ON SELF-RECORDING MAGNETOGRAPHS. 205 



the lens and the mirror 8*1 inches, these being the same as in the declination 

 magnetograph ; but the distance between the centre of the mirror and the 

 cylinder is different, being here 4*885 feet. 



Hence the focal length of the lens for parallel actinic rays is about 14 

 inches. The hemicylindrical lens is in all respects similar to that already 

 described. 



3. Vertical-force Magnetograph. 



This instrument is exhibited in Plate 4. figs. 5, 6 & 7. 



The vertical-force magnet is of the same size as the others, and is balanced 

 by means of a steel knife-edge upon an agate-plane. It is provided (see 

 Plate 4-. fig. 7) at one side with a brass screw working horizontally, and at 

 the other with a similar screw working vertically. By means of these the 

 centre of gravity may be thrown to either side of the centre of suspension, 

 or it may be raised or lowered, and the sensibility of the magnet, when 

 balanced, thereby increased or diminished. 



These screws are arranged so that there is a preponderance of weight 

 towards the south side of the magnet. This is neutralized partly by the 

 magnetic force tending to pull the north end down, and partly by a slip of 

 brass (H) standing out horizontally towards the north side. Let us suppose 

 the system to be in equilibrium at a certain temperature ; if the temperature 

 rise (since brass expands more than steel), the leverage of the weight at the 

 north side will increase more rapidly than that of the weight at the south. 

 There will therefore be a slight preponderance towards the north, and this 

 may be arranged so as to neutralize to a great extent the decrease in the 

 magnetic moment which an increase of temperature produces. 



The plane of the magnet is 15° out of the magnetic meridian (see Plate 3. 

 fig. 1), for the following reason. Had the magnet been in the magnetic 

 meridian, it would have been necessary to have placed the mirror inclined 

 at an angle of 15° to the axis of motion of the magnet. This was tried, but 

 it was found that in this position of the mirror, the correction for tempera- 

 ture was so excessive that the instrument became a thermometer, and not a 

 magnetometer. The mirror was therefore put in a plane passing through 

 the axis of motion of the needle, the needle being made to move in a plane 

 inclined 15° to the magnetic meridian. Its temperature correction is at pre- 

 sent very small. 



The mirror of this instrument is exhibited in Plate 4. fig. 5, one half 

 moving with the magnet, and the other half being fixed to a stand ; I is a 

 lifter which may be inserted from without the glass shade, and which, bv 

 raising three Y s to catch the needle, may remove it from its position of 

 balance when necessary. 



A thermometer is inserted within the glass shade of this instrument, by 

 means of which the temperature both of the horizontal and the vertical-force 

 magnets may be determined with sufficient accuracy. 



In the vertical-force magnetograph, the slit for the light is horizontal, 

 while the hemicylindrical lens and the cylinder are vertical. 



It might be thought that with a horizontal slit the style of burner already 

 described would prove unsuitable, as we here require a horizontal and not 

 a vertical light; but by using a burner twice as large every way as those of 

 the other magnetographs, we obtain a light that is found to answer in prac- 

 tice extremely well. 



The adjustments for regulating the distance between the light and the slit, 

 and between the slit and the lens, are similar to those for the declination and 

 bifilar magnetographs. There is also an adjustment, by means of which the 



