ON THE MECHANICAL EQUIVALENT OF HEAT. 
309 
the lever, is secured without affecting the mean moment on the brake case. And 
this also affords means of adjusting the position of the lever. To admit of adjust¬ 
ment for wear the shaft is coned over that portion which passes through the hushes, 
the bushes being similarly coned, and screwed into short sleeves on the casing, so 
that by unscrewing them the wear can be followed up and leakage prevented. 
The brake levers for carrying the load and balance weight, are such as to allow the 
load to be suspended from a groove parallel to the shaft, at 4 feet from the shaft, by 
a carrier with a knife edge, the carrier and the weights each being adjusted to 
25 lbs. (shown figs. 1 and 4). In addition to this load, a weight is suspended from a 
knife edge on the lever nearer the shaft, this weight being the piston of a dash-]3ot 
in which it hangs freely, except for the viscous resistance of the oil. This weight 
being adjusted to exert a moment of 100 ft.-lbs., and again a travelling weight of 
48 lbs., is carried on the lever and worked by a screw with ^ inch pitch, so that one 
turn changes the moment by 2 ft.-lbs., while a scale on the lever shows the position. 
A shorter lever on the opposite side of the case carries a weight of 74'6 lbs., which is 
adjusted to balance the lever and sliding weight when the load is removed. 
Tlie Accuracy of the Brahe. 
5. The principle of these hydraulic dynamometers is that when moment of 
momentum is introduced into a fixed space without altering the moment of momentum 
within that space, the rate at which moment of momentum leaves the space must 
equal the rate at which it enters. The brake-wheel imparts moment of momentum 
to the water within the case, and the friction of the shaft imparts moment of 
momentum to the case. The water in the case, when its moment of momentum is 
steady, imparts moment of momentum to the case as fast as it receives it, and the 
time mean of the moment of the load is equal to the time mean of the moment of 
effort of the shaft. 
This is not affected by water entering and leaving the case at equal rates, provided 
it enters and leaves radially. 
The condition of steadiness is, however, essential, in order that the moment of 
effort shall be at each instant equal to the moment of resistance on the case ; any 
change in the moment of momentum of the water in the case being the result of the 
difference of the moment of effort on the shaft and that of resistance on the case. 
The Time-Mean of the Moment of Effort. 
6. When, however, the shaft is run over an interval of time, the mean moment of 
resistance on the case, less the difference of the moments of momentum of the water, at 
