a ee 
NEW HYGROMETER OR DEW POINT INSTRUMENT. 25 
of all dew point formule 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. 
Sr 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 occurrence 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. 
