26 [March 13, 



such position, a disk strikes a target not in a vertical plane as fired ; 

 but when the centre of gravity is "above," when the disk is free to 

 roll and not merely slide in the bore, when the sabot, if any, is 

 very light and destructible, as of card, when there is a sufficient 

 charge of powder, and the disk is tolerably symmetrical laterally, 

 and sufficiently eccentric longitudinally (but which eccentricity need 

 not be an amount that causes a dip of more than 1 when the disk is 

 floated in mercury), then the disk, if fired in a vertical plane, is cer- 

 tain to strike a target in that position up to the distances at which I 

 have yet had the opportunity of trying it ; and though such distance 

 (from the land experiments here having been of necessity in a quarry *) 

 has been only from twenty to thirty yards, yet, as in the other posi- 

 tions of the centre of gravity the disk turns over irregularly within 

 such distance, it may be assumed that a rotation in a vertical plane 

 is set up in the one position referred to, viz. " above," and in no 

 other position of the centre of gravity. To this conclusion all these 

 experiments appear to tend. It may by some be questioned whether 

 this rotation is as a wheel, or the reverse way, by the advancement 

 of the lower part of the shot. These experiments do not appear to 

 support the conclusions of M. Magnusf (which have been so very 

 widely adopted, as by Sir Howard Douglas in his fourth and subse- 

 quent edition), viz. that rotation occurs in both positions of "above" 

 and "below," but in the latter only is as a wheel ; while previously, 

 and as expressed in his third edition, Sir H. Douglas entertained the 

 opinion that the rotation was in that direction, or by the advance- 

 ment of the upper hemisphere, when the centre of gravity was above. 

 It is probable that rotation in a disk, in either direction, would keep 

 its plane vertical when the projection had been in a vertical plane ; 

 so that if it strikes upright only when fired with the centre of gra- 

 vity in one position (as when "above"), it seems a fair conclusion 

 that with the other positions of the centre of gravity there can be 

 no rotation imparted. 



This I had reason to suspect, as regards the position of the centre of 

 gravity "below," long before I had an opportunity of proving it with 

 the disk-gun ; for in 1854, the model-mortar experiments referred to 



* Capt. Bent, R.A., of the Royal Laboratory, St. Budeaux, near Devonport, 

 was so kind as to afford the ground for these land experiments, which otherwise 

 I should have been unable to carry out. 



f M. Magnus's Paper on Deviation of Projectiles in ' Taylor's Scientific Me- 

 moirs,' Nat. Phil. Part III. for May 1853. 



