OF ARTS AND SCIENCES. 129 



are hard to manipulate, for they possess the elasticity and brittleness 

 of highly tempered steel, and usually require careful annealing. 



The Galvanometer. 



The galvanometer intended to accompany the heat-measuring device 

 should receive as careful attention as the device itself. The writer 

 had had some previous experience in the construction of sensitive 

 galvanometers before attempting the research of which the present 

 article is an account ; nevertheless, several different forms were tried 

 before one was found exactly adapted to the thermograph. The re- 

 quirements to be met were, in brief, first, considerable sensitiveness, 

 and secondly, rapidity of working, — qualities which are to some ex- 

 tent incompatible, but which have, as far as possible, been combined 

 in the instrument in use. It would have been possible to produce an 

 instrument much more sensitive than the one actually employed, had 

 the rate of working been left out of account; but nothing is more 

 annoying than to be obliged, when time is precious and opportunities 

 for observation are perhaps few, to wait for a galvanometer needle 

 to come to rest ; consequently the element of time has had the first 

 consideration. 



The galvanometer adopted for use with the thermograph has four 

 coils, each containing about four feet of wire 0.063 in. in diameter. 

 The wire was twice shellacked and baked before beinjj wound. The 

 coils are contained between brass plates, which constitute the body of 

 the instrument, the inner faces of the two plates, which are in contact, 

 being grooved for the passage of the staff of the needle and its sup- 

 porting silk fibre. The needle thus swings in an entirely closed space 

 but little larger than itself, and needs no damping vanes to bring it 

 quickly to rest. Two minute scales of mica are attached to the needle 

 to prevent it from being turned completely round. The astatic system 

 has the magnets fixed upon a very slender aluminum staff two inches 

 in length. The magnets are from ] in. to g in. in length, and very 

 slender. The mirror is flat, and | in. in diameter. Much of the suc- 

 cess of the instrument depends upon the perfection of the mirror. It 

 is difficult to obtain perfectly plane mirrors, and. moreover, those to 

 be had in the market are too heavy. They can be made, of great per- 

 fection, by selecting a perfectly flat piece of thin plate glass, cementing 

 it to a thicker piece of glass for a support, and then grinding it down 

 to the thickness of stout letter paper. A very thin sheet of glass is 

 thus obtained, from which circles of the required size can be cut with 

 a diamond. The glass being now silvered upon the Burfaoe, the polish 



VOL. XXIV. (\-. S. XVI.) 9 



