Application of Photography, &¢. 319 
Art, XXXII.—On the Application of Photography to the Self- 
registration of Magnetical and Meteorological Instruments ; 
by Captain J. H. Lerroy, R.A., F.R.S., Director H. M. Magnet- 
ical and Meteorological Observatory at Toronto, Canada. 
Tue successful application of the principle of self-registration 
y means of the action of light upon sensitive paper, or upon 
silvered plates, to observations in magnetism and meteorology 
by 
may be instanced as one of the most important indirect results of 
Magnetism, more particularly, it has been brought to perfection 
at a peculiarly appropriate time. From eight to ten years of la 
observations upon all magnetical disturbances detec d, and of term 
days designed to detect them but on which they seem to have 
made it a point not to occur; in spite of the extraordinary pa- 
hence of Colonel Boileau in observing at Simla, not every hour, 
but every fifteen. minutes, and of the perseverance with which 
Ur. Bache, at Philadelphia, multiplied his observations at the crit- 
ical hours of each of the elements, in spite, in short, of all the 
efforts which have been made to obtain a full knowledge of the 
fluctuations of these most inconstant objects, it cannot be doubt- 
ed that by far the greater, and perhaps the more instructive por- 
tion of all their changes, eluded the vigilance of the observers. 
In this state of things, therefore, a method which secures a minute 
and continuous graphieal record of every change, and which 
can be put in practice with comparatively little difficulty or ex- 
pense, is an acquisition to the science, seco perhaps only to 
the invention by Gauss and Lloyd of the instrumental means 
upon which its previous rapid progress has been so largely based. 
The following description of the Photographical Instrument of 
Mr. Brooke, is. based principally upon that gentleman’s communi- 
