1874.] Notices of Books. 253 
exists between heat and mechanical work. Professor Stewart 
has written an admirable treatise on Heat, and has elsewhere 
discussed the Conservation of Energy with consummate ability. 
We are not disappointed by this more comprehensive treatment 
of the subject. He tells us succinctly in the preface his mode 
of discussing the subject. He divides our knowledge of the 
universe into two branches: the one knowledge of it as a vast 
physical machine composed of atoms swimming intheluminiferous 
ether; the other, the laws which regulate the working of this 
machine; in other words, the laws of energy. 
In the first chapter, energy is defined as ‘‘the power of 
overcoming obstacles, or of doing work,” as instanced by a rifle 
bullet in motion. The work is to be measured by some unit, 
preférably the kilogramme for weight, and the metre for height ; 
and by multiplying a weight raised by the vertical height through 
which it is raised, we get the work done in kilogrammetres. 
Next we have various examples of the change of energy of 
position into energy of motion, and finally into heat. The usual 
and satisfactory examples of the head of water, the bent cross- 
bow, and the wound up watch are adduced, and the advantages 
of energy of position are exemplified by happily comparing a 
water-mill and a windmill with a rich and poorman. In the one 
case we may turn on the water whenever it is most convenient 
for us, but in the other we must wait until the wind happens to 
blow. The former has all the independence of a rich man; the 
latter all the obsequiousness of a poorone. If we pursue the 
analogy a step further, we shall see that the great capitalist, or 
the man who has acquired a lofty position, is respected because 
he has the disposal of a great quantity of energy; and whether 
he be a nobleman or a sovereign, or a general in command, he is 
powerful only from having something which enables him to make 
use of the services of others. When the man of wealth pays a 
labouring man to work for him, he is in truth converting somuch 
of his energy of position into actual energy, just as a miller lets 
out a portion of his head of water in order to do some work by 
“its means.” This (second) chapter continues with an account of 
the functions of machines, the conversion of motion of a mass 
into heat, and the nature of the motion called heat. The next 
chapter discusses the various kinds of energy; gravity, elastic 
forces, cohesion, chemical affinity, electricity, magnetism. The 
classification of elastic forces among the energies is unusual, and 
we have always regarded elasticity as a function of cohesion, not 
as due to any separate and distinctive force. Prof. Stewart 
speaks of the ‘‘ force of elasticity,” but is it not rather a property 
belonging to certain bodies under certain conditions than a force? 
It does indeed require energy to bend a bow, but is not that energy 
expended in partially overcoming the cohesion of the molecules 
in one direction, and in approximating the molecules against 
their molecular motion in another? A useful condensed list of 
