382 
PROFESSOR O. REYNOLDS AND MR. W. H. MOORBY 
point, a suitable correction was made to that already obtained by immersion in the 
mixture of pounded ice and water. 
Boiling-point Thermometers. 
23. In the first instance two similar thermometers were made to order to be ready 
for use in the discharge tube, but on one of these being broken, two additional ones 
were obtained. Only one of the four was, howevei-, used in the research, viz.. Pi. 
This was a chemical thermometer with a ^-inch stem, having the scale engraved as 
already described. The length was IG^ inches over all, the bulb being inches 
long, and a blank space of 5|- inches separating the top of the bulb from the first 
graduation. The scale extended from 200° to 220° Fahr., the 20° occupying 8| inches 
of the stem. 
During the course of an experiment the reading of this thermometer was 
continually altering slightly. This fluctuation made it almost impossible to read the 
temperatures to ^ degree. So that oidy the nearest i\jth of a degree has 
been recorded throuo’hout. 
o 
The English standard boiling point, viz., 212° Fahr., is detined to be the tempera¬ 
ture of saturated steam under a pressure which would sustain a column of mercury 
29-905 inches long at the temperature of melting ice at the sea level in the latitude 
of Greenwich. 
This corresponds exactly, on being corrected for the variation in the value of 
gravity, to the modern definition of the boiling point on the Centigrade scale, the 
pressure in this case being equivalent to a column of mercury 760 millims. lono- in 
latitude 45°, the other conditions being as before. 
It was consequently possible to use Regnault’s steam table in the neighbourhood 
of the atmospheric boiling point as a standard of comparison for the scale of this 
thermometer. 
In order to conduct the comparison in Manchester, a knowledge of the relative 
value of gravity was necessary. 
This was deduced from a formula given in ‘ Memoires sur le Pendule ’ (Societe 
Fran 9 aise de Physique), which is given below, 
(I - 0*00259 cos 2(f>), 
where — is the ratio of the value of gravity in latitude (/> to its value in latitude 45°. 
^ 4:0 
The latitude of Manchester being 53° 29', this gives 
AY = 1-000756. 
