202 report — 1859. 



strike the cylinder, is the conjugate focus to the slit c ; we should therefore 

 have an image of the slit c exhibited on the sensitive paper. As, however, 

 our object is to produce a dot and not a slit of light, a hemicylindrical lens, 

 having its axis horizontal and focus at the cylinder, is placed at g, so that 

 the rays passing through it have the vertical slit of light which they 

 would otherwise have formed on the cylinder compressed into a dot; in 

 which state therefore the light falls upon the sensitive paper. But it is 

 only when both the mirrors, the fixed and the moveable, are in one plane 

 that we shall have one dot upon the cylinder. For if the plane of the one 

 mirror is inclined at an angle to that of the other, the ray from the first 

 mirror will not be reflected in the same direction as that from the second, 

 and will consequently fall upon a different part of the cylinder. Two slits 

 of light will in this case reach the hemicylindrical lens, and two corresponding 

 dots of light will appear upon the sensitive paper which covers the cylinder. 

 The distance between these two dots will be a measure of the angle between 

 the two mirrors, and will consequently (the lower mirror being fixed, and 

 the upper one moving with the magnet) indicate the position of the magnet 

 from time to time. 



The cylinder round which the sensitive paper is wrapped is moved round 

 by clockwork once in every twenty-four hours, so that the dot belonging 

 to the fixed mirror generates a straight line, while that belonging to the 

 moveable mirror will describe a line corresponding to the movement of the 

 magnet. 



The arrangements of the horizontal-force instrument are in all respects 

 similar to those of the declination magnetograph which has just been 

 described, with this exception, that in the latter the magnet is in its natural 

 direction, viz. perpendicular to ef, while in the former it is twisted into u 

 direction at right angles to its natural position, and is now in the line ef. 



The only difference which it is necessary here to notice between the 

 vertical- force magnetograph and those which we have now described, is that 

 in the vertical-force magnetograph the slit c is horizontal and the hemi- 

 cylindrical lens and cylinder vertical, while the axis on which the moveable 

 semicircular mirror, attached to the magnet, turns, is horizontal. The 

 mirror of this magnetograph is exhibited in Plate 4. fig 5. One piece of 

 clockwork is made to drive all the cylinders. 



The principle of construction which we have now described seems to 

 possess the following advantages : — 



1st. The optical arrangements are such as to secure an exceedingly well- 

 defined dot of light, and by means of suitable photographic appliances, an 

 unexceptionable curve and base-line. 



2nd. Should anything occur to change the position of the slit c, both the 

 curve and the base-line will be equally displaced, so that the distance between 

 them (with which only we are concerned) will remain precisely the same as 

 before. 



Thus too, by slightly altering the position of the slit each day, we may put 

 two or even three days' curves on the same sheet. 



3rd. The stone piers, &c. secure perfect steadiness to the apparatus, and 

 the central arrangement presents the advantage that one piece of clockwork 

 drives all the cylinders. 



Section II. Detailed Description of the Instruments. 

 1. Declination Magnetograph. 

 The flame used is that of gas, the supply of which is kept constant by 



