180 



Mr. W. Whitehouse on a New Hygrometer. [Feb. 22, 



tact with a given surface is considered more in detail ; and it is shown how 

 in general such a quadric degenerates into the tangent plane taken twice. 

 To this there is apparently an exceptional case, the condition for which is 

 given and reduced to a comparatively simple form ; but I must admit to 

 having so left it, in the hope of giving a fuller discussion of it on a future 

 occasion. 



The subject of three-pointic superficial contact was considered by Dupin, 

 * Developpements de Geometric/ p. 12 ; and, as I have learnt since the 

 memoir was written, a general theorem connecting superficial contact and 

 contact along various branches of the curve of intersection of two surfaces 

 (substantially the same as that given in the text) was enunciated by 

 M. Moutard*. 



In a corollary to this theorem, M. Moutard states that through every 

 point of a surface there can be drawn twenty-seven conies, having six- 

 pointic contact with the surface. This number is perhaps open to ques- 

 tion ; and I have even reason to think, from considerations stated to me by 

 Mr. Clifford, that the number ten given in my memoir above quoted may 

 be capable of reduction by unity to nine. But this question refers to the 

 subject of that earlier memoir rather than to this. 



II. " On a New Hygrometer." By Wildman Whitehouse, Esq. 

 Communicated by Sir W. Thomson, F.R.S. Received January 

 6, 1872. 



The use of Mason's wet-bulb thermometer as a means of hygrometric 

 measurement, though it be admitted to be the most practically useful, and 

 indeed the only recording instrument for the purpose, has yet this serious 

 inconvenience, not to say defect, viz. that its indications either cease or 

 are valueless at temperatures below 32° F. 



In a conversation which the writer had with the Director of the Meteo^ 

 rological Office some months ago, the question arose whether any thing 

 could be suggested to remedy this inconvenience. 



It was obviously inadmissible to substitute any other fluid for, or to 

 make any addition to, the water employed for the wet bulb, as then it would 

 cease to be a test for the purely hygrometric capacity of the air. It became 

 therefore necessary to fall back in another direction, and to find some 

 hygrometric body which should readily and rapidly absorb moisture from 

 the air, and at the same time afford some means of measuring and re- 

 cording the amount of such absorption. 



Fused chloride of zinc or of calcium seemed promising as very active 

 agents, absorbing rapidly on their surface, and allowing the readiest pos- 

 sible escape of the fluid hydrate for measurement ; yet no means presented 

 itself either of accurately measuring, regulating, or maintaining the exact 

 extent of surface exposed for absorption ; nor could the substance itself be 

 * Poncelet, ' Applications d' Analyse a la Geometrie,' 1864, torn. ii. p. 363. 



