408 Intelligence and Miscellaneous Articles. 



reel. The apparatus is so arranged as to allow the weight a fall of 

 4'50 metres. The friction of the different parts having been re- 

 duced as far as possible, is allowed for by Coulomb's laws. 



On one end of the axle a copper disc is fastened, 8 centims. in 

 diameter and 8 millims. in thickness at its edge. When one of the 

 cords unrolls by reason of the weight which it carries, the reel and 

 its axle turn and give a rotatory motion to the disc. 



A split ring tending to spring open and terminating at both ends 

 in ears, clasps the circumference of the disc, while a rod bearing a 

 screw-thread works in the lower ear of the ring, thus allowing the 

 latter to be opened or shut at pleasure, thereby causing a frictional 

 resistance to the rotation of the disc. This friction is made suffi- 

 ciently uniform by fastening, on the inner circumference of the 

 break-ring, five steel springs at equal distances apart, and with 

 their convex sides turned towards the circumference of the disc. In 

 this way, as long as the ring is open, the springs scarcely graze the 

 disc, but as the ring is closed, the springs are straightened and the 

 friction is exercised on a more extended surface. When the ring is 

 completely screwed up, the five springs coming in contact with the 

 disc almost throughout their entire length, arrest its motion. 



In order to avoid shocks, and to ensure greater uniformity of -mo- 

 tion, on the extremity of the axis opposite to the disc a toothed 

 wheel is fastened, which works in an endless screw turning on a ver- 

 tical axis. This axis also carries a toothed wheel which works in a 

 pinion, the upj^er extremity of whose axis is provided with a ball 

 governor. 



In order to avoid loss of heat by conduction, the key with which 

 the experimenter opens and shuts the break is made of ivory. 



The distance of fall of the additional weight was at least 12 metres. 

 It fell with great regularity and withoat the least shock, its velocity 

 being on the average 20 millims. per second. It is necessary that 

 the fall be corrected for the elongation which the cord undergoes, 

 under the influence of the additional weight. This correction may 

 be performed with great ease and precision. 



The box containing the break is perfectlj'^ enclosed by means of 

 an obturator of cork 15 millims. thick. This obturator only allows 

 the ivory key to pass out, and the cords whose alternate unroUings 

 effect the rotation of the axle of the break. 



The length of the mercurial column, equivalent to one heat-unit 

 on the calorimetric tube, was fixed when the break, fastened in its 

 place, made part of the mass of the calorimeter. 



The greatest pains were taken to avoid all influences tending to 

 render too small the number expressing the heat produced by the 

 desti'uction of the motive power. In fact, in experiments of this 

 kind, the tendency is always to diminish this number, consequently 

 to increase the mechanical equivalent of heat. 



Having, therefore, adopted all possible precautions, I found by 

 experiments agreeing very well with one another, the number 41 3*2, 

 which differs very little from the number 426 which had to be 

 checked.— Co»i/jfes Rendus de I' Acad, de Scien., Feb. 15, 1858. 



