22 TEMPERATURE AND TIME—MACGREGOR. 
of time between two instants, i. e., to compare one interval with 
another as to magnitude ; and when we express the laws of the 
motions of bodies by reference to time, and thus seem to claim 
to be able to measure time, we are in reality only expressing the 
laws of the motions of bodies in terms of the contemporaneous 
motion of some one body. 
The temperatures of bodies may be described by reference 
to any quantity which varies with temperature, as the volume 
of a body under constant pressure, or its pressure under constant 
volume. Thus the temperature of a body is usually described as 
being the same as that of the mercury in a thermometer when 
the apparent volume of the mercury has a specified value. 
Except for the difficulty of making thermometers which are 
exactly comparable, temperatures may be described in this way 
with as great precision as may be desired. 
It is therefore possible, as it is also important, to compare 
the changes of volume, pressure, Wc., of different bodies, involved 
in given changes of temperature. Changes of volume, pressure, 
&e., consequent upon the same change of temperature, may be 
called co-thermal changes, the term co-thermal having the same 
signification with respect to temperature as contemporaneous has 
with respect to time. 
To facilitate the comparison of co-thermal changes, some one 
such change is chosen, and all others are expressed in terms of it. 
Usually the change chosen as standard is the change in the 
apparent volume of the mercury in the ordinary thermometer. 
And when the laws of the variation of the volume, pressure, etc., 
of bodies with temperature are expressed in terms of the 
co-thermal change in the apparent volume of the mercury of the 
thermometer, the laws of the variation of volume, pressure, etc., 
thus expressed, are said to be expressed in terms of temperature. 
They are no more really laws of variation in terms of tempera- 
ture however than laws of the motions of bodies expressed in 
terms of the contemporaneous rotations of the earth are laws of 
their motions with respect to time. When we speak of them as 
laws of variation with respect to temperature, we assume, for the 
sake of a convenient terminology, that increments of the apparent 
