^4)2 Ofi the Construction of Meteorological Instruments. 



When a thermometer is converted into an atmizomic hygrome- 

 ter, it has been the practice to cover the bulb of the instrument with 

 soft cambric or silk, for the purpose of extending the moisture over 

 its surface. But though this contrivance certainly answers the 

 purpose, it is attended with considerable inconvenience ; for if rain 

 or distilled water is not used, the cloth soon becomes loaded with 

 calcareous and other earthy matter deposited from the water * ; 

 and even when the purest Avater is used, the instrument soon be- 

 comes loaded with all kinds of dust, lime, soot, &c. When the bulb 

 has become thus incrusted, it is desirable, if not requisite, to renew 

 the bibulous covering ; a troublesome operation, which cannot be 

 effected without considerable risk of breaking the instrument ; and 

 which is particularly objectionable in the case of slender, and ex- 

 tremely fragile air thermometers. 



This bibulous covering may be got rid of, by having the bulb of 

 the instrument made rough by means of fine emery, with or without 

 the aid of fluoric acid ; and by keeping up a constant supply of al- 

 kalized water, by means of a capillary glass tube, fitted with a soft 

 hair pencil, and connected with a suitable reservoir. 



It would seem, however, that an imperfection attaches to the dif- 

 ferential hygrometer, which does not appear to have been particularly 

 adverted to by writers on meteorology. It has been admitted, that 

 moisture is deposited from the atmosphere on glass at a time when 

 the former is not saturated with aqueous vapour. Hence when a 

 differential thermometer is used as a hygrometer, and when, as has 

 hitherto been the case, the naked bulb of the instrument is left free- 

 ly exposed to the air, it cannot be ascertained, in the evening for ex- 

 ample, whether the air is or is not saturated, for moisture will be de- 

 posited (as it appears) on the naked bulb before the air is saturated ; 

 and when the naked bulb has moisture condensed on its surface, 

 the instrument no longer acts as a hygrometer. Perhaps this im- 

 perfection might be remedied by keeping the surface of the naked 

 bulb coated with a thin layer of a fixed oil that freezes at a low tem- 

 perature ; but this would render it a very complicated instrument. 



* The -deposition, from the water in this cit}^ seems to be much more 

 copious now than it was some years ago. 



