1824.] Mr. Daniett's Reply to X. 257 



valve S. The piston is now at the bottom of the barrel, and the 

 globe is full of mercury, — if the piston be now drawn up, a 

 vacuum would be formed in the barrel, but the mercury in the 

 globe mast descend as it is above the level of the piston the 

 whole height T, and the vacuum in the globe K would be Torri- 

 cellian were there not a communication between it and the 

 receiver R. When the mercury again ascends into the globe, 

 it expels every particle of air provided the mercury rises into 

 the aperture at S ; and to ensure this the cap O is formed into 

 a rim so as always to supply the contraction or waste, and it is 

 admitted towards the end of the exhaustion by raising the valve 

 S with the finger. The air is admitted through a hole a in the 

 cock P, a section is shown, fig. 1 1. The cap O should be strong, 

 and, if brass, should be coated with the cement used in attaching 

 it to the glass (that used for nautical machines is best), the 

 gauge may be attached to the cap, or inclosed in the receiver. 



The stiff wire, with the valve T and the ball U, may be 

 entirely removed ; and for it may be substituted a glass tube 

 open at both ends cemented into the cock P, and reaching 

 almost to the bottom of the globe. The mercury, when it rises 

 to the lower end of this tube, cuts off the communication with 

 the receiver. This will perhaps be the simplest and best plan. 

 It may be made a double pump by connecting the cap O with 

 the barrel G, as on the dotted line b — one valve opening in and 

 one out. The weight of the mercury will be no objection as the 

 machine is small — the diameter of the globe about four inches, 

 the height of the barrel about eight, and the whole height to the 

 plate R, 15 or 20 inches. 



Article VII. 



Reply to the Remarks of X. on certain Subjects in Mr. Darnell's 



Meteorological Essays. By J. F. Daniell, Esq. 



(To the Editors of the Annals of Philosophy.) 



GENTLEMEN, Gowcr-strcct, Sept. 4, 1 85 1 . 



Your correspondent X. has committed a great mistake in his 

 remarks upon my work, which, as having found its way into the 

 Annals of Philosophy, it may not be unnecessary to correct. 



He observes, " It has always been understood that, other 

 circumstances being alike, mercury in the barometer will have its 

 altitude affected by the existing temperature in no other way than 

 as thai temperature alters its specific gravity." It is scarcely 

 worth while, perhaps, to remark the inaccuracy of this expres- 

 sion, but the fact is, the altitude of the mercury in the barometer, 



New Seriei, VOL. VI II. s 



