COMPAUISON OF THERMOMETERS. 
775 
gr. weiglit. 
1 U=Sp + 0-0081 30 
2 U=Sp + 0-0087 14 
3 U=:Sp + 0-0133 6 
4 U=Sp + 0-0024 9 
5 U=:Sp + 0-0261 6 
The mean of all the equations gives 
U=Sp-j-0*0096 grain. 
Excluding the last, which depends upon the weighings in 1824, 
U=Sp+0’0079 grain. 
Excluding all except the result of the comparisons of U with the two platinum troy 
pounds, 
U=Sp+0'0083 grain. 
Comparison of Thermometers. 
The thermometer K was supplied by the Committee of the Kew Observatory. It 
bears the inscription “ No. 43, Kew Observatory, July 1 853.” It is graduated by lines 
etched upon the tube at every fifth of a centesimal degree. The distance between 
the freezing- and boiling-points is about 18’1 inch. Mr. Welsh, under whose super- 
intendence it was constructed, examined it by the method employed by Mr. Sheep- 
shanks, and concluded that the graduation was correct throughout the scale to one- 
tenth of a small division, or 0°'02 C. He obtained the following data for deter- 
mining its boiling-point at the Kew Observatory, the stem being vertical : — 
1853. 
Barom. 
Att. therm. 
Barom. in millims. 
of mercury at 0° C. 
Reading of K. 
Temp, by Regnault’s 
Tables. 
Error. 
July 27. 
30-039 
65-5 
760-47 
99*98 
100-02 
— 0-04 
August l6. 
29*726 
64-3 
752-61 
99*70 
99*73 
— 0-03 
August 17. 
29-640 
63-2 
750-51 
99*38 
99-65 
— 0-07 
Mean — 0-047 
Hence the boiling-point with the stem vertical under the pressure of 760 millimetres of 
mercury at 0°C., is 99°*953. The freezing-point, with the stem vertical, was ~-0°'04, 
before boiling, and — 0°'12, after boiling. 
Assuming 100° C. to be the temperature of steam under Laplace’s standard atmo- 
spheric pressure, or the pressure of a column of mercury at 0° C., the height of which 
in millimetres is 760-1- T946 cos 2 latitude -1-0*0001492 height in metres above the sea, 
the temperature of steam at Kew under the pressure of 760 millimetres of mercury 
at 0°C., will be 100°*016. But the reading of K was 99°*953. Hence, denoting by 
K the reading of No. 43 at the temperature # by a thermometer the freezing- and 
boiling-points of which are accurately determined, when ^=0°, K=-f0°*12, and 
when 100°, K= -l-0°*063. In August 1853 it was heated to rather above 100° C. 
On December 15 the freezing-point had ascended to — 0°*04. The reading was 0°*00 
when the thermometer was surrounded with broken ice. May 26, 1855, and also 
when immersed in pounded ice, July 10, 1855. The determination of the zero of a 
