AND OTHER METEOROLOGICAL INSTRUMENTS, BY PHOTOGRAPHY. 77 
scale is 15' to 1 inch. This register exhibits a very remarkable accordance 
with the extraordinary observations made at the Royal Observatory during 
the latter part of the disturbance. Two sharp cusps at 8 h 2 m 30 s and at 
8 h 25 m may be specially noticed. The advantage of automatic registration 
is here manifest; the magnet having been during several previous bihoral 
observations very steady, as may be seen from the first part of the line, 
there was no previous indication of the disturbance which commenced 
shortly after 6 h : and the attention of the assistant on duty was directed to 
the magnets only by his casually observing that the balanced magnetometer 
had undergone a very remarkable deflection. The sudden breaks in the 
barometer line are due to a small amount of friction that has since been 
obviated. 
Fig. 8. A register of the declinometer from May 1847, l d 23 h 34 m to 3 d 0 h 4 m , made 
in Keppel Street by the apparatus constructed for the Royal Observatory. 
There is no perceptible difference in intensity between the commencement 
and termination of the line. Several shocks may be remarked, the W. de- 
clination having been, as usual, increased by the disturbing cause, which 
appears to have commenced and terminated abruptly. In some instances 
the duration of very transient disturbances may be reasonably conjectured 
from the tint of the marks on the paper, those for instance at a, b have not 
probably exceeded 20 s . The absence of vibrations, as in the shocks repre- 
sented in Plate VII., is probably owing to the inertia of the mercury damper, 
and offers an additional argument in favour of its use. The base-line is 
drawn by a lamp on the opposite side of the cylinder, and shows two of the 
10 s marks previously described. The narrowness and sharpness of the base- 
line will materially assist in diminishing the probable error of reading the 
position of the register by a scale. 
