20 
sun is here invisible, and if we are to study completely the 
action of our atmosphere, we shall have to pay great attention 
to this part, and find out some way of determining the loss in it, 
which will be difficult, for the ultra-red end is not only invisible, 
but compressed, the red end being shut up like the closed pages 
of a book, as you may notice by comparing the narrowness of 
the red with the width of the blue. 
Now refraction by a prism is not the only way of forming a 
spectrum. Nature furnishes us colour not only from the rain- 
bow, but from non-transparent substances like mother-of-pearl, 
where the iridescent hues are due to microscopically fine lines. 
Art has lately surpassed nature in these wonderful ‘‘ gratings,” 
consisting of pieces of polished metal, in which we see at first 
nothing to account for the splendid play of colour apperently 
pouring out from them like light from an opal, but which, on 
examination with a powerful microscope, show lines so narrow 
that there are from 50 to 100 in the thickness of a fine human 
hair, and all spaced with wonderful precision. 
This grating is equal in defining power to many such prisms 
as we have just been looking at, but its light does not show well 
upon the screen. You will see, however, that its spectrum 
differs from that of the prism, in that in this case the red end is 
expanded, as compared with the violet, and the invisible ultra- 
red is expanded still more, so that this will be the best means 
for us to use in exploring that ‘‘dark continent” of invisible 
heat found not only in the spectrum of the sun, but of the 
electric light, and of all incandescent bodies, and of whose 
existence we already know from Herschel and Tyndall. 
Now we cannot reproduce the actual solar spectrum on the 
screen without the sun itself, but here are photographs of it, 
which show parts of the losses the different colours have suffered 
on their way to us. We have before us the well-known 
Frauenhofer lines, due, you remember, not only to absorption in 
the sun’s atmosphere, but also to absorption in our own. We 
have been used to think of them in connection with their cause, 
one being due to the absorption of iron-vapour in the sun, 
another to that of water-vapour in our own air, and so forth; 
but now I ask you to think of them only in connection with the 
fact that each is due to the absorption of some part of the 
original /ig4/, and that collectively they tell much of the story of 
what has happened to that light on its way down to us. Ob- 
serve, for instance, how much thicker they lie in the blue end 
than in the red—another evidence of the great proportionate 
loss in the blue. 
If we could restore all the lost light in these lines, we should 
get back partly to the original condition of things at the very 
fount, and, so far as our own air is concerned, that is what we 
are to ascend the mountain for—to see, by going up through 
nearly half of the atmosphere, what the rate of loss is in each 
ray by actual trial; then, knowing this rate, to be able to allow 
for the loss in the other part still above the mountain-top, and, 
finally, by recombining these rays to get the loss as a whole. 
Remember, however, always, that the most important part of 
the solar energy is in the dark spectrum which we do not see, 
but which, if we could see, we should probably find to have 
numerous absorption-spaces init corresponding to the Frauenhofer 
lines, but where heat has been stopped out rather than light. To 
make our research thorough, then, we ought not to trust to the 
eye only, or even chiefly, but have some way of investigating 
the whole spectrum ; the invisible in which the sun’s power 
chiefly lies, as well as the visible, and both with an instrument 
that would discriminate the energy in these very narrow spaces, 
like an eye to see in the dark ; and if science possesses no such 
instrument, then it may be necessary to invent one. 
The linear thermopile is nearest to it of any, and we all here 
know what good work it has done, but even that is not sensitive 
enough to measure in the grating spectrum, in some parts of 
which the heat is 400 times weaker than in that of a prism, and 
we want to observe this invisible heat in very narrow spaces. 
Something like this has been provided since by Capt. Abney’s 
most valuable researches, but these did not at the time go low 
enough for my purpose, and I spent nearly a year before ascend- 
ing the mountain in inventing and perfecting the new instru- 
ment for measuring these, which I have called the ‘‘ bolometer” 
or “‘ray-measurer.” The principle on which it is founded is 
the same as that employed by my late lamented friend, Sir Wm. 
Siemens, for measuring temperatures at the bottom of the sea, 
which is that a smaller electric current flows through a warm wire 
than through a cold one. 
One great difficulty was to make the conducting wire very 
WA TORE 
[May 7, 1885 
thin, and yet continuous, and for this purpose almost endless 
experiments were made, among other substances pure gold 
having been obtained by chemical means in a plate so thin that 
it transmitted a sea-green light through the solid substance of 
the metal. This proving unsuitable, I learned that iron had 
been rolled of extraordinary thinness in a contest of skill between 
some English and American iron-masters, and, procuring some, I 
found that 15,009 of the iron plates they had rolled, laid one on 
the other, would make but one English inch. Here is some of 
it, rolled between the same rolls which turn out plates for an 
iron-clad, but so thin that, as I let it drop, the iron plate flutters 
down like a dead leaf. Out of this the first bolometers were 
made, and I may mention that the cost of these earlier experi- 
ments was met from a legacy by the founder of the Royal Insti- 
tution, Count Rumford. The iron is now replaced by platinum, 
in wires or rasher tapes, from 1-2009 to 1-20,000th of an inch 
thick, one of which is within this button, where it is all but in- 
visible, being far finer than a human hair. I will project it on 
the screen, placing a common small pin beside it as a standard 
of comparison. ‘This button is placed in this ebonite case, and 
the thread is moved by this micrometer screw, by which it can 
be set like the spider line of a reticule; but by means of this 
cable, connecting it to the galvanometer, this thread acts as 
though sensitive, like a nerve laid bare to every indication of 
heat and cold. It is then a sort of sentient thing: what the eye 
sees as light it feels as heat, and what the eye sees as a narrow 
band of darkness (the Frauenhofer line) this feels as a narrow belt 
of cold, so that when moved parallel to itself and the Frauen- 
hofer lines down the spectrum it registers their presence. 
It is true we can see these in the visible spectrum, but you 
remember we propose to explore the invisible also, and since to 
this the dark is the same as the light, it will feel absorption lines 
in the infra-red which might remain otherwise unknown. zy 
I have spent a long time in these preliminary researches ; in 
indirect methods for determining the absorption of our atmo- 
sphere, and in experiments and calculations which I do not de- 
tail, but it is so often supposed that scientific investigation is a 
sort of happy guessing, and so little is realised of the labour of 
preparation and proof, that I have been somewhat particular 
in describing the essential parts of the apparatus finally em- 
ployed, and now we must pass to the scene of their use. 
(Zo be continued.) 
THE INSTITUTION OF MECHANICAL 
ENGINEERS 
yANG VERY interesting discussion on the merits of the Maxim 
automatic machine-gun, which was described in NATURE, 
vol. xxxi. p. 414, took place at the special meeting of the Insti- 
tution held on the 3oth ult. In reply to Mr. Carbutt, M.P., 
the inventor explained that the recoil of the gun, which is 
utilised for loading and firing, did not interfere with the accuracy 
of aim, and instanced the circumstance that as good target-practice 
was made in firing from the shoulder as with fixed rifles ; whilst 
the energy of recoil was sufficient to carry on the automatic 
action, whether the muzzle of the gun was elevated or depressed. 
As regarded keeping the barrel cool, he found that the water- 
jacket around the body of the gun acted most efficiently, as 
gunpowder in exploding produced very little heat-energy, or, as 
he put it, ‘‘he should not buy gunpowder to raise steam.” 
During all his experiments he had used only Government car- 
tridges, and had never found one to fail; he thought it would 
be an advantage if cartridges were packed in cases containing 
calcium chloride or other hydroscopic material, so that they 
might not be injured by moisture. 
The gun was frequently fired during the meeting, and its 
automatic action was thoroughly shown, as well as its freedom 
from danger should a cartridge hang fire. Mr. Maxim had a 
most enthusiastic reception, the general feeling of the speakers 
and of the meeting being in favour of the gun being taken up 
by the British Government, the President and Mr, Adamson 
giving it as their opinion that, if the necessity should occur, 
1000 of these guns could be produced weekly at a month’s 
notice, when their use might have as material an effect on a 
campaign as the needle-gun had at Sadowa. Mr. Maxim is now 
experimenting with a more recent form of gun of his invention, 
which fires a projectile 3 Ibs. weight at the rate of 120 shots a 
minute ; in this the cartridges are fed from above, which much 
simplifies the mechanical arrangement, as no apparatus has to 
