70 MR. BROOKE ON THE AUTOMATIC REGISTRATION OF MAGNETOMETERS, 
two slips fixed to the surface of the board at such a height that the top of the float 
maybe opposite the middle point between them. To this sliding piece a pulley about 
three inches in diameter, having a fixed axis about 3 inches long, is attached by a 
suitable support ; to this pulley two slender wooden arms are attached, one thirty 
inches, the other five inches long, and fixed at right angles to each other*. A 
piece of wire with an adjustible balancing weight is fixed in the pulley in such a 
position that the axis of the pulley may be the centre of gravity of its appendages. 
The long arm passes through a slit in the stand of the apparatus, and carries a black 
paper screen with a vertical slit in front of the horizontal aperture in the cover above 
described (see Plate V. fig. 2) ; and is so placed that the point at which the slits 
cross each other is exactly thirty inches from the axis of the pulley. The short arm 
rests on the roller at the end of the float, and is marked at the distances of 3, 3 - 75, 
and 5 inches from the axis of the pulley. The mark which rests on the float may be 
changed at pleasure by sliding horizontally the piece to which the pulley is attached ; 
and accordingly as the marks are respectively placed in the above position, it is evi- 
dent that the movement of the point of light transmitted through the slit in the move- 
able screen will be five, four, or three times the variation in the height of the column 
of mercury; and thus by the same lamp the base-line and the barometric curve are 
traced out. Of this, fig. 4, Plate VIII. and fig. 7, Plate IX. are given as examples. 
In these it may be remarked that both the lines are so sharply defined, that by ap- 
plying a scale divided into x&^ths of an inch, the position of both may be read to half 
a division, which is equivalent to O'OOl inch of mercury, if the first scale be adopted, 
which has been the case in these instances. 
A small weight suspended by a string passing round a groove in the pulley keeps 
the short arm in contact with the float, by a constant pressure. There being an 
annulus of mercury rather more than one-fourth of an inch wide between the tube 
and the float, the effect of capillarity is so much reduced as to exert scarcely any 
influence on the variations of the column, the weight of which is sufficient to over- 
come the small amount of friction that exists in the various parts, without sensibly 
influencing its variation, and consequently the barometric curve is frequently con- 
tinuous, and not interrupted by jerks. In one of the registers, not introduced for 
want of space, the passage of an aerial wave is recorded, equivalent to less than g-jj^th 
of an inch of mercury, the duration of which was about 4^ minutes. 
The lamp being placed at a distance of about nine inches from the paper, the 
direction of the small pencil by which the curve is traced varies considerably; hence 
an error is introduced in the register, equal to the distance of the slit from the paper 
multiplied by the cotangent of the angle at which the ray is inclined to it; this how- 
ever may be either allowed for, or obviated by rendering all the rays of the pencil 
parallel to the same vertical plane by means of a cylindrical lens, placed at its focal 
distance from the lamp. 
* The pulley and slide have since been made of brass. — May 1847. 
