% ON THE KEW OBSERVATORY. 83 
clock-work, for imitating long excursions of a magnet in short intervals, in 
order to prove the efficacy of the above-mentioned new arrangements relative 
to light, the slit and the Daguerreotype process in such cases, and adapted 
- it to some horizontal-foree apparatus which was intended for the Toronto 
Observatory ; for it was by far too tedious a task to wait for any disturbance 
approximating in extent to those which occur in Canada. A specimen is 
preserved of the result, which makes out the case (of success) very well. 
But we already begin to contemn these dirty, although efficient, specimens. 
I also began to thik about etching the impressions on the plate itself, and 
received some valuable information on the subject from Mr. Malone, Mr. 
Hodgson of Winchfield, and other gentlemen; and I found that the usual 
cost of plates was somewhat too high. 
Toward the end of May, Daguerreotype apparatus for cleaning, polishing, 
coating, &c. silvered plates of the length required for our purposes claimed 
attention, with special regard to saving of time and labour. 
About the same time Dr. Lloyd visited the observatory, and suggested the 
advantage of procuring a zero line upon the plate formed by the action of 
the same source of light which produced the magnetic curve (as I had from 
the first procured on paper), instead of depending upon the edge of the 
plate for reading off ordinates. This hint appeared so judicious, that 
(although presenting difficulties in contrivance and execution, and thus 
creating delay in the preparations for shipment of the Toronto bifilar appa- 
ratus) I thought it right to try experiments, and attained the object. The 
method will be easily understood presently. 
I had now also hit upon an obvious, but very useful addition to all appa- 
ratus calculated to measure ordinates of magnetic and other curves from a 
given abscissa, within certain but extensive distances, This instrument I call 
the Scale Board (vide Plate IV. figs. 2 and 3), and will describe it below. 
| .. The last-executed improvements have been upon the instruments used in 
__gleaning and coating the plates, in which Mr. Nicklin has materially assisted ; 
and in carefully etching, or rather engraving and etching, the plate 
without using (at first) a “ground,” for which I am chiefly indebted to Mr. 
Wood. The plate which has been thus treated is still capable of receiving 
‘more impressions in the camera, although the first impression is deeply en- 
grayed, and capable of printing any (usual) number of copies. <A printed 
specimen is preserved (vide Plate IV. fig. 4). 
About the first week in June I experienced great satisfaction in receiving 
a visit from Colonel Sabine, to inspect the apparatus (which had been ex- 
perimented upon, improved and tested at our Kew Observatory, under the 
auspices of the British Association) for a horizontal-foree magnetograph, to 
go to the Toronto Observatory. It (excepting the stone pillars) was shipped 
for Montreal, and addressed to Captain Lefroy, Director of the Magnetie 
ine, Toronto, in about the middle of last August, and may be thus 
described. 
_ (Similar letters refer to similar parts in all the figures, excepting in figs. 2 
and 3, Plate IV.) 
_ The figures of Plates I. II. III. and fig. 1 of Plate IV. are drawn to ones 
eighth of size. Fig. 2 and 3 of Plate IV., and all those of Plate V., are one- 
fourth of size. Fig. 4 of Plate IV. is of real size. 
_ V (Plate V. figs. 1 and 2, &c.) is the magnet-box, coated (as usual) inside 
- and out with gold paper, and provided with a short tube (v'), which descends 
and opens into A. 
é A * the camera box. a! is a solid brass casting, forming in part one of 
its ends. 

GQ 
