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POPULAR SCIENCE REVIEW. 
atmospliere_, or the amount of water present, tlii’ougli which 
they pass ; and if so, the proportion of the heat received at 
Mercury and Venus, Jupiter and Saturn, may he the same 
as that received at the earth, if the constituents of their 
atmospheres he the same as that of the earth, and greater if 
the density he greater ; so that the effective solar heat at the 
superior planets, Jupiter and Saturn, may he greater than at 
the inferior planets, Venus and Mercury, notwithstanding their 
far greater distances from the sun. 
If this he true, then there will he no need to refer to the 
law of the decrease of radiant heat, viz., that of the inverse 
square of the distance, as applying to the temperatures at the 
different planets, a law which gives to the surface of the sun 
such an intense heat, far heyond any we have the power of 
producing, and in such amount, that no theory that I have 
seen advanced would seem capable to supply the continual 
demand. 
But few of the heat-rays can he used hy the earth ; yet there 
must he an unceasing flow of such rays in aU directions from 
the sun into space ; not a very large number relatively can he 
used hy all the planets and satellites of our system, and of 
those which are received at the planets as on the earth, 
does the earth absorb them all ? or what part is radiated and 
reflected hack, and spread again into space? We know that the 
presence of water in the air checks both these operations, and, 
as before stated, the air is never free from water up to the 
highest point that I have been, viz., seven miles. We may 
also ask, what becomes of the heat-rays which meet with no 
constituent matter to generate heat ? Where do they go ? Do 
they ever stop ? They are not lost, we may feel certain. These 
and other questions press on us, but with our present know- 
ledge we cannot answer them. 
Let us now turn to another investigation still connected with 
the sun — one of ahsorhinginterest,one promising to tell us some- 
thing about the constituents of the sun itself. The spectroscope 
directed to the sun, as is now well known, shows the solar spect- 
rum crossed hy dark hues. Some observers attribute some of 
these lines to our atmosphere ; and it became a subject of inquiry 
of deep interest, to ascend above the lower portion of the atmo- 
sphere, and examine the solar spectrum there. This was done 
with three different spectroscopes : the one which had been used 
by Professor Piazzi Smyth on the Peak of Teneriffe, belonging 
to the Astronomer-Royal, and lent by him for these experi- 
ments, and two others more convenient for use, and shown in 
the Plate. The spectrum at all times was found to be brighter 
and the colours purer than when viewed from the earth ; also 
every line seen from the earth was seen from the balloon, but all 
