FORCE AND ENERGY. 231 
form so large a part of the food of herbivorous animals. Further- 
more, the difference in the case of thé other common nutrients is so 
slight that the heats of combustion as determined with the bomb- 
calorimeter may be used without appreciable error in computing 
the metabolism of energy in the body. The only substance involved 
in such computations for which the correction needs to be made is 
methane, CH,, the heat of combustion of which is at constant 
volume 13,246 cals. per gram and at constant pressure 13,344 cals. 
Units or MrasuREMENT.—The unit of force is the dyne, which 
is defined as the amount of force required to produce in a mass of 
one gram, in one second, an acceleration of one centimeter per 
second. 
When a .orce acts upon a mass, the amount of work done is 
measured by the product of the force into the distance (measured 
along the direction of the force) through which it acts. The unit 
of work is the erg, which is defined as the work done by a force of 
one dyne acting through one centimeter. 
Energy has been defined as the power of doing work, and is 
measured by the amount of work done, that is, in ergs. Since, 
however, the erg is a very small quantity, it is often more con- 
venient in practice to use a multiple of it. For this purpose the 
quantity 10” erg=1 Kilojoule (J) is a convenient unit. Energy is 
also frequently expressed in units based on weight instead of mass, 
the most common being the gram-meter, the kilogram-meter, and 
the foot-pound. The gram-meter is the work done against gravity 
in raising a weight of 1 gram through 1 meter. Since, however, the 
force of gravity, and consequently the weight of a given mass, varies 
at different points on the earth’s surface, it is necessary to state 
also where the weight is taken. At the level of the sea, in temperate 
latitudes, the force of gravity equals 980.5 dynes. Under these 
conditions, then, doing 1 gram-meter of work would be equivalent 
to exerting a force of 980.5 dynes through 100 em., which equals 
98,050 ergs. The kilogram-meter (kgm.) is the work done against 
gravity in raising 1 kilogram through 1 meter, and is accordingly 
1000 times the gram-meter or 98,050,000 ergs. The foot-pound 
is the work done against gravity in raising 1 pound through 1 foot 
and accordingly equals 13,550,000 ergs. 
In addition to mechanical energy the animal produces heat. 
