346 Chronicles of Science. | April 
succeeds in producing some of the most startling illusions of the day. 
He does not attempt to instruct his audience, but candidly tells them 
that he is going to employ the whole of his complicated electrical, 
voltaic, and optical machinery to deceive their eyes and to astonish 
them. No one can say that he does not succeed in both these attempts. 
His scene of “the violin of Paganini,” and the one in which he repre- 
sents himself as struggling in the embrace of death, are perhaps the 
most real illusions which have ever been brought before the public. 
VII. HEAT. 
Tue determination of the mechanical equivalent of heat has been one 
of the great triumphs of modern times. The numerical relations have 
been obtained by many experimenters, but the methods have been 
liable to very great errors of manipulation. One of the most accurate 
series of determinations has just been completed by MM. Tresca and 
Laboulaye.* The principle upon which they work is to allow a given 
volume of air to expand, and then to measure the amount of heat which 
it has absorbed during the operation. Into a reservoir holding 3°28 
cubic metres, air is forced until it has a pressure of three atmospheres. 
A mercury gauge is connected with the receiver, and a float on the 
upper portion of the mercury registers mechanically its exact height 
at any given moment. ‘This is effected by having connected with the 
float a needle-point pressing against a sheet of glass which is blackened 
with smoke, and carried horizontally forward by clockwork. It is 
evident, therefore, that the variations in height of the mercury column 
communicating a vertical movement to the needle, whilst the glass 
screen is carried forward horizontally, the resulting mark will be a 
diagonal varying in curvature with the variation of height of the mer- 
cury gauge. The exact height of the mercury column can, therefore, 
be ascertained at any desired moment. 
The reservoir being filled with pure dry air at a pressure of three 
atmospheres, and the mercury column being stationary, the needle- 
point registering a perfectly straight line on the screen, a stopcock is 
opened and air is allowed to rush out for a certain time, say five 
seconds; it is then closed. As soon as the air commences to rush out, 
and the gaseous mass suddenly expands, its pressure diminishes, and 
the needle-point consequently gives a downward oblique mark on the 
glass plate. But in expanding, the temperature of the gas sinks, and 
when the stopcock is closed the remaining gas in the reservoir has a 
lower temperature than the reservoir itself or the surrounding bodies, 
The mercury gauge, therefore, stands at a lower point than it other- 
wise would, had the temperature remained uniform. 
Upon closing the stopcock, the gas, absorbing heat from the sides 
of the vessel, gradually regains its original temperature, and the mer- 
cury gauge commences to raise the needle-point, registering an upward 
curve until the temperature is in equilibrium ; it then registers a straight 
* «Comptes Rendus de Académie des Sciences,’ February, 1864. 
