of Dry and Moist Air. 23 



through the tube ; and, on the other hand, dry air brought the 

 needle back to zero*. 



This experiment appeared to me more decisive than any of 

 those in which the rock-salt plates were used, and I therefore 

 owe Prof. Tyndall my best thanks for his courtesy in showing it 

 to me. The needle swung through 30° or 40° ; but whether on 

 leading dry or moist air through the tube I do not remember ; 

 and I further omitted to notice which of the two sources of heat 

 had the preponderance. The result of this experiment was so 

 surprising, and so little in accordance with what I had found by 

 other processes, that when I reached home I determined to repeat 

 it. In my earlier experiments I had used an astatic needle-galva- 

 nometer, because the reflecting galvanometer, as commonly con- 

 structed, has too great a directive force to be deflected suffi- 

 ciently by a weak current like that of the thermo-pile. The 

 needle-galvanometer, however, has the defect that its indications 

 are proportional to the strength of the current only when the 

 deflections do not exceed a few degrees; I have therefore 

 increased the sensibility of the reflecting galvanometer by adding 

 to it a second magnet, exactly similar to the first, and connected 

 with it on the principle of astatic needles, so as to diminish the 

 directive force of the instrument. 



Fig. 1, Plate I. shows the galvanometer thus constructed. It 

 consists of two parallel coils g g, each containing 94 turns of wire ; 

 a is a circular piece of steel placed inside the coils, and serving 

 at once as mirror and as magnet. Between it and the coils there 

 is a copper ring k k, 60 millims. broad and 30 millims. thick, 

 which acts as a damper. This ring carries a small tube cs, 

 which passes up between the coils, and upon which is fastened 

 the cylindrical brass vessel q q, containing a second circular mag- 

 net b, of exactly the same size as the mirror «, and firmly con- 

 nected with it by the wire a b, so that both are in the. same plane, 

 and have their magnetic axes horizontal but in opposite direc- 

 tions. They hang by a cocoon-fibre R R, 0*3 metre in length. 

 In order to be able to put them in their places, after they have 

 been rendered astatic to the proper degree outside the galvano- 

 meter, the copper ring k k is cut through vertically to the extent 

 of half its thickness. With so thick a ring this cutting does not 

 injure its action as a damper. It is plain that this system of 

 mirrors ought not to be quite astatic -, it is, however, only neces- 

 sary to leave it just so much directive force that it shall always 

 come back to the direction of the magnetic meridian. Its posi- 

 tion of equilibrium does not remain constant. Independently 

 of other disturbing causes, such as the torsion of the fibre, &c, 

 the daily variations of the earth's magnetism occasion alterations 

 * Phil. Trans. 1862, p. 92. [Phil. Mag. vol. xxiv. p. 430.] 



