f)2 MR. BROOKE ON THE AUTOMATIC REGISTRATION OF MAGNETOMETERS, 
French shade, about 10 inches high and 14^ in circumference (figs. 1 and 5, Plate VI.), 
the corners being held together by a little gum dissolved in acetic acid. The shade, 
after having been blacked in the inside, is cemented into a cap 1 inch deep in the 
rim, and having a brass pin about 1^ inch long and a quarter of an inch thick fixed 
perpendicularly in its centre. A second shade, a little larger than the former, is 
then placed over it, leaving an annular space about one-eighth of an inch wide ; the 
two cylinders are retained in a concentric position, by plaqing a sufficient number 
of turns of tape or ribbon round the rim of the cap of the inner cylinder, to fill up 
the intervening space. As the sensibility of the paper has been found to be mate- 
rially diminished by allowing it to become dry, the tape or ribbon is saturated with 
water, previously to the outer cylinder being placed over it, and a piece of moist lint 
is placed between the round ends of the cylinders : the paper is thus kept perfectly 
damp until its removal, after having been in action twelve hours. The pin in the 
cap of the inner cylinder rests on a pair of friction rollers, and the cap rests against 
a horizontal roller, Plate V. fig. 8 ; the rounded end of the outer cylinder also rests 
upon a pair of rollers, which are adjustible in the direction of the common axis of the 
cylinders, Plate VI. fig. 1 ; the axis may thus be always rendered horizontal. By these 
means, the cylinders revolve so easily that they have been carried round by the hour- 
hand of a common watch ; as however stoppages occasionally took place, from a de- 
ficiency of motive power, a stronger movement, such as now in use, is recommended*. 
A small collar with a set-screw fits on the pin of the cylinder, into which is inserted 
a bent wire that engages with a fork at the end of the hour-hand of the time-piece 
(Plate VI. fig. 3), the hour-hand being a little longer than the minute-hand. The 
cylinder is thus carried round its axis once in twelve hours, and at the commencement 
of the observation the carrier is so adjusted that the point of light may fall at the top 
of the paper. From the preceding description, it will be readily understood that by 
a combination of the vertical movement of the paper with the horizontal movement 
of the luminous point, the magnetic curve is traced out. The time-scale, which is 
measured lengthwise in the photographs, is evidently q^-th of the circumference of 
the inner cylinder to one hour, and the dimensions above mentioned, which allow 0*1 
inch very nearly to five minutes, have been considered to define the period of any 
* The size of the time-pieces in use is that of an ordinary ship chronometer, and in the construction of these 
a great improvement has recently suggested itself. The motion of the hour-hand is usually reduced from that 
of the arbor carrying the minute-hand by two wheels and pinions, the play of which, although necessary for 
their free action, allows a degree of mobility in the hour-hand rarely less than equivalent to five minutes in 
time, and consequently, without due caution, an error to this amount may be introduced in the register ; more- 
over, the hour-hand is supported by too short an axis to enable it to overcome a considerable resistance applied 
to its extremity, without acting as a drag upon the whole train. To obviate these inconveniences, the hour- 
hand is placed on the arbor of the centre or second wheel, which is driven directly by the barrel, and drives 
the rest of the train. With this and other necessary modifications of the train, the variation of rate, whether 
the hour-hand moves freely, or is engaged in carrying the cylinder, will be scarcely sensible. In carrying out 
these views, the author is bound to acknowledge his obligation to the skill and intelligence of Mr. Eiffe. — 
May 1847. 
