242 Ingvar Jorgensen and Walter Stiles. 
Economy and the production of more efficient engines may 
enable the energy of coal to remain available for a somewhat longer 
period, but this would only postpone and not solve the problem. 
Moreover, probably there is not a great deal to be done in this direc¬ 
tion. Petrol is a better source of energy than coal in regard to its 
efficiency, for a petrol motor transforms 35% of the energy of the 
petrol into useful work, whereas a steam engine only transforms 
from 10% to 16% of the energy of coal into useful work. But 
petrol production is only about 5% that of coal, and although we 
know nothing very certain about the amounts of petrol in the 
world, petrol is not very likely to replace coal as our principal source 
of energy. 
Other sources of energy available for our use are waterfalls, 
winds and waves and tides. Winds and waves are of too unstable 
a character for industry to be based on them. Tides form a more 
regular source of energy, but are scarcely suitable for general use. 
In waterfalls, on the other hand, we have a large source of energy 
of which only a small fraction is utilised at present. It is estimated 
that waterfalls will give at least 200 million horse power while only 
about 100 million horse power is required for industry at present. 
Nevertheless, having regard to the present rate of increase of industry 
and considering that a proportion of the energy of waterfalls will 
not be available for use it would seem clear that some means of 
utilising the sun’s energy such as the plant is able to do, will have 
to be developed if our civilisation is to continue. It is to be noted 
that all our sources of energy except the tides are derived from 
the sun’s rays, and that in the case of coal and probably of petrol 
we are using energy which has been transformed from radiant 
energy by the agency ultimately of plants. 
It has been estimated that the vegetable material formed each 
year by plant growth would give as much heat as 14 x 10 9 tons of 
coal, eleven times as much as the present consumption. But of 
course a good deal of such vegetable matter is used as food for 
animals and as building material. 
However, of the total radiant solar energy only about 0-5% 
becomes stored up in the plant, and only a small part of this could 
be utilised. So the only solution of the energy question of the future 
appears to be in transformation of the sun energy which at present 
is so largely wasted. 
The preceding figures, though very approximate, show the 
importance of the storing up of energy in the plants of the past and 
also of the present. 
