296 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1946: 
Tf all land above and below the ocean waters were leveled off around 
the globe, sea water would cover the land to a depth of 114 miles. 
THE SUN’S ENERGY 
Our weather, as Dr. Abbot has shown us in earlier lectures in this 
series, depends upon solar activity. Without the sun there would be no 
winds, no evaporation, no clouds, no precipitation, no fog or rain, 
sleet or snow, no ocean currents, no springs or streams to return water 
_to the sea to complete the cycle which fertilizes and irrigates the land, 
as well as the sea. 
From the sun the earth receives the energy which warms it and makes 
it a habitable place for living things, and which enables plants to 
grow, to synthesize, to utilize, and to store that self-same energy in the 
form of carbohydrates. This process, peculiar to plants alone, by 
which they can manufacture starches, sugars, and other substances 
from carbon dioxide and water under the influence of sunlight, is 
called photosynthesis. 
It has been estimated that the energy that the earth receives from 
the sun totals 240 trillion horsepower. For all its magnitude, this 
figure represents just about two-billionths of the sun’s total output 
of energy, which, boiled down to simple figures, on a good sunny day, 
amounts to 1 horsepower per square yard of the earth’s surface. 
A very considerable part of the energy received from the sun goes 
into the maintenance of the water cycle between the ocean, atmos- 
phere, and land. In the conclusion of his study of this cycle, Dr. 
George F. McEwen, of the Scripps Institution, remarked, 
* * * The annual precipitation over all the oceans is 80,000 cubic miles, or 
244 units, if the volume of the ocean is 1,000,000, and the annual evaporation 
from the ocean surface is 89,000 cubic miles, or 272 units. Thus a run-off from the 
land to the oceans of 9,000 ecubie miles, or 28 units, is required to balance precipi- 
tation and evaporation, and the whole process involves an expenditure of 500,000 
horsepower from the sun for every square mile of the earth’s surface. * * * 
Five hundred thousand horsepower is perhaps the maximum peak 
load that our local power plant (Potomac Electric Power Co.) could 
possibly deliver ; its actual steady delivery of energy is nearer 380,000 
horsepower. ‘Thus, in order to keep the earth’s water cycle in motion, a 
power plant at least as large as the huge Pepco installation at Ben- 
nings, D. C., operating continuously at peak load—which is impossi- 
ble—is needed, one for each square mile of the earth’s surface. As has 
been already pointed out, there are 197 million square miles of surface 
to the earth. It is inconceivable that man could supply the fuel for 
such a power network. Today you might quickly say atomic energy, 
not realizing the time, money, material, and plant size that is now 
needed for the production of a very limited supply of man-made atom 
bombs. 
