134 



Lord Rayleigh and Dr. A. Schuster. [May 5, 



followed again by a positive one, had been taken, a regular displace- 

 ment such as that we are discussing would not have affected the mean. 

 This latter plan was adopted on the last night. In the measurements 

 undertaken by the British Association Committee, the deflections in 

 one direction were generally greater than in the other, and the 

 difference was ascribed to a considerable torsion in the fibre of sus- 

 pension, which, in order to explain the discrepancy, must, as pointed 

 out by Rowland, have displaced the magnetic axis considerably 

 out of the meridian. The differences in the readings taken when 

 the coil was spinning in opposite directions were, on the average, 

 about 3 per cent., and amounted in one case to 8 per cent. They 

 show no regularity dependent on the speed of rotation. "We also 

 observed some slight differences of the same nature ; but they are 

 very small, and always remain within such limits that they may easily 

 have been produced by a displacement due to air currents. On the 

 last night, when more frequent zero readings were taken, the 

 differences were, it is true, not much reduced in amount, but their 

 sign was reversed. It is, perhaps, worth remarking that, owing to 

 the absence of any controlling instrument equivalent to our auxiliary 

 magnetometer, the Committee of the British Association had no 

 opportunity of discovering the presence of these air currents, as any 

 changes in the zero position would naturally have been ascribed by 

 them to a casual change in the direction of the earth's magnetic force. 

 Owing to the different shape and material of the box containing the 

 mirror, it seems possible that the effect of air currents may have been 

 considerably larger than it has been in our experiments. 



'Reduction of Observations. 



Wcale Corrections. — The first step in reducing the observations 



£ - ^^ating the value of 2 tan from the observ ?/* ^ ec ^- 0I1 



sists in caiocLLc applied to the reading in order t, ^ a u , ■ 



.- x- h mf i pin i • . , , ! obtain numbers 

 The correction to uJ 1 - ot deflection, it the posit ^ ^ e 



proportional to the tangents c** ' * 1 ssJ^ok ^ — ° ° 

 LJnet is at the centre of the scale, wou^L .... ^ - d being the 

 observed reading, and D the distance of the mirror from the scale. If 

 the zero, howe.fr, is at a point B of the scale, the correct™ becomes 

 _( cJ-aW + SWtD 2 , where « is to be reckoned positive when m toe 

 same direction as A. For the higher speeds a second correctmn 

 to 4-d«/8D* was applied, which comes just withm the limits ot 

 aeenrae; aimed at in the actnal readings. The corrected deflections 

 I obtaLd are equal to 2D tan 0. Small errors due to a M y 

 division of the seale, onght also to be applied. It is difficult to obtain 

 a proper scale in one piece of sufficient length to be nsed m these 

 LLLents; and the one in actnal nse consisted of three parts, 

 cemented with caoutchouc cement to a thick piece of deal. No appre- 



