460 



As it is necessary to know the temperature of the air at that 

 moment, in order to determine the fraction of saturation, Eegnault 

 places another Thermometer in a second silver thimble and glass tube 

 like the first, but unconnected with the aspirator, and not containing 

 ether. Both thimbles being placed near each other on a proper stand, 

 and being equally polished, the constrast presented by one, as soon as 

 the dew begins to be formed on it, renders the deposition evident. 



With regard to the advantages of this Hygrometer, M. Eegnault 

 observes, that the temperature of the ether must be uniform on account 

 of the continual passage of air bubbles ; the very thin silver thimble 

 must be of the same temperature ; the observer may be at a great 

 distance, viewing the process through a small telescope, and hence 

 exhalations are avoided. 



Notwithstanding the advantages of this Hygrometer, it is not 

 without some inconvenience, which, to be fully appreciated, must be 

 experienced during its management. The cork, which admits the 

 stem of the Thermometer and the air tube, being only f -inch diame- 

 ter, and perforated by two holes, is much weakened, and is thus ren- 

 dered difficult to be withdrawn without being broken, along with the 

 Thermometer, so often as experiments may require. The junctures 

 must be secured with cement, and re-cemented as often as it is neces- 

 sary to renew the spoiled residue of ether. Any leakage would defeat 

 the object. All this implies risk to so delicate an apparatus, as well 

 as much trouble in its management. 



There is another inconvenience which Eegnault himself points 

 out — namely, the bulk of the aspirator, and the necessity of procuring 

 water enough to supply it, which might be difficult in an open coun- 

 try on an expedition of research. He observes, however, that the 

 aspirator may be dispensed with, if a mouthpiece and stop-cock be 

 affixed to the leaden tube. The operator thus breathes {souffle) through 

 the ether, and thus produces cold; but, considering the anaesthetic 

 effect of the ethereal vapour, the inspiration by this method might 

 have disagreeable consequences, and the expiration would blow out 

 the ether. 



I have mentioned these inconveniences as my apology for venturing 

 to propose a modification of an instrument coming from so high an 

 authority. I now proceed to a description of it. On a circular brass 

 foot is erected a brass pillar, twelve inches in height, carrying on its 

 top a horizontal piece, at the under surface of which is screwed a 

 brass socket pointing downwards. Into this socket is cemented a 

 depending vertical glass tube, open at both ends, seven inches in length. 

 The outside diameter of this tube must be such that it will fit into 

 a silver cylinder made as thin as possible ; into this cylinder the tube 

 enters about half an inch, and is firmly fixed there by gluing. The 

 silver cylinder, closed at the bottom, will contain ether without the 

 possibility of escape in any other manner than in vapour through the 

 top of the tube, when the pressure of the air is withdrawn. The 



