Notices of Boohs. 
5 6 5 
1878.] 
ever, not exactly constant, but increases with the temperature. 
The density of steam is not at present exadtly ascertained, but 
the general results of experiments show that the weight of a 
cubic foot increases nearly in proportion to the pressure, but at 
a somewhat slower rate. 
In the next chapter, on the convertibility of heat and work, 
it is asserted that “ energy when exerted is not destroyed, but 
simply transferred from one body to another.” It is shown, 
however, what a very small proportion of the heat expended 
in the process of evaporation produces an equivalent of work, — 
hardly one-twelfth, — the greater part of the heat having been 
employed in producing changes within the water itself, in over- 
coming the molecular cohesion of its particles, which resists its 
conversion into steam. The heat expended is thus converted 
into two classes of work, viz., external work, or that which is 
appreciable to the senses, and internal work. In applying the 
foregoing reasonings to the steam-engine it is demonstrated 
that, in a non-expansive engine, only from 5k to 7I- per cent of 
the heat expended is converted into useful work, but that the 
efficiency may be more than doubled by expanding the steam 
five times in the cylinder, and still further increased, though 
only to a small extent, by a further increase in the expansion. 
Chapter IV. deals with physical properties of the permanent 
gases, in which the theory of a heat-engine worked with a 
perfect gas, under conditions of maximum efficiency, is consi- 
dered, and it is explained that under certain conditions 56 per 
cent of the heat expended may be transformed into mechanical 
energy. We have next a consideration of the circumstances of 
“ a perfedt heat-engine,” and here we find that “ in the best pos- 
sible steam-engine, unless the steam be superheated considerably, 
at least two-thirds of the whole heat expended is wasted, the 
waste arising from no fault in the construction or nature of the 
engine, but solely from the narrow limits of temperature within 
which we are restricted to work. Again, the consumption of 
steam in a perfedt steam-engine is much less than that of an 
adtual engine under the same circumstances, showing that 
faults in the construdtion of the engine or the treatment of the 
steam must exist, which, at least theoretically, are remediable.” 
The expansion of steam, the effedt on engines receiving heat 
at varying temperature, the consequences of “ clearance ” and 
“ wire-drawing,” and the adtion of the sides of the cylinder and 
of water remaining after exhaust, are dealt with at some length 
in the succeeding chapters, whilst the last one is devoted to the 
application of the theory of the steam-engine, already discussed, 
in further detail to the working of steam-engines in pradtice. 
Here we learn that “ Experience appears to show that 18 lbs. of 
steam per I.H.P. per hour is about the minimum consumption 
of steam commonly reached in adtual engines ; and the preceding 
calculations indicate that, in condensing engines, this is equiva- 
