NEW HYGROMETER OR DEW POINT INSTRUMENT. 25 



of all dew point formulae and tables of factors is to be determined. The grounds 

 of my dissent from such a view have been already given. 



There is a slight modification of the manner of applying the ivory intercepting 

 portion, by placing it as an ivory collar in the neck of the bottle at the point 

 P, instead of at N, as formerly described. It ought to be | of an inch broad. In 

 this case the valve piece N is of brass, so that the risk of its accidental fracture, in 

 screwing or unscrewing, is avoided, but in these operations the ivory portion of 

 the neck of the bottle should not be grasped. I am not yet, however, prepared to 

 recommend either of these modifications in preference to the other. Both, gene- 

 rally speaking, answer perfectly well. 



St Andrews, 25th February 1854. 



Postscript. — After the paper was read at the meeting of the Royal Society, I 

 was asked by a gentleman present, whether the instrument would answer for an 

 Indian climate, where extreme cases of great dryness of atmosphere sometimes 

 occur ? I, of course, answered that I had had no opportunity of making experi- 

 ments under such circumstances as an Indian climate might occasionally present ; 

 but that if the present size of the syringe was found not to render it sufficiently 

 powerful for such extreme cases, I had little doubt that by some such increase 

 of the size of the syringe as formerly suggested, the necessary augmentation of 

 power might be attained. I may add, that in the event of the occurreDce of very 

 extreme cases, where all ordinary hygrometers might fail to give the necessary 

 indication, the object, there is little doubt, might be accomplished, by means of 

 the arrangement which I suggested several years ago, as mentioned in the com- 

 mencement of this paper, employing instead of a bottle of brass, one of thin glass, 

 or still better, of platinum, somewhat larger than that described in the notice re- 

 ferred to, and introducing into it some mixture capable of producing great cold. 

 One of phosphate of soda, nitrate of ammonia, and diluted nitric acid, is capable 

 of reducing the temperature from 50° to — 20°; and from higher temperatures 

 would produce a proportional reduction ; and if pounded ice with the proper acid 

 were used, still greater cold would result. 



