OF THE MECHANICAL EQUIVALENT OF HEAT, 
367 
hydraulic supporter, w v, was not employed in the first two series of experiments, and 
will be described further on. Three sides of the frame are boxed in permanently ; the 
fourth, or front, has shutters with windows which can be removed at pleasure. A 
delicate thermometer, suspended within the frame, is observed through a telescope, 
as is also the thermometer employed in reading the temperature of the calorimeter. 
Fig. 2 represents the section of the calorimeter, with its paddle, all of stout sheet 
brass ; and fig. 3 gives a plan of the same. The dotted lines in the latter show the 
position of the fans in the upper part. The axle of the paddle works easily in the 
collar m, and is screwed into the boxwood piece n. There is another boxwood piece, 
o, fig. 1, placed to prevent any considerable quantity of heat arising from the friction 
of the shaft being conducted downwards. This friction was, however, so small that the 
precaution was afterwards found to be needless. 
It will be seen in figs. 2 and 3 that there are four stationary vanes in the calori- 
meter, and two sets of rotating vanes, each of five arms, the upper set being fixed on 
the axis 9° behind the lower set. Hence no two vanes pass the fixed ones at the 
same moment, and inasmuch as the momentary alteration of resistance at crossing 
takes place 40 times in each revolution the resistance may be considered as practically 
uniform. 
The circumference of the groove of the calorimeter was found by measuring its 
diameter in various places, and also by measuring it directly with a fine wire, allowing 
for the thickness of the latter. The results, obtained with a rule verified by the 
Warden of Standards, are — 
Diameter in inches. 
Circumference. 
10-5850 . . x 7t=33'2538 
10-5855 . . x 7t=33"2553 
10-5855 . . Xtt=33-2553 
Measured by wire yA ( y in. cl i am etcr = 3 3 ' 2 5 3 8 
„ „ tv „ =33-2563 
Average 33'2549 inches= 2‘77l24 feet. 
The diameter of the silk cord, which was the finest that could be used with safety, 
was exactly y^th inch. Hence the distance to be considered as run against the 
weights of the scales was, for each revolution, 277386 feet. 
When a silk thread with a weight of 11,000 grains at each extremity was thrown 
over the small pulleys, 30 grains added to one of the weights was sufficient to keep 
both in motion. This friction, which includes the rigidity of the silk cord, taken with 
the distance traversed by the weights in their slight upward and downward motions 
during an experiment, gives the loss of work on the calorimeter from this cause. It 
did not amount to more than ijotK) o o subtractive from the equivalent, and could there- 
fore be neglected. 
3 n 
MDCCCLXXVTIT. 
