357 



touch of the apparatus for regulating the more delicate movements 

 of the telescope. 



In order to protect it from the weather, which was found to injure 

 its action and derange its adjustments, the author erected an obser- 

 vatory to contain it, consisting of a light piece of carpentry, 1 6 feet 

 in diameter, with a revolving conical roof rising 9 feet above the 

 walls, containing about 360 square feet of surface, and weighing 

 about 10 cwt. It is moveable by a simple apparatus, made to re- 

 volve and open to any required azimuth, by the application of a force 

 of about 10 or 12 pounds. 



His first object in the preliminary experiments was to ascertain 

 the best position of the lenses for diminishing as much as possible 

 the secondary spectrum. For this purpose he reverts to the formulae 

 given in a preceding paper, whence he deduces equations applicable 

 to this object. The mode of constructing different parts of the tele- 

 scope is then particularly described, especially that of the fluid lenses, 

 and of effecting the proper centering, and other adjustments. He 

 then describes its power when applied to several double stars. By 

 its means 77 Persei, marked as a treble star in South's and Herschel's 

 catalogue, is seen distinctly sextuple ; four of the smaller of these 

 stars, together with a larger one, form a miniature representation of 

 Jupiter and his satellites. Of the planets, he has only had opportu- 

 nities of trying the telescope on Venus, Saturn, and Mars, all of 

 which appear with remarkable brightness and distinctness. The moon 

 is also remarkably beautiful ; every minute distinction of figure and 

 shade being brought into view. The paper concludes with a detailed 

 description of the various parts of the telescope and stand, illustrated 

 by a drawing. 



On the Dip of the Magnetic Needle in London, in August, 1828. By 

 Captain Edward Sabine, of the Royal Artillery, Sec. R.S. Read 

 January 8, 1829. [Phil. Trans. 1829, p. 47.] 



This paper commences by noticing that the Philosophical Transac- 

 tions contain the record of observations on the dip of the needle in 

 London, from the early part of the last century to the present time. 

 That these observations all concur in showing a progressive decrease 

 of the dip during the whole period in question, but that they are in- 

 sufficient in number and frequency, and the earlier ones particularly, 

 in the required accuracy to enable us to determine whether the an- 

 nual decrease has been uniform or otherwise. 



The author having taken much pains to obtain a correct determi- 

 nation of the dip in the Regent's Park, in August 1821 (published 

 in the Philosophical Transactions for 1822), repeated his observations 

 in August 1828, at the expiration of seven years from the former 

 determination ; an interval which he considered sufficient to throw 

 light on the rate at which the dip is at present diminishing. In con- 

 sequence of the increase of buildings in the Regent's Park, he was 

 induced to change the place of observation to the Horticultural So- 



