differ considerably in different portions of the square. On these grounds, it would be a mistake, scientifically, to correct the averages of the actual observations on the large chart, in the present state of our knowledge. These should be printed with none except instrumental corre ctions; and as we have already said, the mean hour of the day and the mean day of the month of each average should be given; for if this be not done, the resul's of the discussion can be turned to no s'rict scientific use whatever.

But it is quite otherwise in discussing the data entered on the large chart, with the view of arriving at some knowledge of the distribution of pressure over this important part of the ocean. As we stated before, "such a discussion necessarily calls for a preliminary preparation of the results by the application of such approximate corrections for range as we are in possession of," and of these corrections Fitzroy's, to which Mr. Symons refers, are among the most valuable. To have attempted such a discussion, di-regarding the correction for range, is a grave mistake; and we can scarcely suppose the Meteorological Committee will sanction it when they ultimately decide on the met hod of discussion to be adopted.

Fitzroy recognised the vital importance of range corrections in such discussions; and with this view the monograph above referred to was published under his direction upwards of eleven years ago. It would be well if a series of such monographs were prepared under the direction of the Meteorological Co mmittee, as necessary preliminaries, which indeed they are, to the discussion of the meteorology of each portion of the ocean they under'ake to discuss. YOUR REVIEWER

66

'During August we had an influx of fresh water all along our southern coast, and throughout the whole extent the sea eggs crawled ashore, and died in great numbers. No one has seen the like before. I have no doubt the fresh water was the cause of the mortality, and that o.her shells also suffered."

I have not the means of ascertaining the rainfall of the basins drained by the Orinoco and Amazon, but we in Barbados, and most of the islands in these seas, have been suffering for many months from a protracted drought. Have there been excessive

rains on the Continent?

THERE has been no doubt as to the author or authors of the

"Treatise on Probability," published under the superintendence of the Society for the Diffusion of Useful Knowledge, since 1844. In that year the "Value of Annuities and Reversionary Payments," by David Jones, was issued in two volumes by Robert Baldwin, of 47, Paternoster Row, and the tide-page states"To which is appended a ‘Treatise on Probability,' by Sir John William Lubbock, Bart., F. R.S., and J. E. Drinkwater Bethune, Esq., A.M." Sir John Lubbock's name also appears on the oposite page, with his first Christian name properly affixed, and this is repeated at the end of the volume in a catalogue of the works published by that society. The treatise consists of 64 octavo pages, and was one of the best on the subject at the time it was first issued. The late Prof. De Morgan alludes to it in the English Cyclopædia, and Mr. Todhunter quotes "Lubbock and Drinkwater no fewer than ten times in his "History of Probability," published in 1865. T. T. WILKINSON

THE HAWAIIAN VOLCANO, MAUNA LOA

THE following condensed account of the visit of a

party to the summit of the Hawaiian Volcano, Mauna Loa, at present in a state of fearful activity, appears in the Times of November 23, from the pen of Prof. F. L. Clarke.

"From Kaalualu, on the southern side of Hawaii, where we left the steamer on the afternoon of the 4th, we procured horses and proceeded to Wiohinu, where we remained for the night, and started next morning; and, after travelling a distance of twenty-five miles over a very rough road, although it is considered one of the best, we reached Lyman's ranch, where we were kindly received, and passed the night. The following morning, at daylight, our friends having exerted themselves in procuring the services of an experienced guide, we resumed our journey, and after stopping at several ranches for rest and refreshment, during the forenoon of the 6th, we emerged from the woods, which opened upon an immense field of pa-hoc-hoe. The lava fields in this region exceed in wildness and confusion the most extravagant imagination. For miles around, as far as the eye could reach, great masses of once molten lava were tossed into a thousand grotesque shapes. After travelling several hours over the roughest kind of ground imaginable, we reached a rude kind of gateway that was formed by gigantic columns of lava rock, through which we passed, and reached the edge cf a rough pali, from whence we were able to look out upon the summit. To our right rose a remarkable pillar, towering high up black against the sky, and on every hand yawned deep crevices and spent lava waves which bad dashed together in various shapes and

cooled.

"After reaching a favourable spot, where we left our animals secured for the night, we procceded about 500 yards over a narrow strip of rugged lava, when we suddenly found ourselves upon the edge of the crater of Moku-weo-weo, on the very summit of Mauna Loa, situated about 1,400 feet above the sea level. Before us yawned a fearful chasm, with perpendicular black walls some 800 feet in depth, carrying the eye to where, in the darkness of the lower basin, there sprang up in a glori ously brilliant light a mighty fountain of clear molten lava, and looking across and below us, at a distance probably of three-quarters of a mile, there arose from a cone in the south-vest corner of the lower basin a magnificent column of liquid lava, about seventy-five feet in diameter, that sent its volume of molten matter to a height of nearly 200 feet in a compact and powerful jet. The axis of this gigantic fountain inclined somewhat toward us, so that the descending cascade fell clear and distinct from the upward shooting jet, forming a column of continuous liquid metal surpassingly bright and beautiful to gaze upon. Flowing down the sides of the symmetrical cone, which the falling stream of lava was rapidly forming, were numerous rivers of liquid light, that as they flowed away, spreading and crossing, formed a lake of rivulets constantly widening and interlacing, which presented a beautiful and unique appearance.

"When we reached the summit of the mountain, the subdued roar of the pent-up gases was fearfully distinct as they rushed through the openings which their force had rent in the solid bed of the basin, and when we were in full view of the grand display our ears were filled with the mighty sound as of a tremendous surf rolling in upon a level shore, while now and again a mingled crash would remind us of the heavy rush of ponderous waves against the rocky cliffs of Hawaii."

Since the return of the party to Honolulu later advices state that the crater is increasing in action, and reflecting at night a light of unusual brilliancy, which reaches many miles off shore. The crater in Kilanea, since the present eruption of Moku-weo-weo, has been very irregular in its action, which leads to the supposition that the two alternate, that when one is active the other is passive.

may almost say for certain that Newton and his successors would have brought a great deal more out of the attention to it, and had tortured it as they did other prism than they did, if they had given a little more things; that those who follow us will point to us and say men of science, in working and experimenting, saw a the same; they possibly will say that in the 19th century, great many things, and chronicled them, but did not care result is, that work is not done which might be done if to go any further with them. This is very true; and the we were more receptive and original in our methods of investigation; that is to say, if we trusted Nature more and ourselves less.

FIG. 1.-Geometrical form of the prism.

being made, the science of spectroscopy must still be considered in its infancy. And yet, so far as one can see now-it is always very easy to prophesy after the event there seems very little reason why lectures on the spectroscope should not have been given two centuries ago; for

FIG. 3.-Refraction of light.

I propose that the first part of this lecture should in the main consist of an account of the prism and the principles of the spectroscope, and then of a description of the various kinds of spectroscópes which are now employed. I hope afterwards to go somewhat in detail into the applications of the spectroscope, not only with regard to terrestrial matters, but also with regard to those problems which we may possibly consider much grander, problems dealing with those celestial bodies which are sufficiently our neighbours to send us light.

Obviously, the first question we have to answer is this, What is a spectroscope? This I answer by saying that

FIG. 2.-Prism mounted on a stand.

nearly two centuries have elapsed since the immortal Newton made his classical researches on the action of a prism upon sunlight. You may, perhaps, be inclined to ask, how it could take 200 years for the knowledge of the prism, and of the wonders that can be worked by it, to become part and parcel of our common stock of information? If you ask me to explain this, I tell you candidly that I cannot; but there is this grain of comfort connected with it which none of us should forget

Revised rom the series of Cantor Lectures, delivered in 1869.

we

FIG. 4.-Explanation of the bent stick.

a spectroscope is an instrument in which the action of a prism or a combination of prisms is best studied. The next question, then, that arises, is, What is a prism? The accompanying figures (Figs. 1 and 2) will give a good idea of what is meant by a prism, and little time need be spent in description. It is usually a piece of glass-though it need not necessarily be so-bounded by five surfaces, two of which are parallel to each otherthough they are not necessarily so-and three of which, bounded by parallel edges, cut each other at different angles; it is in reality shaped like a wedge. The importance of these different angles you will see by-and-by.

The discoveries of Newton, to which I have already alluded, were connected with prisms, and were based on well-known properties of light.

If a beam of light, as for instance sun-light or an artificial white light, be allowed to enter a dark room from a round hole in a shutter, it will simply travel in a straight line from its source; and to make it deviate from this straight line one of two things must be done. The beam must either be reflected or refracted.

The reflection of light is of very ordinary occurrence, for when light strikes any polished metallic surface, or in fact a surface of any kind, it is more or less reflected by it. The phenomena of reflection are so well known, the use of the mirror or looking-glass being perhaps one of

at which a ray of light strikes the surface of water, or, in fact, any transparent surface, the greater will be the proportion of light reflected at its surface.

Refraction may be clearly studied by plunging a stick into a vessel of water: the stick will appear bent at the point where it enters the liquid, as in Fig. 4, thus giving the appearance as if the stick were lifted or bent upwards. Another very instructive experiment is to place a coin at the bottom of a vessel, and then, standing so that the coin is just hidden by its edge, to gradually fill the vessel with water; the coin will appear to rise with the bottom of the vessel, and will become visible, as shown in Fig. 5. The amount of refraction varies with the medium emploved, and also with its temperature. The effect of different media can be clearly seen by passing a ray of

FIG. 5.-Refraction of light. Apparent elevation of the bottoms of vessels.

the most tangible, that no detailed reference need Le made to them. The refraction or bending of light takes place when the ray passes obliquely from one medium to another of different density, as from air into water, or from water into air. A simple experiment may be made by passing the beam of light from above into a glass vessel containing water. If the ray strikes the surface perpendicularly, it will be seen that no visible change takes place, the ray simply proceeds directly into the water without altering its direction. If, however, the beam be allowed to fall on the surface of the water, say at an angle of about 45°, two things may be observed. In the first place a reflection will take place at the surface of the water-that is to say, the light will appear reflected at the surface, and it will be noticed incidentally that the angle at which the reflected ray leaves the water is precisely equal to that at which the incident ray strikes the surface, thus proving the rule that "the angle of incidence and of reflection

FIG. 6.-Light passing through plate of glass.

are equal." The second thing to be noticed is that on entering the water the direction of the beam of light will not be the same as it was in the air. In Fig. 3, the ray RI, striking the water at I, instead of proceeding to R', is deflected or refracted to S; that is, the ray will be bent downwards, or, what is the same thing, towards a line, I P, perpendicular to the surface, to a definite extent, depending on the angle of the incident ray. The experiment may be varied by allowing the light to fall on the surface at various angles, when it can be shown that the angle formed by the ray refracted in the water varies in proportion to the angle of the incident ray, and that the angles formed are bound together by a regular law. Another fact may be observed, that the smaller the angle

FIG. 7.-Deviation of luminous rays by prisms.

light into a vessel containing a liquid such as bi-sulphide of carbon, with a layer of water floating on the top. The ray will be seen to be bent on entering the water, and still more bent on passing from the water into the layer of bisulphide of carbon.

We have now to see what takes place when a ray of light enters a piece of glass. We will take first the case of glass with parallel sides. The ray on entering the glass at the upper surface is refracted downwards, as in the case of water, and travels through the glass until it reaches the under surface. Here we have precisely the reverse condition holding-that is, the ray of light passes from a dense medium to a rarer one. The ray is refracted upwards or away from the perpendicular line, and thus will exactly neutralise the previous refraction, and the beam of light will come out in a direction parallel to its original path, though not quite in the same straight line; as shown in Fig. 6, the ray, instead of proceeding in the direction of S', proceeds in the direction of S.