CHEMICAL AND PHYSICAL NOTES. 123 
ture will rise at unequal rates. These unequal masses may be 
thermometers with similar but unequal bulbs. Under the same 
conditions they will lose or gain heat at unequal rates. If, at any 
epoch, they and the air in which they hang are at the same 
temperature, and the temperature of the air is changed to another, 
which is kept constant, it is clear that at equal intervals of time 
from the initial epoch the two thermometers will have and will show 
different temperatures, although their scales may be without error. 
Further, in the ordinary course of diurnal variations of temperature 
these two thermometers will never again show the same temperature 
except for a moment of time when, in the exigencies of their different 
rates of cooling and heating, their thermal paths cross. 
The importance of self-recording thermometers for supplying a 
continuous record of the temperature of the air and of its variations 
is well understood. The form of instrument most commonly used is, 
on account of its compactness and comparative cheapness, that 
known as Richard’s recorder. It is usual to control its indications 
by reading a standard mercurial thermometer hung along side of it, 
and it is a very common practice to make this reading at 9 am.,, 
when the temperature of the air is changing most rapidly. When 
the paper is removed, the temperatures taken from the curve traced 
will be corrected so as to bring them into harmony with the 9 a.m. 
readings of the mercurial thermometer. Except in the very unlikely 
case of the mercurial and the recording thermometers having the 
same rates or terms of cooling, error will be introduced and not 
eliminated by this proceeding. The temperature of the air is as 
good as constant for a considerable time at the hour of the diurnal 
maximum, which is usually about 2 pm. This is the hour to 
compare the recorder with the mercurial thermometer, 
Again, if the wet and dry bulb thermometers are not exactly 
equal and similar—and they never can be—their indications are not 
exactly comparable in air of changing temperature or humidity. 
These few remarks will show the practical importance of a 
knowledge of the rate or term of cooling of every thermometer used 
in a meteorological observatory, and of the application of it to the 
correction of observations. They also show the supreme technical 
importance of making such thermometers according to a perfectly 
uniform pattern. 
When the thermometer was a novelty and philosophers studied 
its resources and applications in every direction, the importance of 
this constant was fully recognised, and the application of it to 
' observations was insisted on. 
