Chemical and Physical Notes 69 



want of knowledge of this constant and of its application 

 may, and in point of fact does, frequently introduce error and 

 confusion into meteorological results. 



When two unequal masses of the same substance have 

 the same temperature and hang in the same medium, the 

 temperature of which is constant and lower than their own, 

 their temperature will fall at unequal rates. If, being at the 

 same temperature, they are hung in a medium of constant 

 but higher temperature, their temperature will rise at unequal 

 rates. These unequal masses may be thermometers with 

 similar but unequal bulbs. Under the same conditions they 

 will lose or gain heat at unequal rates. If, at any epoch, 

 they and the air in which they hang are at the same 

 temperature, and the temperature of the air is changed to 

 another, which is kept constant, it is clear that at equal 

 intervals of time from the initial epoch the two thermometers 

 will have and will show different temperatures, although their 

 scales may be without error. Further, in the ordinary course 

 of diurnal variations of temperature these two thermometers 

 will never again show the same temperature except for a 

 moment of time when, in the exigencies of their different 

 rates of cooling and heating, their thermal paths cross. 



The importance of self-recording thermometers for sup- 

 plying a continuous record of the temperature of the air and 

 of its variations is well understood. The form of instrument 

 most commonly used is, on account of its compactness and 

 comparative cheapness, that known as Richard's recorder. 

 It is usual to control its indications by reading a standard 

 mercurial thermometer hung alongside of it, and it is 

 a very common practice to make this reading at 9 a.m., 

 when the temperature of the air is changing most rapidly. 

 When the paper is removed, the temperatures taken from 

 the curve traced will be corrected so as to bring them into 

 harmony with the 9 a.m. readings of the mercurial thermo- 

 meter. Except in the very unlikely case of the mercurial 

 and the recording thermometers having the same rates or 

 terms of cooling, error will be introduced and not eliminated 

 by this proceeding. The temperature of the air is as good 



