ON THE KEW OBSERVATORY. 363 



The number of daily curves tabulated and traced before the experimental 

 trials commenced on April 1st, is not considerable. The number since then 

 to the present time is about 200. 



It only remains here to annex lithographed specimens of printed curves 

 and their abscissae (or zero lines), Plate XXIII., which specimens were printed 

 in the manner described at p. 353, ante. It is not pretended that the litho- 

 graph is equal, in sharpness and accuracy, to the original impression from 

 the gelatine. It is made use of merely because a sufficient numbor of im- 

 pressions for this Report could not be obtained probably from the gelatine. 



Impressions made in September from the gelatine on India-paper, were 

 very good ; but the use of bank-note paper was preferred, on account of 

 its very superior toughness and less liability to adhere to the gelatine under 

 the press. The impressions on bank-note paper were very nearly equal to 

 the India-paper impressions. The number which could be "pulled" from 

 the gelatine itself would be ample for distribution to other observatories, &c. 



On February 18, a specimen of the action, on Talhotype paper, of the 

 horizontal-force magnetograph sent to the Toronto Observatory, arrived 

 here. About half of the curve had been produced by day-light and the re- 

 mainder by lamp-light. The outline, as viewed by the naked eye, seemed 

 tolerably well defined (in both cases) ; but when examined under the same 

 magnifying power as that used in tabulating the curves received upon silver, 

 was found to be much less perfect than in these curves : it exhibited a little 

 fringe, evidently due to the spongy texture of the paper, and was quite in- 

 adequate to measurements, equally minute, with those of lines received 

 upon a silver surface*. 



The Magnetographs and Magnets in general. 



In September 1850, it was ascertained that the cost of Lucca oil consumed 

 in the Rumford polyflame lamps used with these instruments, was about 0*8 

 penny per hour for each. 



In December, and at long intervals subsequently, the improvements upon 

 the sliding-frames (H), as regards the scales (vide p. 350), were prosecuted. 

 The first were engraved by Ross with a diamond point. 



By the end of March it had been found that the etching process, by means 

 of hydrofluoric acid, produced scales on the ground glass well-adapted to the 

 purposes; and the dividing machine of M. Perreaux was very advantage- 

 ously employed in this operation. 



On January 24, Mr. Welsh explained his plan for applying his sliding 

 rule system to computations relative to magnetic curves. 



About June 15 a working drawing of the dip and total-force sliding-rule 

 was made. It was put into Mr. Adie's hands, who constructed it in box- 

 wood ; it has, together with Mr. Welsh's description of it, been exhibited 

 and delivered to the Secretary of the Physical Section. 



In March, and subsequently, the Toronto vertical-force magnetograph 

 was examined and corrected. [A screw had become loose and the knife- 

 edge a little damaged.] 



* I cannot help adverting here to an evident mistake in the Athenaeum of July 12, 1851, 

 p. 784, where the Astronomer Royal is made to imply that my method of self-registration 

 does not embrace the use of the paper process ; whereas it has been clearly shown that, be- 

 tween April 1844 and February 1849, 1 used no other ; that specimens, so procured, have been 

 approved by himself, and that the Kew magnetographs were always equally applicable 

 to either the Talbotype or the Daguerreotype process at the pleasure of the observer or pho- 

 tographist. They can be as easily applied to any new photographic process I believe. 



