1823.] the Barometer to the Measurement of Heights, 165 



of the corrections which the heights of the mercury must undergo 

 on account of the changes in the temperature of the instrument, 

 sufficiently remind us that we must employ none but the best 

 thermometers. The same remark will apply to the thermometers 

 employed for the temperature of the air ; but thermometers 

 perfectly to be depended upon are, perhaps, of all the apparatus, 

 the most difficult to procure. When the observer does not 

 construct them himseli, he cannot be too careful in employing 

 none but the best artists ; and even then it is not prudent to 

 take their instruments without examination. Often the fixed 

 points have been inaccurately determined ; if these are exact, 

 the calibre of the tube is often unequal, and equal degrees do 

 not correspond with equal dilatations. In some instruments, 

 this defect in the tube is to a certain extent corrected by an 

 inequality in the divisions. These compensations, however, are 

 not to be relied on, as being obtained generally from a very de- 

 fective method ; namely, the comparison of a small portable 

 thermometer with a large standard one ; but it is very difficult to be 

 certain that the temperature communicated is, at the same time, 

 exactly the same, in two thermometers, of very different volumes, 

 and which gain and lose the same degree of heat in very different 

 times. The artist desirous of making a really good instrument 

 will never fail to verify the calibre of his tubes by the known 

 methods, and absolutely to reject all those which do not stand 

 test. But those comparisons just alluded to, which ought not 

 to be employed for the construction of thermometers, may be 

 employed to try them, at least approximately in the case of 

 instruments where the mass is equal, and the structure similar. 

 My mode of proceeding is this : I unite two or three together, 

 the thermometers most similar in figure and dimensions, and 

 subject them together to the heat of boiling water in a vessel 

 placed on a chafing dish. This point, as is well known, is only 

 fixed in respect to a certain pressure of the atmosphere ; and in 

 order to avoid having to make a reduction, it is convenient to 

 make the trial under the standard pressure ; that is to say, when 

 the barometer is at 29*921 inches. This first test will infallibly 

 detect the presence of small air bubbles otherwise imperceptible, 

 and which mterrupt in whole or in part the continuity of the 

 thread of mercury. The heat dilates them, and we are at once 

 warned of their existence by the rapidity with which the mer- 

 cury rises above the boiling point. These bubbles altogether 

 imperceptible ordinarily conceal themselves about the neck of 

 the bulb, and this is a common fault in thermometers whose 

 tubes are contracted in that part. Such must be rejected as the 

 evil is irremediable. The point of ebullition being verified, I 

 leave the apparatus to a well-regulated cooling process, and 

 follow with my eye the progress of the thermometers. This 

 trial would be very defective if the bulbs were of very different 

 capacities J or if the hec^,t diminished with too great rapidity \ but; 



