428 PROFESSOR A. SCHUSTER AND MR. W. GANNON ON A 
being dependent on the temperature to which the thermometer was previously 
exposed. It is found, however, that consistent results can be obtained by referring 
the reading of a thermometer not to a fixed zero, but to the position of the mercury 
thread, observed when the thermometer is plunged into ice directly after being 
exposed to the temperature to be measured. Calling T,/ the zero, as thus determined, 
after exposure to the temperature ¢, that temperature measured in the mercury 
thermometer is now defined as 
ESC es 
The two definitions of temperature indicated by (1) and (2) are identical if the 
so-called depression of the zero T,° — T;’ is proportional to the temperature ¢, as with 
thermometers made of the Jena, or French standard glass. Writing in this case 
Ty? — Ty! == @ (T; — Ty’) 
or 
1 
T, — Ti =~ (1 — 1,9), 

and similarly 
. i 
Tyo — T= iene (Tyo ne T.), 
we obtain by substitution in (2) 
t = 100 (T, aE Ty) /(T roo — T,%), 
which is identical with (1), if the zero in that equation is meant to signify the 
position which the thermometer would take up if plunged into ice for a considerable 
time. 
If the observations are made with English glass of recent manufacture, the 
identity of equations (1) and (2) does not hold, as the depressions of the zero are 
nearly proportional to the squares of the temperatures. It then becomes of import- 
ance to define clearly how a temperature is taken. Thus Wrese finds that ther- 
mometers made of English glass read /ower than the air thermometer by as much as 
0°:047 at 40°, while Rowxanp states that a// thermometers examined by him, 
including those of English manufacture, read higher than the air thermometer. The 
two statements are not contradictory; the discrepancy is explained by the fact that 
Wiese refers all temperatures to the zero observed directly after each reading, while 
at the time Rowianp’s work was done, this practice, which alone converts the 
thermometer into an instrument fit for scientific research, and is now uniformly 
adopted, had not yet come into use. 
It is an essential point in thermometric measurements that the work done by 
different observers should be comparable, and some uniform scale should be adopted. 
