Pee μὰ oak 
Tae ar 
bE; Eas 
ON THE KEW OBSERVATORY. 181 
« The mode of procedure here spoken of is chiefly applicable to serene 
weather, including fogs, mists, &c. If rain or snow, impending or more 
distant clouds, or sudden changes from a positive to a negative state of charge 
should occasion difficulties or impossibilities of observing frequency, these 
circumstances to be noted in the electro-meteorological journal and frequency 
paper.” 
III. ExpERIMENTS. 
The first subject of consideration, as respects instrumental experiments, 
after the last annual meeting, was a better mode of mounting the standard 
thermometer and wet-bulb hygrometer. 
In October, a revolving stand, on the Greenwich plan, was erected at the 
north entrance of the building ; but objections exist to this, and in fact to all 
“thermometer-stands” hitherto invented. Either the sun or the wind has 
injurious influences, which it should seem are hardly to be got rid of. I trust, 
however, the method will be improved under the suggestions of Col. Sykes. 
In November 1849 some work and preparations were executed here for 
the vertical-force magnetograph (vide Plates I. & II.), alluded to in my last 
Report as being in an advanced state for the Toronto Observatory. In De- 
cember some principal parts of it arrived from Mr. Ross and Mr. Newman, 
and its completion was proceeded with. 
A successful attempt to improve this sort of apparatus was that of fitting 
a sliding plate to the frame containing the Daguerreotype plate, in such 
manner that it completely excluded light from the latter, whilst it was not 
in its place in the instrument, But allowed the focus to act upon it when 
properly placed there. This contrivance (a modification of one commonly 
practised by photographists) precludes entirely the necessity of operating 
upon the plate or paper in a dark room before the mercurializing part of the 
Operation is performed. 
An improvement in the mouth-piece permits a much greater facility and 
accuracy of adjustment in the breadth of the slit than had been before 
attained, a matter of some importance, when the delicate and rapid changes 
of the magnet’s position are required to be registered. A chain was substituted 
for a gut-line for suspending the sliding-frame, which somewhat improves 
the accuracy of the magnetic curve produced on the contained metallic plate 
or paper. A little frame, containing ground glass, was added, in order to 
save time and trouble in examining the image of the slit in the shield. A 
screen, placed temporarily in the place of the fixed shield, was used with 
advantage for dividing the aberration of the lenses between the central and 
the outer parts of the range of the said image. . This screen was provided 
with a series of slits, in lieu of the one slit only of the fixed usual shield. 
An improvement applicable to this and all photo-registering instruments of 
similar construction was adopted, consisting in a sliding shutter, which, by a 
simple, small, rotatory movement, given by the fingers to an arbor passing 
through the clock-plates, is opened in order to expose the Daguerreotype 
plate to the focus of light, and at the same moment to set the clock in nio- 
tion, and vice versd. 
In order that the new arrangements may be clearly comprehended, and 
trouble saved in recurring to former descriptions, it will be convenient to 
place the whole apparatus before the eye as finally constructed. 
Description of the Vertical-Force Magnetograph. 
Similar letters;refer to similar or analogous parts infthe figures of this in- 
strument, as well as of the horizontal-force magnetograph described at page 80 
of the British Association’s Report for 1849. 
