SUNLIGHT AND THE EARTH'S ATMOSPHERE. 651 



as well as the visible, and both with an instrument that would dis- 

 criminate 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 

 four hundred 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 Captain 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 ascending the mountain in inventing and 

 perfecting the new instrument 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 friend, Sir 

 William 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 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 unsuit- 

 able, 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 fifteen thousand 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 ironclad, but so thin that, as I let it drop, the iron plate flut- 

 ters down like a dead leaf. Out of this the first bolometers were 

 made, and I may mention that the cost of these earlier experiments 

 was met from a legacy by the founder of the Royal Institution, Count 

 Rumford. The iron is now replaced by platinum, in wires or rather 

 tapes from 2( ) i 6 to g0 ^ 60 of an inch thick, one of which is' within 

 this button, where it is all but invisible, being far finer than a human 

 hair. I will project it on the screen, placing a common small pin be- 

 side 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 Fraunhofer line) this feels as a narrow belt of cold, so that when 

 moved parallel to itself and the Fraunhofer lines down the spectrum 

 it registers their presence. 



