26 Photography at Greenwich Observatory. 



the value of the base line, the epoch of the sheet, and other 

 data, negatives are prepared in the ordinary way of photo- 

 graphic printing, from which secondary photograms positive 

 tertiaries can be printed, which are faithful copies of the 

 original sheets that were placed round the cylinders, and 

 such copies are distributed when necessary. The paper used 

 for the printing process is made by Rive, and the salting solu- 

 tion is chloride of ammonium, and the sensitizing one ammonia- 

 nitrate of silver. Having by the kindness of Mr. Airy, the 

 Astronomer Eoyal (who has supplied every information necessary 

 for this article), been furnished with a duplicate of one of the 

 photographic records, and permission to use it in any way 

 thought desirable, we are enabled to offer our readers a reduced 

 facsimile of the trace on the horizontal cylinder which records 

 the variations of the declination and bifilar magnetometers. 

 The date is 4th September, 1859,* and the wood engraving, 

 although unable to give the full effect of the photographic 

 curve as to its minute inflections and transparency, is most 

 curious and interesting, and, we believe, the first opportunity 

 most of our readers have had of inspecting these beautiful 

 tracings by the pencil of light, of the occult workings of one 

 of the most important, and at present obscure, physical forces. 

 We have hitherto spoken of the instruments as placed upon 

 the ground level of the observatory, which position they 

 occupied from its establishment until last year, when in conse- 

 quence of the difficulty of keeping the temperature nearly con- 

 stant (and unless this be effected the variations are influenced 

 by it) an excavation was made under the building to the extent 

 of the three arms of the cross occupied by the magnets, and 

 this being well bricked round, the instruments have been 

 placed in these vaults, which are lighted by sunken windows of 

 yellow glass ; and as the temperature now rarely varies more 

 than 10° from 60°, the indications which have been obtained 

 this year when the observations were recommenced after the 

 necessary interruption, are considered better than the former 

 results. The original declination magnetometer, however, re- 

 mains above in the southern arm of the cross building, for the 

 sake of still observing the transit of circumpolar stars through 

 the roof-slit, to obtain the astronomical meridian, and read off 

 the departure of the magnet from the true north. An exact 

 duplicate being mounted below the observations made by the 

 theodolite above and the photographic ones in the apartment 

 below are strictly comparable. 



* The curve displays some very considerable perturbations, and it may be 

 remarked that on 1st September, 1859, Mr. Carrington and Mr. Hodgson -wit- 

 nessed the violent outburst of light on a solar spot, which affected magnetic in- 

 struments simultaneously in all parts of the globe. 



