These trials consist in examining the comparative degrees of elasticity and tenacity.

The manner in which they are conducted in the French navy appears practical and easily followed. The first test consists in cutting from the sheets samples, which are left in a steam-boiler under a pressure of 5 atmospheres for 48 hours. At the end of this time, the pieces should not have lost their elasticity. The specimens may then be placed on the grating of a valve box, under a pressure from above of 85.5 lbs. per square inch, and should withstand 9100 stroke- at the rate 0f 100 per minute. Specimens not boiled should withstand 17, 100 strokes. Thongs of rubber boiled, and having a section 0.6 inch square and a length of 8 Inches, fixed between supports and elongated 3.9 inches, should resist, without breaking, a further elongation of 8 inches, repeated 22 times a minute for 24 hours. Thongs not boiled, under the same conditions, should resist for 100 hours. These extra elongations may be easily made by a wheel, to the periphery of which one end of the thong is fastened, while the other extremity may be attached to a support. By turning the wheel, any determined elongation may be given at the rate of from 20 to 25 times per minute. Under the above conditions, bands of first quality rubber, perfectly pure and well vulcanized, break after 180 or 200 elongations of 8 times the initial length. Bands cut from pure rubber, but of secondary quality, break after 50 or 60 elongations. Inferior caoutchouc, containing mineral matters or residue of old vulcanized rubber, gives no results at all.

M. Ogier has investigated the properties of rubber belts made of repeated layers of cloth covered with prepared rubber. Through the adhesive nature of the caoutchouc, the superposed tissues form, after vulcanization, a homogeneous substance, comparable, in M. Ogier's opinion, to the best curried leather. His experiments, in order to obtain the coefficient of friction of these belts on cast-iron pulleys, give us results varying from 0.42 to 0.84, as against the coefficient for leather, 0.28. The minimum value corresponds to canvas and rubber belts without an exterior rubber coating. On pulleys of various forms, the maximum value of the coefficient of friction was found on those slightly convex and presenting a roughly turned surface, this result being inverse to that obtained with leather belts. Similarly the presence of fatty bodies has an opposite action on the cloth and rubber belts to that which it has on- leather. On covering the former with a light varnish of half olive-oil and half tallow, the adhesion was found to be considerably augmented. (1) The resistance to traction of rubber and canvas belts per square millimeter (0.0009 square inch) of section, is at least equal to that of leather belts. (2) This resistance per square millimeter is independent of dimensions-length, breadth, or thickness. Such is not the case with leather belts, and therefore preference should be given to rubber belting whenever the conditions of the power to be transmitted necessitate the employment of very long, very wide, and very thick belts. (3) From two trials it appears that the external covering of caoutchouc adds nothing to the resistance, and hence it is advantageous to use covered belts which, at equal weights and prices, give a superior resistance. (4) Under the same weight, the elastic elongation of leather belts is double that of rubber ones. The permanent elongation, under a change of 0.55 pound per square millimeter, reached 2 per cent in the former and nothing in the latter.