of Magnetic Moments in Absolute Measure. 323 



to a focus on the scale by sliding L to the proper distance from 

 the mirror, and served as an index by means of which the de- 

 flection was read. The distance of any point on either of the 

 arms A A from the mirror was found by referring it to a fine 

 line marked on the upper surface of each of the arms by a 

 sharp point attached to a fixed support above in such a 

 position as just to bear on the upper surface of the arms when 

 they were turned under it. The distance of this line from the 

 axis was evidently the same for each of the arms ; and half 

 the total distance between the two lines thus drawn gave 

 the distance of either. 



The plane of the magnetometer-needles was made to pass 

 through the axis by first placing the magnetometer in such 

 a position on the stand that this condition was approximately 

 fulfilled, and then adjusting it by means of the levelling- 

 screws at the base of the instrument until the deflections given 

 on the scale by a magnet placed in the V-groove on one of the 

 arms, when the arm was turned so that the magnet was alter- 

 nately due east and due west of the centre of the mirror, were 

 equal. In order that the magnetometer might be removed 

 from the stand when desired, and replaced in exactly the same 

 position, Sir William Thomson's geometrical arrangement was 

 employed. One of the three rounded feet of the instrument 

 was made to rest in a conical hollow cut in the upper surface 

 of the stand, another in a V-groove cut with its axis in line 

 with the centre of the conical hollow, and the remaining foot 

 on the plane upper surface of the stand. 



The mode of experimenting was as follows : — A large 

 number of cylindrical steel bars were cut from the same bar, 

 the diameter of which was *097 centimetre, its weight per 

 metre 5*77 grammes, and its density 7*83. 



Before being tempered the bars were carefully filed to a 

 uniform length of five centimetres. Their lengths were com- 

 pared with a scale of half-millimetres by means of a lens. 

 About sixty of these bars, in order that they might be heated as 

 nearly as possible to the same temperature, were spread on the 

 bottom of a small thin iron tray, and the whole raised to a bright 

 red heat in the heart of a glowing fire. To temper the bars glass- 

 hard, the tray with its contents was quickly removed from the 

 fire and plunged into water at about 15° Centigrade. The bars 

 were then made up into parcels of five each, and placed in a 

 vessel containing oil. The whole was then heated by means 

 of a Bunsen lamp, and parcels of the bars removed at each of 

 the following temperatures— 100°, 150°, 200°, 240°, 250°, 

 260°, 270°, 280°, 300°, 310°. While this process was going 

 on, the heated oil was taken advantage of to temper a number 



Y2 



