RECENT PROGRESS IN PHYSICAL SCIENCE. 107 
not within the province of this paper, but it is desired to 
describe a new form of apparatus that has recently been de¬ 
veloped, which appears to possess peculiar merits and striking 
advantages in point of accuracy, portability, and convenience 
of observation. It may be remarked at this point that in¬ 
struments for the measurement of solar radiation belong to 
one or the other of two distinct classes. The one, and chro¬ 
nologically the older class, comprises instruments which are 
acted upon directly by the gross radiant energy of all wave 
lengths. In the later instruments comprising the second 
class the radiant energy is first resolved into its elementary 
wave lengths, by means of a rock salt prism for example, 
and the energy over all portions of the spectrum thus ob¬ 
tained measured and mapped out. 
The numerous so-called actinometers, pyrheliometers, etc., 
belong to the first class. Professor Langley’s well-known 
spectro-bolometer remains unsurpassed as the type of instru¬ 
ment belonging to the second class. 
The instrument to which special attention will now be 
given is called by Angstrom, its inventor, the electric-com¬ 
pensated pyrheliometer.* Two very thin and narrow strips 
of platinum, with blackened surfaces, are exposed side by 
side in a tube. A screen, with double walls, is arranged so 
that first one and then the other of the strips can be shaded 
alternately from the radiation it is desired to examine. 
Each strip is backed by a delicate thermo-electric couple of 
“ constantan ” and copper or nickel and copper. The strip 
which is exposed to the solar or other radiation is heated 
thereby. In order to determine the amount of this heating, 
the shaded strip is likewise heated by an electric current to 
an exactly equal temperature, as shown by the thermo-electric 
elements. This equality of temperature having been secured, 
and both strips being otherwise equally affected by losses of 
heat from radiation, convection, conduction, etc., it is plain 
that the radiant heat absorbed by the exposed strip is equal 
to that produced by the electric current in the shaded strip. 
* Wied. Ann. 67, p. 636, 1899. 
