of Magnetical and Meteorological Instruments. 333 
or five feet. It acts therefore by refraction, instead of reflection. 
The ray which passes through the screen is received upon a sil- 
vered plate prepared by the common daguerreotype process, some- 
what modified to adapt it toa slow and long continned action. 
The plate is made to travel slowly before the light in a vertical 
plane, by the action of a time piece: the distance of its surface 
rom the centre of the magnet being about thirty-four inches, a 
sufficiently large scale is allowed to render sensible very stall 
movements of the horizontal force, provided a due degree of sen- 
sibility is given to the balance of forces by which the magnet it- 
Self is held in equilibrium. In other words, provided the ratio @ 
in the common notation, is made to approach nearly to unity. Mr. 
Ronalds has succeeded, in the climate of London, in producing 
effects by natural light so late as 8 p.m. in the s:mmer. For the 
nocturnal portion of the curve a powerful argand lamp is employed. 
_ The relative merits, in practice, of the two valuable and ingen- 
1ous inventions now described, can scarcely be stated at present. 
That of Mr. Brooke has probably the advantage in economy and 
facility ; that of Mr. Ronalds will, it is expected, prove capable of 
a higher degree of precision, and it offers a convenience of which 
_the inventor has already availed himself. Any trace of unnsual 
terest can at once be engraved on the plate, thus giving the ut- 
Most possible accuracy and facility to graphical comparisons. It 
1S Not intended however under ordinary circumstances to retain 
the impressions, but after recording every particular of interest, 
and tabulating the hourly or other ordinates, or taking a copy of 
the traces, to clean them off and make use of the plate again as 
long as the silvering lasts. 
In conclusion the writer begs leave to add that shonld the fore- 
S0ing account lead to the establishment of any instruments of the 
kind, he will have pleasure in giving any further information in 
is power in answer to personal enquiry. Without presenting 
facsimiles, it is difficult to convey an idea of the interest attaching 
to many of the movements which have been registered, but the 
important information which such records are calculated to afford 
as to the periodicity of certain movements, the nature and degree 
of local anomalies in disturbances of the magnetical elements, 
the effect of Aurora, and many other enquiries, will occur imme- 
diately to any one interested in terrestrial magnetism, and it is 
' hoped secure the adoption of a register upon one or the other 
Principle, by more than one of the numerous scientific establish- 
ments in the United States. 
Toronto, January 21, 1850. 
