S38 On the Hygrometer, 



m 



There was no barometer at hand when the observations were 

 made ; but the variations of pressure at the surface of the earth 

 are too small to occasion any sensible error, except when the 

 air is of a low temperature, and very dry. 



From an inspection of the table, it appears that the maximum 

 errors in excess and defect were -|- 1.25 and — 1.3. Also by 

 comparing the 4th and 5th observations together ; the 6th and 

 9th; the 15th and 30th ; the 22d and 25th ; the 29th and 33d ; 

 the 36th and 37ih, — it will be seen that the observed dew- 

 point varied considerably, without a corresponding change in 

 the dry and wet thermometers, sufficient to account for the 

 difference. It is true that this circumstance does not determine 

 on which side the error lies ; but if the comparative difficulty of 

 making an observation in the two cases be considered, it seems 

 fair to attribute it to the uncertainty attending the use of Da- 

 niell's hygrometer. While the writer makes this remark, he is 

 of opinion that the original form of DanielPs hygrometer is de- 

 cidedly preferable to those proposed by Jones and Adie, from 

 the circumstance that the dew-point is confined to a zone of the 

 bulb only — the contrast between the clear and dim surface en- 

 abling the eye to mark the commencement of the deposition 

 much more accurately. The invention of M. Pouillet, noticed 

 by Professor Forbes in his Report on Meteorology, seems likely, 

 from its construction, to combine the separate advantages of the 

 above-mentioned instruments. 



In a note at p. 107. of Mr Meikle's paper, already alluded 

 to, there is a remark regarding the effect of exposing a wet ther- 

 mometer to the sun. It happened, that, just before seeing that 

 observation, the writer had been making one or two experiments 

 on the same subject. Two thermometers, similar in every re- 

 spect, and having their bulbs covered so as to possess the same 

 power of absorbing and radiating heat, were exposed to the sun. 

 The bulb of one of them was then moistened, and when the 

 maximum cold was established, the temperatures of the dry and 

 wet thermometers, and also those of the air and of the dew- 

 point, were observed ; the latter by means of DanielPs hygro- 

 meter. The results are exhibited in the accompanying Table. 



