t 69 ] 
VIII. On the Automatic Registration of Magnetometers, and other Meteorological 
Instruments, by Photography. By Charles Brooke, M.B., F.R.C.S.E. Sup- 
plement. Communicated by G. B. Airy, Esq., F.R.S., Astronomer Royal. 
Received November 23, — Read November 26, 1846. 
DURING the period of the summer recess, the system of automatic meteorological 
registration by photography has been rendered complete by the adaptation of the 
barometer and thermometer to the apparatus previously described. It having been 
found a matter of much difficulty to obtain a photographic base-line from the lamp 
already described as being placed near the magnet, the idea naturally arose that the 
base-line might be simultaneously described by a second lamp placed on the opposite 
side of the cylinder, as represented in fig. 1, Plate V. A pencil of light proceeding from 
this lamp through a horizontal slit in the chimney is received by a cylindrical lens 
placed, as before, horizontally, and the focal line of light thus formed is allowed to 
pass through a corresponding slit in the covering of the cylinder. A small section 
only of this focal line is transmitted through a vertical slit in a piece of thin sheet 
brass attached to the stand on which the cylinders rest, and placed very near the 
surface of the outer cylinder. Aline thus described maybe seen in Plate VIII. fig. 4, 
and Plate IX. figs. 6, 7, 8, and the same light has been by the following means rendered 
available for the registration of the barometer. A siphon barometer has been con- 
structed with a column of mercury a little more than one inch in diameter, Plate VI. 
figs. 1 and 2. As the weight of an entire column of this size would be inconvenient, 
and as it would be difficult to obtain a tube more than three feet long of so large a 
bore, both ends of which were of the same internal area, two adjacent short pieces of 
a very nearly cylindrical tube have been united to the extremities of a tube of small 
bore, and form the ends of the instrument which contain the surfaces of the mercury. 
A wooden cap about two inches high is fitted to the open end of the tube, at each end 
of which are fixed three small friction rollers, placed radially, vertical, and equidistant 
from each other. The stem of a glass float, having a bulb about half an inch in 
diameter, resting on the surface of the mercury, passes up vertically between these fric- 
tion rollers, by which the free vertical movement of the float is much facilitated. At 
the upper end of the stem is a cap containing a small grooved roller. The barometer 
tube is attached to a board by two clamps, so as to be capable of being raised or 
lowered at pleasure, and the bend at the lowest part rests on a piece of wood, w 7 hich 
is likewise capable of a vertical adjustment. Another piece of wood, about half an 
inch thick, two inches wide, and five or six long, is made to slide horizontally between 
