86 Prof. A Connell on a new Hygrometer, 



piston and valves should always be renewed before they get 

 dry. 



Although the various joinings must be air-tight, great care 

 should be taken in screwing and unscrewing the bottle and 

 ivory valve-piece that too great force shall not be used, as the 

 fracture of the latter is otherwise risked. If at any time the 

 screws get fixed, the best course is to tie cotton around them and 

 moisten it well with olive oil, and leave them in this state for a 

 couple of days till the washers get softened, when the joinings 

 may be unscrewed with little risk of fracture. And it is expe- 

 dient every time the instrument has been used, to loosen the 

 screws a little till the next observation is to be made, when of 

 course the necessary degree of tightness must again be cautiously 

 given to them. 



There is a slight modification of the form of the instrument, 

 in which, instead of the ivory valve-piece, a collar of ivory is 

 introduced into the neck of the bottle at P to cut ofi* the com- 

 munication of heat from the syringe. The terminal valve-piece 

 of the syringe is in that case made of brass, and there is no risk 

 of fracture of that portion of the instrument in screwing or un- 

 screwing ; but the bottle itself requires great care to be taken 

 in handling it, as this point of junction may otherwise receive 

 injury and become not air-tight. With this form of the instru- 

 ment, and the external air at 74°, I effected one day in London 

 a reduction to 31°, i. e. 43° below the temperature of the air, 

 the dew-point being at 58°. Either modification, if in proper 

 order, answers the purposes of the instrument sufficiently well*. 



The instruments of Doebereinerf and Dr. Cummingof Chester J, 

 with which I have become acquainted since I had that here de- 

 scribed constructed, produce the evaporation of the aether in a 

 different way, viz. by passing through it a current of air by a 

 proper condensing apparatus. Regnault's apparatus for the 

 dew-point also acts in the same way by the aid of an aspirator §. 



It is thought that the instrument described in this paper will 

 present some advantages in point of security from injury in tra- 

 velling and in general use ; the matter chiefly requiring some 

 attention in this particular being the ivory intercepting portion, 

 as already noticed. 



* Messrs. Kemp of Edinburgh have undertaken to prepare the instru- 

 ment with bottles for sether, measure, &c., packed in a portable case. 

 t Gilbert's Annalen, tenth volume. 



t Library of Useful Knowledge, Article Thermometer and l*yrometer. 

 § See Ganot's Traits de Physique. 



