was known in some of its effects-at all events to every one who had seen a glacier-for hundreds of years; but it was only within comparatively recent times that attention was directly called to it. The first who seems to have done so was Dollfuss-Ausset, in his experiments upon the Swiss glaciers, where he showed that by compressing a number of fragments of ice in a Bramah press, it was possible to melt them; and when pressure was taken off them, to allow them to revert again into a solid block. But he found that with very cold ice the experiment did not succeed. In fact, as we now see, even with his Bramah press, he could not apply pressure enough. Another form in which it must have been well known for hundreds of years is the form in which we try the same experiment every time we make a snowball. Schoolboys know well that after a very frosty night the snow will not make :'-their hands cannot apply sufficient pressure. But if the snow be held long enough in the hands to be warmed nearly to its melting point it recovers the power of 'making,' or rather of 'being made.' Every time we see a wheel-track in snow we see the snow is crushed, and even after one loaded cart has passed over it, certainly after two or three have passed, -the snow has been crushed into clear transparent ice. The same thing takes place by degrees after people enough have walked over a snow-covered pavement; and in all these cases this minute lowering of the freezing point has led to the result. And now we see how it is that the enormous mass of a glacier moves slowly on like a viscous body, because in consequence of this most extraordinary property it behaves under great pressure precisely as if it were a viscous body. The pressure down the mass of a glacier must of course be very

great, and as the mass is-especially in summer-freely percolated through by water, its temperature can never (except on special occasions, and then near the free surface) fall notably below the freezing point. Now, in the motion of the mass on its journey, there will be every instant places at which the pressure is greatest,—where in fact a viscous body, if it were placed in the position. of the glacier ice, would give way. The ice, however, has no such power of yielding; but it has what produces quite a similar result-wherever there is concentration of pressure at one particular place it melts, and as water occupies less bulk than the ice from which it is formed, there is immediate relief, and the pressure is handed on to some other place or part of the mass. The water is thus relieved from the pressure by the yielding caused by its own diminution of bulk on melting. The pressure is handed on; but the water remains still colder than the freezing point, and therefore instantly becomes ice again. The only effect is that the glacier is melted for an instant at the place where there is the greatest pressure, and gives way there precisely as a viscous body would have done. But the instant it has given way and shifted off the pressure from itself it becomes ice again, and that process goes on continually throughout the whole mass; and thus it behaves, though for special reasons of its own, precisely as a viscous fluid would do under the same external circumstances.

LECTURE VI.

TRANSFORMATION OF ENERGY.

Further consequences of Carnot's ideas. Anomalous behaviour of water and of india-rubber. Application to rock masses, and the state of the earth's interior. Availability of energy, and loss of availability. To restore the availability of one portion of energy, another portion must be degraded. Dissipation of energy. Sources of Terrestrial and Solar Energy. Energy of plants and animals. Measure of the Sun's Radiant Energy. Energy now in the Solar System.

I SHALL commence this afternoon by taking a few further consequences of the grand ideas of Carnot, which I developed at full length in my last lecture. Whereever, in fact, we meet with any one anomalous physical result, we almost invariably find that it is associated with other anomalous results; and perhaps it is in this respect that Carnot's ideas have been of the greatest use in giving us new information.

Let us take, for instance, what I incidentally mentioned in connection with thermometers in my last lecture, the fact that water would be an exceedingly bad substance to employ for the purpose of filling a thermometer bulb, because, even supposing that it did not burst the bulb when it froze,-supposing that we were using it from zero of Centigrade scale up to 100°, it would begin when first heated to contract, and would continue to do so up to the temperature of 4° C., and then it would expand like most other liquids. Now, here is a substance, which, when heated, becomes less

in bulk it contracts instead of expanding. We should expect, therefore, to find that water exhibits some other anomaly, really the same thing if we could understand exactly all about the physical question involved, but appearing very startling to us when presented as something apparently quite new and different.

Let us look closely into the circumstances of this question. We are applying heat to water, and in consequence the water is contracting instead of expanding. Suppose, then, that we take water at a temperature between zero and 4° C., and apply pressure to it, what should we expect? Pressure applied to water at any temperature above 4° C., and to most other liquids at any temperature whatever, develops heat. Now Carnot's reasoning shows that just for the same reason that pressure in a liquid which expands by heat produces a development of heat, so in a liquid such as water between zero and 4° C., a liquid which contracts on being heated, pressure produces cold, so that water taken at any temperature between these narrow limits and squeezed in a Bramah press becomes colder in consequence of the forced contraction in bulk.

Another very startling result is derived from the anomalous behaviour, which I daresay is familiar to most of you, of an india-rubber band. I daresay you all know that an india-rubber band suddenly stretched and applied to the lip feels warmer than before. Most bodies when extended become colder, as air does when it expands. If you pull out a steel spring it becomes colder, as Joule showed by direct experiment; but india-rubber is an exception: it not only becomes warmer when it is pulled out, but if,-keeping it still pulled out-you allow it to cool to the temperature of

the air, and then suddenly allow it to contract again, it is very much colder than before, as you feel by applying it again to your lip.

Now these other bodies, such as air and the steel spring, when heat is applied to them, expand. A steel spring supporting a weight, and with heat applied to it, will expand, and allow the weight to descend. On the contrary, as I hope to be able to show you by a simple arrangement, when you apply heat to stretched india-rubber, instead of expanding it contracts, and perfectly in accordance with the theoretical prediction of Sir William Thomson from Carnot's reasoning applied to this case.

I suppose the spot of light crossed by a sharp horizontal dark line is visible to all of you near the top of the scale. The light from an incandescent lime-ball passes through a lens, and (after reflection from a plane mirror) is brought to a focus on the scale. The horizontal dark line is the image of a wire stretched in front of the lime-ball. This is our index, not the vaguely-defined spot of light. I have here suspended a piece of vulcanised india-rubber gas-tubing, with the spiral wire-core removed from it. Its lower end has a scale-plan attached, and is also fastened to a lever which moves the plane mirror. In order to show you what the movements of the apparatus indicate, my assistant will put one or two additional weights into the scalepan hanging from the tube, and you will notice that the effect of the additional weights, which is of course to extend the india-rubber, produces a movement of the reflected light up the scale. Hence, if this indiarubber were to expand further by the application of heat, we should see the spot of light on the scale move