xlviii REPORT — 1870. 



be again mentioned in the second division of this Report. These arc in the 

 charge of Mr. Baker, the photography being superintended by Mr. Page. 



3. Photoheliogvajih. — The Kew Heliograph, in charge of Mr. Warren De La 

 Rue, continues to be worked in a satisfactory manner. During the past year 

 351 pictures have been taken on 237 days. 



It was considered desirable that sis prints should be obtained from each 

 of the negatives of the sun-pictures taken at the Observatory during the 

 whole time that the Photoheliograph should remain at work, Avhich will pro- 

 bably be from February 1862 to February 1872. 



In order to accomplish this, an outlay of ^120 spread over two years was 

 found to be necessary, and this sum has been voted from the Donation Fund 

 of the lioyal Society. 



A large number of these prints has already been obtained, and it is pro- 

 posed to present complete sets to the following institutions : — 



The Royal Astronomical Society, 



The Imperial Academy of Paris, 



The Imperial Academy of St. Petersburg, 



The Royal Society of Berlin, 



The Smithsonian Institution, United States, 



leaving one set for the Royal Society. 



A paper embodying the positions and areas of the sun-groups observed at 

 Kew during the years 1864, 1S65, and 1866, as well as fortnightly values 

 of the spotted solar area from 1832 to 18G8, has been communicated to the 

 Royal Society by Messrs. Warren De La Rue, Stewart, and Locwy. 



This paper is in the course of publication in the Philosophical Trans- 

 actions, aud wUl shortly be distributed. 



A Table exhibiting the number of sun-spots recorded at Kew during tho 

 year 1869, after the manner of Hofrath Schwabe, has been communicated to 

 the Astronomical Society, and published in their Monthly IS'oticcs. 



M. Otto Struve, Director of the Imperial Observator}- at Pulkowa, visited 

 England in the month of August last. He brought with him, for the Kew 

 Observatory, some sun-pictures made at Wilna with the photoheliograph, 

 which, it will be recollected, was made some years ago, under the direction 

 of Mr. De La Rue, by Mr. Dallmcyer. This instrument combines several 

 important iuaprovemeuts on the original Kew model, the value of which i.s 

 forcibly brought out in the superior definition of the Wilna sun-pictures. As, 

 however, the series of the ten-yearly record at Kew was commenced with 

 the instrument as originally constructed, it was not deemed desirable to alter 

 it in any way initil the series had been completed and reduced, and the 

 corrections for optical distortion ascertained and applied. In the event of 

 the sun-work being continued after 1872, it wiU be desirable to do so with 

 a new and improved heliograi)h. 



M. 0. Struve proposed to exchange tlie complete series of pictures obtained 

 at Wilna for that made at Kew. He also stated that it is contemplated to 

 erect a second heliograph at the Central Observatory at Pulkowa. 



4. Miscellaneous Worlc. — A few experiments have been made on the ro- 

 tation of a disk in vacuo. By an arrangement devised by Mr. Beckley, a 

 very perfect carbonic-acid vacuum has been obtained, the residual pressure 

 being 0-02 inch as indicated by a mercurial gauge with a contracted tube, but 

 it was believed that the vacuum was even more perfect. 



A disk of paper and one of ebonite gave very sensible heat effects in such 

 a vacuum, and it was hoped that the experiments might have been satisfac- 



