GENERAL MEETING. 25 
287TH MneEtINnG. May 8, 1886. 
The President in the Chair. 
Thirty-five members present. 
The Chair announced the election to membership of Messrs. 
JosEPH Hammond Bryan and Merwin MARIE SNELL. 
Mr. Tuomas RussELu made the following communication on 
TEMPERATURES AT WHICH DIFFERENCES BETWEEN MERCURIAL 
AND AIR THERMOMETERS ARE GREATEST. 
Glass and mercury do not expand uniformly. An increase in 
temperature of one degree at one hundred degrees causes greater 
changes of volume than the same increase at zero. (In all refer- 
ences here to degrees and temperatures the centigrade scale is to be 
understood.) Normal mercurial thermometers, when corrected for 
their various ‘errors of construction, differ among themselves and 
also from the air-thermometer. 
At 40° the mercurial thermometer reads about 0.°2 higher than 
the air-thermometer. At— 38.°8, the melting point of mercury, it 
reads about 0.°2 lower. The quality of mercury in a thermometer 
has an influence on its reading. A thermometer containing roo00 
of lead in the mercury will read 0.°5 lower at 50° than if the 
mercury is pure. [H. J. Green.] 
Comparisons have been made at the Signal Office between an air- 
thermometer and a number of mercurials. Some deductions have 
been made from the results of this work as to the temperatures at 
which the differences between the two thermometers are greatest. 
From the same results there have also been derived values of the 
coefficients of expansion of glass dependent on the second and third 
powers of the temperatures. It is to these I wish to call your at- 
tention. 
The air-thermometer used was of the kind that measures tem- 
peratures by the varying pressure of a quantity of air kept at a 
constant volume. Five Tounelot mercurial thermometers were 
compared with this air-thermometer at temperatures from 0° to 
55°. Two Baudin thermometers were compared with it from 0° 
to — 38.°8. 
The freezing points of mercurial thermometers rise with age. A 
few days after a thermometer is filled this rise may amount to a 
