PROGRESS IN PHYSICS—THOMSON. 191 
How great is the supply the sun lavishes upon us becomes clear 
when we consider that the heat received by the earth under a high 
sun and a clear sky is equivalent, according to the measurements of 
Langley, to about 7,000 horsepower per acre. Though our engineers 
have not yet discovered how to utilize this enormous supply of power, 
they will, I have not the slightest doubt, ultimately succeed in doing 
so; and when coal is exhausted and our water power inadequate, it 
may be that this is the source from which we shall derive the energy 
necessary for the world’s work. When that comes about, our centers 
of industrial activity may perhaps be transferred to the burning 
deserts of the Sahara, and the value of land determined by its 
suitability for the reception of traps to catch sunbeams. 
This energy, in the interval between its departure from the sun 
and its arrival at the earth, must be in the space between them. 
Thus this space must contain something which, like ordinary matter, 
can store up energy, which can carry at an enormous pace the energy 
associated with light and heat, and which can, in addition, exert the 
enormous stresses necessary to keep the earth circling round the sun 
and the moon round the earth. 
The study of this all-pervading substance is perhaps the most 
fascinating and important duty of the physicist. 
On the electromagnetic theory of light, now universally accepted, 
the energy streaming to the earth travels through the ether in electric 
waves; thus practically the whole of the energy at our disposal has 
at one time or another been electrical energy. The ether must, then, 
be the seat of electrical and magnetic forces. We know, thanks to the 
genius of Clerk Maxwell, the founder and inspirer of modern elec- 
trical theory, the equations which express the relation between these 
forces, and although for some purposes these are all we require, yet 
they do not tell us very much about the nature of the ether. 
The interest inspired by equations, too, in some minds is apt to be 
somewhat platonic; and something more grossly mechanical—a model, 
for example, is felt by many to be more suggestive and manageable, 
and for them a more powerful instrument of research, than a purely 
analytical theory. 
Is the ether dense or rare? Has it a structure? Is it at rest or in 
motion? are some of the questions which force themselves upon us. 
Let us consider some of the facts known about the ether. When 
light falls on a body and is absorbed by it, the body is pushed for- 
ward in the direction in which the light is traveling, and if the body 
is free to move it is set in motion by the light. Now, it is a funda- 
mental principle of dynamics that when a body is set moving in a 
certain direction, or, to use the language of dynamics, acquires mo- 
mentum in that direction, some other mass must lose the same amount 
of momentum; in other words, the amount of momentum in the uni- 
