Two needles carefully placed on water near each other will attract each other; and in like manner two iron balls placed on mercury attract each other.

692. There are numerous processes in nature and art where the influence of the forces may be traced which are concerned in the production of capillary phenomena. Thus water is supposed to rise from wells and reservoirs below the surface of the earth to the roots of plants which are nearer the surface in the same way as it rises in fine tubes. Moisture deposited on the surface of fibrous bodies is transmitted through the interior by capillary attraction; and in consequence an increase of volume occurs which may lead to striking results. Thus let one end of a rope be fixed at a point, and the other end fixed to a weight vertically below the point; the rope being just stretched tight. Let the rope be wetted; then it swells in bulk, and in the act of swelling it shortens its length and raises the weight. Considerable weights may be raised in this manner. Another illustration is furnished by a process used in France for splitting off mill-stones from a block; it is thus described in Sir J. Herschel's Discourse on Natural Philosophy. "When a mass of stone sufficiently large is found, it is cut into a cylinder several feet high, and the question then arises how to subdivide this into horizontal pieces so as to make as many mill-stones. For this purpose horizontal indentations or grooves are chiselled out quite round the cylinder, at distances corresponding to the thickness intended to be given to the mill-stones, into which wedges of dried wood are driven. These are then wetted, or exposed to the night dew, and next morning the different pieces are found separated from each other by the expansion of the wood, consequent on its absorption of moisture..."

693. Endosmose. There are phenomena somewhat resembling capillary elevation and depression, but which at present have not been well connected with them. The general fact involved is this: when two different liquids are separated by a thin porous partition, either of organic or inorganic substance, currents arise between them in opposite directions. M. Dutrochet having introduced into the swimming bladder of a carp a thin syrup, carefully closed up the aperture by which he introduced it, and placed the

bladder in a vessel of water. After a time he found that the weight of the bladder had increased considerably; the water had passed through the pores of the bladder, and become mixed with the syrup. In another experiment he filled the bladder with water, and then put it in a vessel of syrup; in this case the weight was diminished after a time the water passed out of the bladder and became mixed with the syrup. He gave the name endosmose to the first process, and exosmose to the second. At present the former word alone is found sufficient to enable us to describe all the phenomena; and it is applied to the current which increases the volume: so that in both experiments there is endosmose from the water to the syrup.

694. The experiment is usually performed in the following way. Take a long tube open at both ends; to one end fasten a membranous bag containing a strong syrup: then immerse the bag in a vessel of water, supporting the tube in a vertical position. It is found that some of the syrup passes out into the vessel, but at the same time more of the water passes into the bag, so that the liquid will rise in the tube to the height of several inches. The experiment may be changed by putting water into the bag, and syrup into the vessel. Then again more water passes through the membrane than syrup: so that the level of the liquid in the vessel rises. In both experiments endosmose takes place from the water to the syrup. Instead of syrup other liquids may be used, as milk or albumen; and, in general, endosmose takes place towards the denser liquid.

695. For the production of endosmose the following conditions are necessary: (1) The liquids must be different but yet capable of mixing, as spirit and water; there is no endosmose between oil and water. (2) The liquids must be of different densities. (3) The membrane must be such that at least one of the liquids can pass through it. The ascent of the sap in plants seems to be a case of endosmose.

696. The phenomena of endosmose are seen in the case of gases. If two different gases are separated by a porous partition currents are produced both ways; and finally the composition of the mixture on both sides of the partition is the same.

LVIII. ANIMAL MECHANICS.

697. In the structure and in the movements of living creatures numerous interesting illustrations of mechanical principles have been pointed out by philosophers. In order to appreciate these fully some knowledge of anatomy and physiology would be required; but a few remarks may be made which will be easily intelligible.

698. The long bones of men are hollow, in agreement with the principle that they are stronger than solid bones of the same weight and length would be: see Art. 679. At a joint of two bones a tough elastic substance called cartilage is always interposed to break the force of shocks, like the buffers attached to railway carriages: see Art. 577. And moreover a joint is always provided with an apparatus by which a certain viscid liquid can be spread over the surfaces in contact. This somewhat resembles the white of an egg, and is hence called synovia; it is perpetually renewed as required, and acts like the oil and unguents which are used to prevent friction in machinery; see Art. 329.

699. Numerous examples of Levers of the third kind occur in the animal frame. One is found in the human fore arm when applied to raise an object. The fulcrum is at the elbow; the Power is exerted by a muscle which comes from the upper part of the arm, and is inserted in the fore-arm near the elbow; the Weight is the object raised in the hand. The muscle is a strap capable of extension and contraction, after the manner of an indiarubber band.

700. The pressure of the atmosphere plays an important part in keeping together the mechanism of the joints. Thus the head of the thigh bone cannot be separated by the mere weight of the limb from the surface of the cavity in the adjacent bone to which it is accurately fitted; in all motions the contact is maintained by the pressure of the atmosphere: the muscles which surround the hip joint may be divided, but still the weight of the

limb does not move the head of the thigh bone from the cavity. But if the cavity be exposed to the air by boring a small hole, or if the pressure of the atmosphere be removed by the aid of an air pump, the separation takes place. In ascending high mountains the pressure of the atmosphere is much diminished, and thus more stress is thrown on the muscles in order to maintain the contact between the convex and concave bones: this appears to be one cause of the peculiar fatigue felt in a laborious ascent. Dr Arnott seems to have been the first to draw attention to this example of the pressure of the atmosphere; he estimated the pressure at the knee joint to be about 60 pounds.

701. It is owing to the pressure of the atmosphere that various animals can sustain their bodies in opposition to the force of gravity. A fly on the ceiling of a room is an obvious example. The feet of the creature are furnished with a contrivance like a boy's sucker; so that a vacuum can be formed at the extremity of each foot, and the pressure of the atmosphere retains the foot in contact with the ceiling. It is said that the structure can be perceived "by looking at the movement of the feet of any insect upon the inside of a glass tumbler through a common magnifying glass; the different suckers are readily seen separately to be pulled off from the surface of the glass, and reopposed to another part." The same contrivance is found in the feet of other creatures, especially in the feet of the walrus, where it can be easily examined on account of the large size of the animal.

702. There are two kinds of motion of animals on the land. In one the effort consists in pressing the ground in the direction opposite to that in which the motion is to take place; the pressure is produced by internal muscular effort, and the reaction of the ground yields the force necessary to give forward motion: this is the mode of walking of man and quadrupeds. The other kind of motion may be called creeping, and is seen in the case of a snail: the animal here lays hold of an external fixed point, and clings to it by a part of his body; then he drags the mass of his body towards this

point. The motion of a snail may be watched by putting the animal on a piece of transparent glass, and looking through it from below.

703. The motion of birds is produced by the reaction of the air which they beat with their wings; so that the resistance of the air is essential to them, and they could not fly in a vacuum. The most arduous part of a bird's motion is the rising from the ground; the bird often runs for a short distance, or throws itself into the air by a sudden leap: the process resembles that of starting a boy's kite. Long-winged oceanic birds appear to use the tips of their wings as levers to raise their bodies. Birds which have a large surface of wing, as eagles, give only slight strokes in their flight. Birds on the contrary which have little wings, as pigeons, move them to a great extent, and thus compensate for the slight resistance which they experience from the air.

704. The motion of a fish is usually produced by lashing the water with its tail. The cuttle fish compresses forcibly its pouch which is full of liquid, drives out this liquid in one direction, and thus propels itself in the opposite direction. Fishes are furnished with an airbladder which they can compress by muscular action; this accordingly they do when they want to sink, for so they render themselves heavier than water, bulk for bulk: when they want to rise they allow the air-bladder to expand. As a fish is nearly of the same specific gravity as the medium in which it moves, there is no need for constant exertion, as in the case of the bird to prevent sinking; all that is necessary is to overcome the resistance of the medium.

705. The structure of the wings of insects has received much attention. It appears that under all modifications two elements are essential, namely a rigid mainrib, and a flexible membrane. If the rigidity of the former is destroyed flight is prevented; and so also it is if the membrane be covered with a varnish which hardens as it dries.