cloud, about 2° in apparent diameter, which exhibited the colours of the least refrangible portion of the spectrum, commencing with the red on the south end nearest the sun, succeeded by orange, yellow, and pale greenish yellow, fading into white on the north edge, the rays being perpendicular. This appearance continued for about five minutes or upwards while we viewed it, and then faded away. Though the phenomenon appears simple, the light cloud merely refracting the sun's rays, it is not evident why the com. plementary colours of the more refrangible portion of the spectrum should not have been visible; and, as far as I am aware, a similar appearance has not been recorded before. G. F. D.

IN NATURE of August 31 there is a note headed, "A Rare Phenomenon," from Magdeburg. Your correspondent, I think, evidently refers to what in India, or at any rate in Ceylon, is called "Buddhu's Rays," an appearance in the sky very com. monly observed here, and for which I have never heard any scientific explanation attempted. I regret to say that hitherto I have never taken any exact notes of the position of these rays. They generally occur, I think, when the sun is low, sometimes in the west at sunset, but also occasionally in the east. colour and blue radiating from one point on the horizon, and pearance presented is that of alternate broad streaks of rose extending, I should say, for about thirty or forty degrees. I will, whenever I see them in future, take exact notes of their position, &c. At present I can only say that I certainly think that dust in the atmosphere can take no part in their production. Colombo, October 1871

The ap

BOYD MOSS

Crannogs in the South of Scotland

It may interest some readers of NATURE to learn that a considerable number of crannogs, various articles of the New Stone Period, and some "kitchen-middens" have been discovered in connection with the small lochs which stud the surface of Wigtonshire and Dumfriesshire. Dowalton Loch, Machermore Loch, and the lochs which surround Castle Kennedy in Wigtonshire, have been examined within the last few years, and have disclosed ancient lake-dwellings. The Black Loch of Sanquhar and Lochmaben Loch in Dumfriesshire contain platforms of wood and stone. In some cases canoes and causeways connecting the arti ficial islands with the adjacent shores have been traced. Sir William Jardine, in his presidential address to the Dumfries Natural History Society, 1864-5, gives an interesting account of the crannog discovered at Sanquhar Black Loch; and recently the Rev. Geo. Wilson, Glenluce, read a detailed description of the crannogs in his vicinity to the Scottish Antiquarian Society. J. SHAW

Freshwater Lakes without Outlet

IN your notice of Morelet's "Central America" (NATURE, December 28, 1871) you speak of the water of the lake of Peten as fresh, though without an outlet. This is uncommon, but not unexampled. The lake of Araqua in Venezuela, described by Humboldt, is of this kind, and so are the lakes near Damascus, into which the Abana and Pharpha respectively discharge. The best account of these latter is, I believe, in Mr. Macgregor's work, "The Rob Roy on the Jordan."

JOSEPH JOHN MURPHY Old Forge, Dunmurry, Co. Antrim, Jan. 1

Pupa of Papilio Machaon WHILST working at the colour patterns of Insects in November 1867, I very carefully dissected off a portion, about oneeighth of an inch square, of the hard integument from the side of a pupa of P. Machaon, near the anterior extremity. The portion of the interior thus displayed was filled with a clear colourless fluid, in which was floating a delicate membrane, to which were attached several tubes, trachea, formed by a spiral fibre. In the fluid were floating many roundish grains. Another pupa of the same brood was examined January 15, 1868, and another on April 15. The floating grains were now evidently made up of ganglia of the spiral fibre of the trachea, and were connected with the tube by long pedicels of the same kind of fibre. On May 20 the tubes had enlarged to such an extent that they were almost contiguous, and were covered with minute granules, apparently incipient scales; in fact, a few small but well-formed scales appeared on one portion. The specimen examined in November was laid in cotton; a perfect cicatrice was formed, and the butterfly in excellent condition appeared at the usual time. Rainhill, December 23, 1871 HENRY H. HIGGINS

Lunar Calendars

In reply to "Myops" in NATURE, No. 111, p. 123, the English New Moon of the Jews is really the Month-Head (Caput mensis), formed from an artificial system. The true mean conjunction derived from the 19-year cycle is called the Molad or MoonBirth, and generally differs from the festival-day.

Said artificial system consists in combining AZ, BY, CX, &c., as follows:

1st Day of Passover has Black Fast (9th Ab) on same week day.

2nd

1st of Pentecost.

Ist of New Year (Tishri).

4th

5th

do. do.

Last of Tabernacles-Rejoicing of Law. do.
White Fast (Atonement Day). do.

,, Preceding Purim (Esther's Feast. do.

Hints to Dredgers APPEALED to by name-spirits from the vasty deep-I have waited for my elders, also named, to answer Mr. Hennah's queries about dredging, and, failing to see anything more, I venture to trouble you with a few lines, the more so as I felt the want of advice when I was fitting out the Norna in 1870. Details would be out of place here; I will only at present give a few hints. And first-to repeat Punch's advice to those about to marry—if about to buy a yacht, DON'T! Begin by hiring one of the tonnage you require, the proper price being 1. 5s. per ton per month, including the wages of skipper and crew, but rarely of cook or steward. After your first season buy by all means if you like.

If bound on a long cruise your craft should not be under 80 to 100 tons. But for dredging in the Channel or round our coasts 25 tons and upwards are sufficient; but not on any account under that. A little boat of 25 tons makes up two good berths and two more possible ones, exclusive of the crew's sleeping quarters, and being decked stands a good chance in a gale of wind.

Beware the discomfort of a half-deck and a small boat, remembering that you may unavoidably have to face some nasty breezes which an ordinary yachtsman would run away from.

You may,

for instance, be caught in a bay offering rich results, and have to thrash out of it.

Hire a man knowing the locality in which you desire to try your fortune.

Take a particular line, say the comparative life on the borders of fresh and salt water junctions, or at spots where the depth suddenly increases. No better locality, with a good pilot, could be picked out to begin with than the Channel Islands.

Especially note the submarine geology. Exactly fix the spots you dredge in by cross bearings. A small prismatic compass is invaluable, both afloat and ashore. Take carefully temperature, current, tidal observations, a multitude of soundings, and keep specimens of all. Fill a private log-book with the most trivial and infantile details. You will afterwards laugh at much you have noted; but it is a great gain, and, unlike partridges, impressions are best fresh.

*For Mahommedan Calendar inquire of a Moslem, or such an authority as Capt. R. J. Burton, the famous Hajji El-Iraki, and Consul to El-Sham,

One

This is not the occasion to go into matters of outfit. thing I must name, on no account let any man on board be without a life-belt for his own use.

Any intending dredger writing to me at this club will be cordially answered. A small squadron of yachts working together under a commodore of their own election would partition the labour, and produce a little emulation among the crews. Make a rendezvous every few days, and talk results over. MARSHALL HALL

New University Club, St. James's Street, S. W., Jan. 6

Anacharis Canadensis (A. Alsinastrum)

I SHOULD esteem it as a favour if you would allow me to ask, through the medium of NATURE, if there be any published account of observations, confirmatory or otherwise, of Mr. Wenham's notes on the free-cell formation which he has described as being carried on at the terminal growing point of Anacharis, quoted by Dr. Carpenter in "The Microscope and its Revelations," p. 405, et seq. (3rd ed.) H. POCKLINGTON

FIGHT BETWEEN A COBRA AND A

MONGOOSE *

THE HE snake was a large cobra 4 ft. 10 in. in length, the most formidable cobra I have seen. He was turned into an enclosed outer room, or verandah, about 20ft, by 12 ft., and at once coiled himself up, with head erect, about ten or twelve inches from the ground, and began to hiss loudly. The mongoose was a small one of its kind, very tame and quiet, but exceedingly active.

When the mongoose was put into the rectangle, it seemed scarcely to notice the cobra; but the latter, on the contrary, appeared at once to recognise its enemy. It became excited, and no longer seemed to pay any attention to the bystanders, but kept constantly looking at the mongoose. The mongoose began to go round and round the enclosure, occasionally venturing up to the cobra, apparently quite unconcerned.

Some eggs being laid on the ground, it rolled them near the cobra, and began to suck them. Occasionally it left the eggs, and went up to the cobra, within an inch of its neck, as the latter reared up; but when the cobra struck out, the mongoose was away with extraordinary activity.

At length the mongoose began to bite the cobra's tail, and it looked as if the fight would commence in earnest. Neither, however, seemed anxious for close quarters, so the enclosure was narrowed.

The mongoose then began to give the cobra some very severe bites; but the cobra after some fencing forced the mongoose into a corner, and struck it with full strength on the upper part of the hind leg. We were sorry for the mongoose, as but for the enclosure it would have escaped. It was clear that on open ground the cobra could not have bitten it at all; while it was the policy of the mongoose to exhaust the cobra before making a close attack. The bite of the cobra evidently caused the mongoose great pain, for it repeatedly stretched out its leg, and shook it, as if painful, for some minutes. The cobra seemed exhausted by its efforts, and putting down its head, tried hard to escape, and kept itself in a corner. The mongoose then went up to it and drew it out, by snapping at its tail, and when it was out, began to bite its body, while the cobra kept turning round and round, striking desperately at the mongoose, but in vain.

When this had continued for some time, the mongoose came at length right in front of the cobra, and after some dodging and fencing, when the cobra was in the act of striking, or rather, ready to strike out, the mongoose, to the surprise of all, made a sudden spring at the cobra, and bit it in the inside of the upper jaw, about the fang, and instantly jumped back again. Blood flowed in large drops from the mouth of the cobra, and it seemed much

*The following interesting narrative has been obligingly forwarded to us by Prof. Andrews, of Queen's College, Belfast.

I HAVE long held that a vital property of "irritability," or "tonicity," was unnecessary in muscular action. As it seemed to me, the state of relaxation in living muscle was to be accounted for by the mutual repulsion of molecules arising from the presence in the muscle at the time of a charge of electricity, sometimes positive, sometimes negative; as it seemed to me, muscular contraction, whether in ordinary muscular action or in rigor mortis, was nothing more than the result of the operation of the elasticity of the muscle upon the discharge, sudden or gradual, of the charge which had previously kept up the state of relaxation. And I still hold that the state of relaxation is caused by the presence in the muscle of a charge of electricity, and that muscular contraction is brought about by the elasticity of the muscle coming into play upon the discharge of this charge; but, since I began to work with the new Quadrant Electrometer of Sir Wm. Thomson, I have been obliged to take a different view of the way in which the charge operates in causing relaxation. The fact, discovered by means of this instrument, that there are two charges of electricity in muscle, positive and negative, was fatal to the idea that the state of relaxation was due to the mutual repulsion of molecules consequent upon the presence in muscle of a single charge, positive or negative. With either charge singly the idea might be entertained, though it was not easy to understand how, wanting effectual insulation, the electricity could be kept to its work; with two opposite charges, on the contrary, the attraction of each charge for the other must .neutralise the repulsion arising from the presence of either singly. Nor did I find a way of escape from this difficulty until I began to seek it in a totally different direction, even in the theory according to which the sheath of muscular fibre during rest is charged as a leyden-jar is charged. Is it possible, I asked myself, that the two opposite charges, disposed leyden-jar-wise upon the two surfaces of the sheath, may cause elongation of the fibre by compressing between them the elastic sheath? Opposite charges of electricity must attract each other; that was plain enough. Opposite charges attracting each other across an elastic sheath may compress that sheath in such a way as to cause elongation of the fibre; that was not impossible. Upon this view, too, there was no difficulty in understanding how each charge was prevented from escaping, and made to work in this manner, by the mutual attraction of each for the other. In a word, the idea that the two charges might act in this way in causing muscular relaxation was far more easy to realise than that which regarded the state of relaxation as the result of the muscular molecules being kept in a state of mutual repulsion by the presence of one charge in the muscle. And so it was that it became necessary to look into this matter a little more closely-to put it to the test of experiment, as best I could.

In order to this, I began by inquiring whether the idea in question was possible or not. I wanted to be certain that the mutual attraction of two charges of electricity, dispersed leyden-jar-wise upon the two surfaces of the sheath of the fibre, would cause elongation, and that the discharge of this charge would be followed by contraction; and, after several abortive attempts, I found what I wanted, and more than I expected at first, by the means which are represented in the accompanying figure.

Vulcanised india-rubber sheeting being at once elastic

and dielectric, it occurred to me that this material was the very thing for putting to the test of experiment what I believed might happen in the elastic and dielectric sheath of muscular fibre. I therefore took a band of this sheeting, provided it with the conducting surfaces necessary and discharged, and had constructed an apparatus for for charging and discharging it as a Leyden-jar is charged showing whether or not the anticipated changes in length were produced by this charging and discharging. The band (which is to be regarded as the counterpart of a strip of the actual sheath of the muscular fibre) is 14 in. in length by 2 in. in breadth, the commercial number of the india-rubber sheeting being 30. The necessary conducting surfaces to allow of the charging and discharging are made by painting the band on each side with fluid dutch-metal, care being taken to leave at the edges a sufficient unpainted margin to secure the necessary insulation of the two painted surfaces. The frame-work of the apparatus consists of two strong brass pillars, 18 in. in height, and 4 in. apart, rising from a flat brass stand. Across these pillars work two axles, horizontal in direction and parallel to each other-the one at the top, the other near the base, immediately above the stand. At the middle of the upper axle, midway between the pillars, is a wheel with a grooved edge, 2 in. in diameter, which may be called the driving-wheel; at one end, which projects beyond the pillar on that side, is another and larger wheel, 6 in. in diameter, also with a grooved edge, which may be called the multiplying-wheel. At one end of the lower axle, beyond the pillar on that side and immediately under the multiplying-wheel is a collar with a grooved edge; at the other end, also beyond the pillar on that side, is a socket for carrying a long index, of which the free end moves backwards or forwards before a graduated arc fixed immediately over the socket upon the same pillar near its top. The two axles move together, the upper telling upon the lower by means of an endless band which at one and the same time bites in the grooved edge of the multiplying-wheel at the end of the one, and in that of the collar at the end of the other; and thus the movements of the index before the graduated arc are made to represent a very considerable exaggeration of the movements of the upper axle. The india-rubber band is clipped at each end in a clamp, acting by screws, and having a hook on its free edge; and, being so clipped, it is fixed in a a vertical position by passing the hook on the clamp at its lower end into a socket provided for it on the stand, and by attaching the hook on the clamp at its upper end to a string which passes over the grooved edge of the driving-wheel to a short hanging rod with a button at its lower end, upon which rod are to be slipped coin-like weights, notched in the centre for this purpose, which weights have to be so adjusted as to put the band gently upon the stretch. In this way the band is so fixed that it cannot lengthen or shorten without these changes being made to tell upon the index, for as it lengthens or shortens, the driving-wheel which moves the index must be made to turn this way or that by the string which bites into its grooved rim in passing from the band to the weights. For charging and discharging, two short pillars are fixed to the stand in front of and at a short distance from the bottom of the band, that for the former purpose having an ebonite shaft, that for the latter being altogether metal; and through holes in the caps of these pillars the rods which are intended to serve as the actual channels for the charge and discharge are made to slide horizontally backwards or forwards in a suitable direction. In charging, the electricity is supplied to the metallic surface on the front of the band by pushing forwards the charging rod so as to touch this surface, and at the same time taking care that the discharging rod is drawn back so as to leave the necessary break in the circuit. In discharging, the discharging rod is pushed home so as to complete the circuit between the two opposite