AIR. JOULE ON THE MECHANICAL EQUIVALENT OF HEAT. 
77 
The effect of each degree of difference between the temperature of the laboratory 
and that of the apparatus being 0°'18644, 0°‘9157 — 0°'0606+0°'000488=0°'855588, 
will be the proximate mean increase of temperature in the above series of experi- 
ments. The correction, owing to the mean temperature of the mercury in the friction 
experiments being 0°'013222 higher than appears in the table, will be 0°’002452, 
which, being added to the proximate result, gives 0°'85804 as the true thermometrical 
effect. This, in the capacity of 2207 T68 gi’S. of water, is equal to 1° in 2'70548 lbs. 
of water. 
The leaden vveights amounted to 137326 grs., from which 1040 grs. must be sub- 
tracted for the friction of the pulleys, leaving 136286 grs. as the corrected weight. 
The mean height of fall was 1293‘532 inches, from which 0*047 inch, subtracted on 
account of the velocity with which the weights came to the ground, leaves 1293*485 
inches. This fall, combined with the above corrected weight, is equivalent to 
2098*618 foot-lbs., which, with 1*654 foot-lb., the force developed by the elasticity 
of the string, gives 2100*272 foot lbs. as the mean force employed in the experiments. 
2100*272 
2-70548 
= 776*303, will therefore be the equivalent from the above series of ex- 
periments, in which the amount of friction of the mercury was moderated by the use 
of lighter weights. 
Ath Series of Experiments. — Friction of Cast Iron. Weight of cast iron apparatus, 
44000 grs. Weight of mercury contained by it, 204355 grsi Weight of the leaden 
weights and string attached, 203026 grs. and 203073 grs. Average velocity with 
which the weights fell, 3*12 inches per second. Time occupied by each experiment, 
38 minutes. Thermometer for ascertaining the temperature of the mercury, C. 
Thermometer for registering the temperature of the air, A. 
