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flow of hot or cold water, or similar applications for regulation 
of temperatures. The vapour of aether has been used for 
such purposes when confined in tubes by mercury, but in such 
apparatus the amount of power to be obtained is much limited 
by the weight and consequent expense of mercury required. 
He makes use of vessels of thin metal, of which the 
ends or sides being discs of metal, are so thin and flexible as 
to be changeably either convex or concave, and in which 
the vapour of sulphuric aether (or of suitable volatile liquid), 
along with a portion of such liquid, is enclosed ; the discs or 
ends of such vessels acting as diaphragms, separating the 
vapour from the external atmosphere, will, at temperatures 
under the boiling point of the fluid employed, be pressed 
inwards by the atmosphere, but with a pressure varying 
according to the temperature. He therefore attaches a 
counterpoise of sufficient weight partially to counteract the 
pressure of the atmosphere, so that the differential pressure 
of the sethereal vapour above or below the degree at which 
the pressure is counterbalanced, gives motive power. And 
as the extent of motion from each of such moveable sur- 
faces is very slight, he combines several of such vessels 
together, so as to have several moveable ends or discs ; 
and as there is a peculiar action when a weight is employed 
to extend such discs, it is advisable to use a combination of 
two pulleys, placed eccentrically on the same axis, so that a 
descending weight may operate in a rapidly increasing ratio. 
Motion being thus produced, it is obvious that if the Ther- 
mostat be geared to the ventilators of hothouses or of build- 
ings, and to pipes supplying hot or cold water, or to dampers 
of furnaces, and to numerous similar purposes, by a sufficient 
size and number of flexible surfaces, any reasonably required 
power and extent of motion may be obtained. 
Mr. Ward then exhibited the apparatus, of which a draw- 
ing is annexed. 
