64 SOUTHERN CALIFORNIA ACADEMY OF SCIENCES 
focal length of the lens is the known ratio of the Sun’s diameter 
to the lens’ diameter. 
At the focus of the largest lens yet constructed the high 
temperature produced instantly melted and vaporized the most 
refractory materials—platinum, fire-clay, carbon—everything 
tested. Its space-point of equal temperature is about two 
hundred fifty thousand miles from the Sun’s surface—a little 
further from the Sun than the Moon is from us. 
If then, our solid Earth were placed at that distance from tie 
Sun—a quarter of a million miles—it would quickly melt, vapor- 
ize, vanish! That certainly is the temperature at one-fourth of 
a million miles from the Sun. Still higher is it at the Sun’s 
surface—and inconceivably higher, the internal temperature. 
Source of the Heat. 
How does the Sun produce its enormous output of heat, and 
maintain its inconceivably high temperature? Of four theories 
to be considered—if the newest suggestion can be ealled a 
theory—only one is perfectly satisfactory. 
1. The Cooling Theory. 
It is certain the Sun is not simply an intensely hot body 
slowly cooling, for in that ease, its materially lowered tem- 
perature after thousands of years of human history would have 
eaused decided climatic changes, but we know that the vine 
the olive and the palm are fruitful now just where they flouri- 
shed in the days of classie writers. 
2. The Combustion Theory. 
It is equally certain that the Sun is not simply burning up— 
that its heat does not result from ordinary combustion. As 
estimated by Young, its continuous great yield of heat could 
be produced by the consumption every hour of a layer of coal 
all over the Sun’s surface nineteen fet deep. That is, by burn- 
ing one ton of coal hourly on each square foot of the Sun’s 
surface. 
Lord Kelvin computes that were its vast mass solid coal envi- 
roned by pure oxygen, and yielding its heat by combustion, it 
would be utterly consumed in five housand years. It would 
then, have dwindled more than one-third since Ptolemy’s day. 
Yet in the nearly two thousand years elapsed, it has certainly 
not diminished appreciably. 
3. The Radium Theory. 
Though not produced by combustion, the Sun’s heat is now 
