THE MECHANICAL EQUIVALENT OP HEAT. 
429 
CoMPAEisoN of the Reading 87'5 on with P (March 15, 1893). 
Temperature reading by P. 
Barometer (corrected). 
H^O pressure (expressed in terms of Hg) 
Total external pressure 
Observed reading. 
Calibration correction. 
Internal 
Zero 
Fundamental error correction 
Sum corrections 
millims. 
754-1 
12-1 
Zero point, 
millims. 
754-1 
4-4 
766-2 
758-5 
14-025 
— -036 
— -026 
0 
— -001 
0 
+ -026 
-f -008 
+ -028 
— -028 
— -006 
.+ •021 
Correction for stem [see (a), suprci] — ’002. 
Hence reading on the mercury scale = 14’044. 
Correction to H scale — ’067 ; to N scale — ’059. 
Hence 
Reading E„.. 
87-5 
Hydrogen scale. 
13-977 
Nitrogen scale. 
13-985 
I confess that I did not know until I adopted in detail the precautions advised by 
Dr. Guillaume in his “ Traite de Thermometrie,” that it was possible to regard the 
mercury thermometer as an iiistrument of precision, and I had considered such correc¬ 
tions as too refined for so rough an instrument. I am, however, now convinced that, 
given the right conditions, it is possible to ascertain temperatures by a mercury 
thermometer with a limit of error of 0°-002 or 0°-003. It is rarely, however, that such 
conditions prevail. The labour involved in the previous standardization of the, 
thermometer is enormous, and there are but few men capable of constructing ther¬ 
mometers on which such labour may be profitably expended. The actual operation of 
observing and reducing the results is also considerably greater than is the case when 
platinum thermometers are used. 
The following table gives the results of all observations made with the micrometer 
eye-piece. I had previously made several comparisons whose mean result is in fair 
agreement with those here given; but, as the last two figures of the readings had to 
be estimated, the discrepancies are greater, and the results of less value. 
