Oct. 8, 1885 | 
NATURE 
551 
tion by Professor Langley of his Bolometer, which 
is an instrument for detecting and measuring small 
quantities of radiant heat much more sensitive than the 
thermopile. It depends upon the fact that the electrical 
resistance of a metalis increased as it rises in temperature. 
Suppose, now, that two circuits conveying equal and 
Opposite currents meet in a galvanometer, the needle will 
of course remain at rest. If, however, a portion of one 
of these two circuits be heated, its resistance will be 
increased, and the current passing through it will thus be 
diminished. The two opposing currents will now no 
longer balance each other, and in consequence the gal- 
vanometer needle will be deflected. 
In the bolometer the two circuits each contain a sheet 
of extremely thin platinum foil, so that a very small 
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shaded curve above the spectrum represents the obser- 
vations made by Professor Langley with his Bolometer 
at the foot of the mountain. We have next a dotted 
curve derived from observations at the top of the moun- 
tain, and, finally, another representing the probable curve 
of solar energy above the limits of the atmosphere. It 
follows from these curves that if we could view the sun 
beyond the limits of the atmosphere the light would be 
decidedly blue. 
There can be no doubt that the improved process of 
photography devised by Captain Abney, and the Bolo- 
meter of Professor Langley, furnish us with excellent 
differential instruments by which we may compare at 
any place and moment the relative distribution of solar 
energy over the various parts of the spectrum. 
If either of these observers could produce such a 
uniformity in his process that his results of to-day should 
be exactly comparable with those ten or twelve years 
afterwards, then his method would go far to obtain for us 
the requisite information regarding solar variability. But 
I fear that we cannot expect this, at any rate for some 
time to come. As it is, we learn by the foregoing diagram 
what are the regions of the solar spectrum most affected 
by the selective absorption of the components of the 
earth’s atmosphere, fer Professor Langley imagines that 
the gaps in the shaded curve are caused by this means. 
Let me now venture, in conclusion, to make the following 
suggestion. By aid of the information furnished by the 
instruments now described, let us select certain regions of 
the spectrum for which in the shaded curve there are no 
gaps, and in the spectrum below it no corresponding dark 
lines ; that is to say, regions for which there is no selective 
absorption. Now let us throw the energy from these 
selected regions either upon the standard sensitive paper 
of Roscoe’s actinometer, or upon the thermometer of a 
suitable heat-actinometer, or upon both. 
We shall by this means greatly simplify the problem 
under consideration, since these instruments will now be 
recording the intensity from year to year of those por- 
tions of the solar spectrum which are not subject, as 
far as we know, to selective absorption from the variable 
constituent of the atmosphere of the earth. 
It is possible that the rays which blacken chloride of 
silver are rays on which this variable constituent exercises 
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little or no selective absorption, although the general 
absorption of these rays is no doubt very considerable : 
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Fic. 10. 
quantity of radiant heat falling upon these may produce a 
considerable result. These sheets may be compared to 
the two faces of the pile, and if the one be heated we 
shall have a current in the one direction, while if the 
other be heated we shall have a current in the opposite 
direction. By this instrument Professor Langley has 
determined with much precision the exact distribution of 
energy in the solar spectrum. But he has done more 
than this: he has carried his instrument up to the top of 
Mount Whitney, in America, and has thus procured us 
much information regarding the absorbent effect of the 
various constituents of the earth’s atmosphere. 
The following diagram (Fig. 10) exhibits the result of 
his researches. In it the lower band represents the 
solar spectrum as obtained by a perfect method. The 
in this case no special adaptation to the chemical actino- 
meter would be necessary. 
To conclude, I think we may entertain a well-grounded 
hope that by patience and persistence in these or similar 
means, we shall ultimately arrive at a definite solution 
of this very interesting and important problem. 
BALFOUR STEWART 
(To be continued.) 
NOTES 
THE Geological Congress met last week at Berlin. England 
was represented by Messrs. Geikie, Hughes, Bauermann, Hinde, 
Marr, Topley, White, Woodall, Lieut.-Col. Tabuteau, and 
