44 
Steam Engine. 
atmosphere by a quantity which has a certain relation witli 
the excess of the temperature above those just stated. 
Till lately there was wanting on this important subject 
series of exact and direct experiments by means of 
which, having given the temperature of the heated fluid, 
the expansive force of the steam rising from it might be 
known, and vice versa. There was likewise wanting an 
analytical theorem expressing the relation between the 
temperature of the heated fluid and the pressure with 
which the force of the steam was in equilibrio. These 
desiderata have, however, been lately supplied by M. Bet- 
tancourt, an ingenious Spanish philosopher, after a method 
which we shall now concisely explain.'^ 
M. Bettancourt’s apparatus consists of a copper vessel 
or boiler, with its cover firmly soldered on : this cover 
has three orifices which close up with screws : at the first 
the water or other fluid is put in and out ; through the 
second passes the stem of a thermometer which has the 
whole of its scale or graduations above the vessel, and its 
ball within, where it is immersed either in the fluid or in 
the steam according to the different circumstances ; 
through the third hole passes a tube, making a commu- 
nication between the cavity of the boiler and one branch 
of an inverted syphon, which contains mercury, and acts 
as a barometer for measuring the pressure of the elastic 
vapour within the boiler. In the side of the vessel there 
is a fourth hole into which is inserted a tube with a turn- 
cock, making a communication with the receiver of an 
air-pump, in order to extract the air from the boiler and to 
prevent its return. 
Tlie apparatus being prepared in good order, and dis- 
tilled water introduced into the boiler at the first hole, and 
then stopped, as vrell as the end of the inverted syphon or 
barometer, M. Bettaneourt surrounded the boiler with ice. 
'■* 1, Phil. Mag. 345. T. C. 
