70 Chemical and Physical Notes 



as constant for a considerable time at the hour of the diurnal 

 maximum, which is usually about 2 p.m. This is the hour to 

 compare the recorder with the mercurial thermometer. 



Again, if the wet and dry bulb thermometers are not 

 exactly equal and similar and they never can be their 

 indications are not exactly comparable in air of changing 

 temperature or humidity. 



These few remarks will show the practical importance of 

 a knowledge of the rate or term of cooling of every thermo- 

 meter used in a meteorological observatory, and of the 

 application of it to the correction of observations. They 

 also show the supreme technical importance of making such 

 thermometers according to a perfectly uniform pattern. 



When the thermometer was a novelty and philosophers 

 studied its resources and applications in every direction, the 

 importance of this constant was fully recognised, and the 

 application of it to observations was insisted on. 



With the prevalent dilettante character of meteorology 

 its existence was forgotten, and its application fell into 

 desuetude. 



The subject is dealt with in great detail and quite ex- 

 haustively in the works of Newton, Lambert, Leslie, and 

 others. For profound but, at the same time, simple treatment 

 of this interesting subject, the writings of these great men 

 cannot be surpassed. 



The method of determining the rate or term of cooling of 

 a thermometer is simplicity itself; indeed, it can be carried 

 out even in a shop, so that we need never buy a thermometer 

 in ignorance of what may be termed its thermal nimbleness. 



The necessary observations are best made in a dwelling- 

 room of fair size, in which the air remains for at least a 

 considerable time at a constant temperature. The thermo- 

 meter, which is not attached to any backing, is whirled in the 

 air until it assumes a constant temperature. This is noted as 

 the temperature of the air at the time. The thermometer is 

 then hung up in the middle of the room, and a reading 

 telescope should be set up at a little distance from it, so that 

 its scale can be read without approaching too near. Except 



