134 DR. WOLLASTON ON A DIFFERENTIAL BAROMETER. 
variations would of course be more perceptible ; but the instrument, from its 
length, would be exceedingly incommodious. The corresponding advantage 
and disadvantage would be proportionally greater in employing a column of 
alcohol ; and by having recourse to ether, we should arrive at the limit of 
inconvenience, as of sensibility, in any simple column. 
The instrument which I have employed for this purpose is, on the contrary, 
very compact in its form, and the principle on which it is constructed is such, 
that any assignable degree of sensibility may be given to it. It consists of a 
tube of glass, having its internal diameter at least a quarter of an inch, bent in 
the middle into the form of an inverted siphon, with the legs parallel to each 
other. The extremities of the legs are cemented into the bottom of separate, 
but equal, cisterns, about two inches in diameter ; one of these cisterns being 
closed on all sides, excepting by a small horizontal pipe opening laterally from 
its upper part ; while the other cistern remains open. 
