152 $ANNUAL REPORT SMITHSONIAN INSTITUTION, 1946 
sition of the solar atmosphere into clouds with the emission of light 
and heat; or worse still, the suggestion by Sir John Herschel that the 
vital energies of monstrous creatures—the willow leaves of Nasmyth— 
are the true source of luminosity! Two famous theories based upon 
the mechanical equivalence of heat originated about the middle of 
the nineteenth century. Although discarded long ago, they still are 
accorded respectful mention because they contain at least some basis 
of sound physical reasoning. 
The first theory of the meteoritic origin of the sun’s heat was an- 
nounced by J. Robert Mayer? in 1848. Mayer claimed to have been 
the first to enunciate the principle of the conservation of energy, but 
seems to have discussed it in such obscure terms that he never received 
clear title to the distinction. There is no question, however, that he 
anticipated Helmholtz by 2 years in this respect. 
Mayer pointed out that space is known to be filled with innumerable 
small bodies many of which must be drawn into the sun with a speed 
approaching the parabolic velocity of 380 miles per second. As they 
plunge into the sun, their energy of motion would be converted into 
heat. He calculated that a total mass equal to 144 of the earth’s mass 
striking the sun annually would be sufiicient to maintain its observed 
output of energy. At first glance this appears quite reasonable, and 
for a few years Mayer’s theory enjoyed considerable success. Closer 
examination, however, revealed that on this basis the earth should 
receive from meteoritic bombardment millions of times more heat than 
could possibly be admitted, so that the theory soon became of historical 
interest only. 
A theory ascribing the source of the sun’s heat to friction but in a 
wholly different way was first described by Helmholtz * in a popular 
lecture delivered on February 7, 1854. He showed that as the sun 
loses heat by radiation it must contract, and this contraction 1s equiva- 
lent to the fall of particles by various amounts depending upon their 
distance from the center of the sun. Making the most unfavorable 
assumptions, Helmholtz found that a shrinkage of 250 feet per year 
would be enough to supply the sun’s annual output of energy, an 
amount too small to be detected for 10,000 years. 
One of the necessary conditions which any theory of solar radia- 
tion must fulfill is that the temperature of the earth has not sensibly 
diminished in historical times. As evidence of this condition, Helm- 
holtz cited the cultivation of grapes and olives, plants extremely sen- 
sitive to temperature changes, over the same regions that prevailed in 
the days of Homer. Against this evidence it was argued that formerly 
the German knights had made their own wine and drunk it, but the 
2 Republished in Philos. Mag., vol. 25, p. 241, 1863. 
§ Philos. Mag., vol. 11, p. 489, 1856. 
