We can now understand why a water-beetle is able to run across the surface of a pond, without wetting its legs or running any risk of sinking. Each of its legs produces a dimple in the surface, but the pressure on any one leg is not sufficient to break through the skin. We can imitate this by bringing the point of a lead pencil gently to the surface of water, when a dimple is produced, but the skin is not actually penetrated. On removing the pencil, the dimple immediately disappears, just as the depression caused by pushing the finger into a stretched sheet of indiarubber becomes straight immediately the finger is removed. *Elastic Skin of other Liquids—Minimum Thermometer.*—The possession of an elastic skin at the surface is not confined to water, but is common to all liquids. The strength of the skin varies with different liquids, most of which are inferior to water in this respect. The surface of petroleum, for example, is ruptured by a weight which a water surface can readily sustain. But wherever we have a free liquid surface, we shall always find this elastic layer at the boundary, and I will now show, by the aid of lantern projection, an example in which the presence of this layer is utilized. On the screen is shown the stem of a minimum thermometer—that is, a thermometer intended to indicate the lowest temperature reached during a given period. The liquid used in this instrument is alcohol, and you will observe that the termination of the column is curved (Fig. 2). In contact with the end of the column is a thin piece of coloured glass, with rounded ends, which fits loosely in the stem, and serves as an index. When I warm the bulb of the thermometer, you notice that the end of the column moves forward, but the index, round which the alcohol can flow freely, does not change its position. On inclining the stem, the index slides to the end of the column, but its rounded end does not penetrate the elastic skin at the surface. I now pour cold water over the bulb, which causes the alcohol to contract, and consequently the end of the column moves towards the bulb. In doing so, it encounters the opposition of the index, which endeavours to penetrate the surface; but we see that the elastic skin, although somewhat flattened, is not pierced, but is strong enough to push the index in front of it. And so the index is carried towards the bulb, and its position indicates the lowest point attained by the end of the column—that is, the minimum temperature. Obviously, a thermometer of this kind must be mounted horizontally, to prevent the index falling by its own weight. [Illustration: __Fig._ 2.—Column and index of minimum thermometer._]