190 S. P. Langley—The Actinic Balance. 
technical language have a small probable error. I com: 
menced, guided by these considerations, and with the aid of 
Mr. F. W. Very, in December, 1879, to experiment in the 
following direction. The principle has been employed by 
Jamin, by Siemens and by others, before. The following appli- 
cation is, I believe, new. Let us suppose that from a battery 
two wires of equal length and equal section pass to a differen- 
tial galvanometer so that one current tends to move the ne le 
to the right, the other and equal current, tends to move it to 
the left, and the needle solicited in opposite ways by equal 
forces, remains motionless at zero. Suppose now a ray from 
the sun, from a vessel of hot water, from a candle, or from any 
source, of radiant energy of higher temperature than the wires 
themselves, to fall on one of them; this wire becomes heated 
and therefore a worse conductor than before, and as its resist- 
ance increases in nearly the ratio of its increased temperature, 
there is less current through the heated wire, and the needle 1s 
deflected by a force which is strictly proportional in theory t0 
the energy in the original ray, to the energy of the battery, and 
to certain constants of the galvanometer and the rest of the clr- 
cuit. In what has just been said, it is temporarily assume 
that all the energy of the original ray is represented by heat 1» 
the wire, and that none of it has been lost by conduction, conve®” 
tion, or by re-radiation. It is also supposed that no large 
change of temperature has taken place in the wire, but that 
the heating energy of the original ray is small. The last con- 
dition is only too easily met in practice. The consideration 
of the first will be resumed in another place. : 
We have just indicated merely the fundamental conceptlo 
which is to guide our search for an actual instrument. ¥& 
tween this conception and the partial realization a great many 
months of assiduous and often disheartening labor have been 
expended, and complete success is far from having been reached 
even now, but as I believe it certain that the instrument 10} 
actual stage of progress has been already successful in doing 
important work quite out of the thermopile’s reach, I shall de- 
scribe briefly the considerations which have led to its exec” 
tion in its present form. Some of these are obvious. The 
