228 On the Measure of 
This principle of moving force may perhaps 
be illustrated in some degree, by comparing 
the phenomena of force with those of heat.— 
‘Metals and fluids having been observed to 
expand and contract according as their tempe- 
rature is increased or diminished, it was for a 
long time understood that temperature was the 
measure of heat. After it had been proved 
by Dr. Black that bodies of equal tempera- 
tures contain unequal quantities of heat, it 
was no longer eontended that temperature 
eould be taken generally as the measure of 
heat. Yet temperature is a most important 
property of heat, and in cases where the tem- 
perature and the heat increase and diminish in 
the same ratio, the temperature may be used 
as the measure of the heat.—In cases of mov- 
ing force, where the space described by a 
constant pressure, and its duration increase 
in the same ratio, the duration may be taken 
as the measure of the moving force.—Of abso- 
lute motion or of absolute heat, we know 
little—our researches are chiefly directed to 
relative heat and to relative motion.—In the 
estimation of deflecting forces, the duration 
becomes an important element.—In investi- 
gating the phenomena of liquefaction and 
evaporation, temperature becomes an essential 
consideration, Yet there appears to be no more 
