296 



deviation of the compass on ship-board by an iron plate ; for shouli 

 circumstances require the removal and replacement of the compen- 

 sating plate in high northern latitudes, its magnetism might be so 

 altered by the effect of rotation as materially to injure its compen- 

 sating property. The means of avoiding this disagreeable conse- 

 quence are pointed out. 



Observations to determine the Amount of Atmospherical Refraction at 

 Port Bowen in the Years 1824-25. By Captain W. E. Parry, R.N. 

 F.R.S. ; Lieutenant Henry Foster, R.N. F.R.S. ; and Lieutenant 

 J. C. Ross, R.N. F.L.S. Read June 15, 1826. [Phil. Trans. 

 1826, Part IV. p. 206.] 



The author commences by observing, that on attempting the va- 

 rious methods proposed by astronomers for ascertaining by actual 

 observation the amount of atmospherical refraction at low altitudes, 

 they all proved impracticable at Port Bowen, by reason of the in- 

 tense cold, which rendered it impossible to use the repeating circle 

 or other similar instruments. The method therefore proposed by 

 Lieutenant Foster, and modified by Captain Parry, which was found 

 successful, was, to place a board edgeways and truly horizontal on 

 that part of the high land behind which a given star set, and observe 

 the moments of its disappearance. Then, determining at leisure the 

 zenith distance of the upper edge of the board on the return of the 

 sun, and in weather better fitted for delicate observations, the stars 

 fixed on were a Aquilae and Arcturus ; and the paper before us gives 

 a detailed account of a series of observations of the moments of dis- 

 appearance of both these stars, and also of the zenith distances of 

 the boards employed by the several observers enumerated in the title. 

 In some cases also, the reappearance of the star below the board was 

 observed, thus giving an observation at another altitude, and the an- 

 gular breadth of the board was afterwards measured by a micrometer 

 from the station of observation. 



Description of a Percussion Shell, to be fired horizontally from a com- 

 mon Gun. By Lieutenant Colonel Miller, late of the Rifle Brigade, 

 and now unattached. Communicated by R. I. Murchison, Esq. F.R.S. 

 Read November 16 and 23, 1826. [Phil. Trans. 1827, p. 1.] 



In this paper, the author first considers the theory of rifles, with 

 which the subject of it is intimately connected ; and regarding it as 

 an admitted principle, that irregularities in the flight of shot arise 

 from irregularities either in their surface or substance, shows how 

 the rotatory motion of a rifle ball, by presenting every part uniformly 

 to the action of the resisting medium, obviates the effect of these, 

 irregularities. The spiral or rotatory motion of the ball in rifles, is 

 generally supposed to arise wholly from the re-action of the grooves 

 in the barrel, or from the indentations made by them in the surface 

 of the ball ; but the author, taking into consideration the powerful 

 action of the air on projectiles, is led to conclude that the rotation 



