144 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1914. 
ones below. It is easy in that manner for the papers in the upper 
drawers to fall into the lower drawers, but work has to be done in 
order to get the papers from the lower drawers into the upper ones 
again. So with energy—all forms of energy may easily be trans- 
formed into heat, which is the lowest type of energy, but heat energy 
can only partially be transformed back again ito the higher types. 
Of these types radiation is one of the very highest. 
Now it is on the sun’s radiation that a temperature suitable 
for life upon the earth depends. Not only that, but the peculiar 
properties of certain wave lengths of the solar radiations are re- 
quired for supporting plant growth, with its complex chemical 
reactions. All sources of energy upon the earth have been directly 
or indirectly produced by solar radiation. A good many investiga- 
tors, among them Mr. Shuman, of Philadelphia, have endeavored 
to use the solar radiation commercially for the production of power, 
and, in fact, very satisfactory results are being obtained in this 
way, under Mr. Shuman’s direction, from a plant in Egypt. 
Evidently it is of the greatest interest to measure the quantity 
of the solar radiation, the distribution of it in the spectrum, the 
hindrances which it suffers in passing through the earth’s atmos- 
phere, and the quantity of it available to warm the earth after it 
reaches the surface. This has been the principal work of the 
Astrophysical Observatory of the Smithsonian Institution for the last 
12 years. 
In the first place, we have to deal with the measurement of 
the solar radiation as a whole. For this purpose we employ what 
is called the pyrheliometer, a name first devised by Pouillet, about 
the year 1835. He employed a blackened box, filled with water, 
and containing a thermometer for observing the rise of tempera- 
ture in the water due to the absorption of the solar rays upon 
the blackened box. In our.practice we have considerably devel- 
oped the instrument of Pouillet, until now it comprises a silver 
disk inclosed in a chamber provided with a vestibule for the ad- 
mission of the solar rays. The disk has inserted in it a thermometer, 
which is bent at right angles for convenience, and on which the 
rise of temperature of the silver disk due to the absorption of solar 
radiation is observed. The instrument is shown in figure 2. 
It is not possible to obtain the correct heat capacity of the pyr- - 
heliometer in this form, so that we have reduced its measurements 
by comparison with what is termed the standard pyrheliometer, in 
which the heat produced by the sun’s radiation is carried off by 
flowing water. The rise of temperature in the water, due to the 
absorption of the solar radiation, is determined by means of an elec- 
trical thermometer. In this apparatus it is possible to introduce 
electrically known quantities of heat, and to measure them as if it 
