AND OTHER METEOROLOGICAL INSTRUMENTS, BY PHOTOGRAPHY. 
67 
of which has frequently been noticed, and which are very conspicuous in fig. 20, a 
portion of a register purposely taken during the most quiescent period of the week. 
Notwithstanding the local sources of error above alluded to, it is very satisfactory 
to observe the close agreement between the photographs and several hundreds of the 
observations made at the Royal Observatory, which have been very carefully laid 
down upon them. These automatic registers include the two last term-days, and 
several periods of unusual disturbance, during which extraordinary observations 
have been made at Greenwich at intervals of one, two, or three minutes, and through- 
out the whole series the number of discrepancies is very small. An almost con- 
tinuous registration has been maintained during the present year; and it is due to 
the praiseworthy vigilance of the observers engaged at the Royal Observatory to 
state, that very few extraordinary disturbances have escaped their notice. 
Of those which have been compared with the Greenwich observations, fig. 20, 
Plate VII., the register of April 6th, 1846, and fig. 1, Plate VIII., that of April 16th, 
exhibit the greatest disturbance ; in both instances it may be remarked that the 
greatest variation has occurred between 7 h and 1 l h . It may here be conveniently 
remarked, that in accordance with the practice of the Royal Observatory, the time 
adopted is Gottingen mean astronomical time, which is 39 m 46 s in advance of Green- 
wich mean time. 
Fig. 17, Plate VII., the register of April 15th, exhibits a brief disturbance, occurring 
between 8 h and 9 h , in which the photograph differs more essentially from the Green- 
wich observations than in any other instance : a sharp cusp in the latter, correspond- 
ing at two points to a mere bend in the former, would lead to the inference, that the 
disturbing cause must have been so near to Greenwich as to have exerted a mate- 
rially greater influence on the magnet there under observation. 
A singular fact is established by some of these photographs, namely, that after a 
certain space of time the actinic influence spontaneously decays ; for in some in- 
stances the paper has been suffered to remain on the cylinder more than twelve hours ; 
and consequently a second impression is made upon the paper within a very small 
distance of that which had been made twelve hours previously, and while the latter 
impression is distinctly developed, the former is very faint : fig. 6, Plate VII. is a good 
illustration of this fact. The succeeding portions however of the first impression 
become more and more distinct, thus showing that the decay is gradual. It appears 
to commence after a period of from ten to fourteen or fifteen hours ; but the condi- 
tions on which this variation of time depends have not been ascertained*. 
* Subsequent observation has led to the opinion that this decay of the impression depends on the paper having 
been originally prepared with too weak a solution of bromide of potassium. The solution now used contains 
4 grains of isinglass, 16 grains of bromide of potassium, and 4 grains of iodide of potassium in one fluid-ounce 
of distilled water. The paper prepared with this solution appears to retain the actinic impression unimpaired for 
a period of more than 24 hours. Consequently two registers, each of 12 hours’ variation, may be obtained on 
the same paper by allowing the cylinder to go twice round. An example of this may be seen in Plate IX. 
fig. 8, in which it may be observed that the occasional crossing of the lines does not at all interfere with their 
K 2 
