374 M. CLAPEYRON ON THE MOTIVE POWER OF HEAT. 
mosphere, is to that disengaged from the same volume of iron under 
the same circumstances as 0°00375, the cubic dilatability of the air, is 
to 0:00003663, the cubic dilatability of iron. For the second term of 
the proportion we find the number 0°0007718. Nowa volume of 077 
of iron weighs 5996 kilogrammes ; the heat disengaged by one kilo- 
: 0°007718 
gramme will therefore be 5906? for the pressure of an atmosphere 
it will be 267 times more considerable, or equal to 0:00003436 ; the 
division of this number by the specific caloric of the iron referred to 
that of water gives the quantity of the elevation of the temperature of 
the iron by the pressure of an atmosphere ; it is, we see, too feeble to be 
appreciated by our thermometrical instruments. 
§ VIII. 
We shall not further insist upon the consequences to the theory of 
heat of the results enunciated in this Memoir; but it may, perhaps, be 
useful to add a few words upon the employment of heat as a motive 
force. M.S. Carnot, in the work already cited, appears to have esta- 
blished the true basis of this important part of practical mechanics. 
High or low pressure engines without detent (détente) bring into use 
the vis viva developed by the caloric contained in the vapour, in its pas- 
sage from the temperature of the boiler to that of the condenser; the 
high pressure engines without condenser act as if provided with a con- 
denser at a temperature of 100°. In the latter, therefore, all that is 
brought into use is the passage of the latent caloric contained in the 
vapour, from the temperature of the boiler to the temperature 100°. As 
to the sensible caloric of the vapour, it is entirely lost in all the engines 
without detent. 
The sensible caloric is in part utilized in the engines with detent, 
in which the temperature of the vapour is allowed to sink: the cy- 
lindrical envelope, the use of which in Woulf’s engines with two cy- 
linders is to maintain the vapour at a constant temperature, though very 
useful to diminish the limits of the variation of the motive force acting 
upon the pistons, has an injurious influence as to the quantity of effect 
produced compared to the consumption of fuel. 
To render useful all the motive force at our disposal, the detent 
should be continued until the temperature of the vapour be reduced to 
that of the condenser; but practical considerations, suggested by the 
manner in which the motive force of fire is employed in the arts, pre- 
vent the attainment of this limit. 
We have elsewhere shown that the employment of gases, or of any 
other liquid than water, between the same limits of temperature, could 
add nothing to the results already obtained; but from the preceding 
considerations it follows, that the temperature of the fire being from 
