586 MR EDMUND J. MILLS’S 
RECKNAGEL, and amply confirmed by myself) that the contents of the air- 
reservoir are continually undergoing diminution by escape of air against the 
mercurial pressure, and can scarcely be depended on for more than a single 
day.* They may also be in part due to the difficulty experienced by air in 
leaving, or being returned to, the reservoir through the capillary tube, which 
is probably nearly always too narrow. ‘The latter fault is capable of easy 
remedy ; but it unfortunately did not occur to me until the experiments were 
complete, and I have since had no leisure to repeat them with the new con- 
dition. 
According to the equation expressing the relation between thermometer 2 
and the air-thermometer, the maximum difference between the two lies at 
34°47, when it amounts to 0°198. The numbers on which this equation is 
based have received PoGGENDORFY’S correction. 
VI. CoMPRESSION. 
The stem of an ordinary thermometer may be regarded, for all practical 
purposes, as incompressible. The bulb, however, is always thin, and has yield- 
ing sides ; it is therefore affected by external pressure. 
The first observation with regard to the compression of the bulb appears to 
have been made by GourDoN, who, according to EcEn,t made the experiment 
of fracturing a thermometer of known positive zero-error. After fracture, the 
zero fell by a measurable amount. Ecen himself repeated this experiment, 
and obtained the following results with spherical bulbs :— 
Thermometer. ap VI. Tx. 
Depression due to 1 atmosphere, , ‘205 080 369 
Relative thickness of the bulbs, . og Peat! i Zo 1:0 
Here the effect evidently tends to be inversely proportional to the thickness 
of the bulb. Four of my own thermometers were opened, and the zero- 
depressions observed were — 
Thermometer. 500. 501. 502. 20. 
Observed depressions, . d *285 199 193 180 
Depressions corrected for the \ 
295 ‘200 “199 ‘5 
pressure of a little air, 
Thermometer 20, which was cylindrical, contained no appreciable quantity of 
air. The other thermometers had spherical bulbs. On the whole, it appears 
* My own experiments were for the most part performed at the rate of a “ series” per day. 
+ Pogg. Ann, xi. p. 283 (1827). 
