which determine the Indications during Absence. 345 



at successive intervals of time, the application of the same prin- 

 ciple to the registering of the hygrometer will not require much 

 illustration : For, if it be admitted that the atmizomic hygrome- 

 ter (that is, a hygrometer constructed on the Huttonian princi- 

 ple), may be depended upon, all that is requisite to procure an 

 accurate register is, to attach two thermometers to one slip of 

 metal, on which is engraved a scale for each, and to keep one of 

 the bulbs moist with water. When at any instant the instru- 

 ment thus constructed is brought into the horizontal position, 

 the index in the one tube will indicate the temperature of the 

 air, and that in the other the temperature produced by evapora- 

 tion. Nothing is more simple than this modification of the re- 

 gistering apparatus, for nothing can be more easily effected than 

 keeping one of the bulbs moist with water, and in this only does 

 it differ from that fitted to register the atmospheric temperature 

 alone. It therefore appears, that, with very little trouble, we 

 can ascertain the hourly variation of the temperature and humi- 

 dity of the atmosphere ; and we have the means of greatly facili- 

 tating other thermometric and hygrometric investigations. 



Hitherto (I speak to the extent of my own information) 

 there has been no method devised for registering even the ex- 

 tremes of the barometric changes, which does not infer a very 

 considerable increase of mechanical friction ; and which, conse- 

 quently, does not include a degree of inaccuracy no way consist- 

 ent with the present advanced state of meteorological science. 

 For it is admitted, that, at the present day, a variation in the 

 elevation of the mercurial column to the five hundredth part of 



an inch, must be attended to by those who aim at scientific ac- 

 curacy. 



The principle of the method for registering the indications 

 of the barometer which I was led to adopt, consists in cutting 

 off, at a given instant, all communication between the atmos- 



x x 2 



