On the Resistance of the Air to Rifled Projectiles. 463 



dispersions, which should be sought for in plates normal to the two 

 bisectrices, are, on the contrary, but slightly evident. However, on 

 examining iu oil some thin plates placed so as to have the plane of 

 their optic axes horizontal and perpendicular to the plane of polariza- 

 tion, the author observed in the plates normal to the obtuse positive 

 bisectrix some faint blue and red fringes, dispersed in contrary di- 

 rections above and below the bars which traverse the two systems 

 of rings, indicating the existence of an appreciable twisted dispersion. 

 In the plates normal to the acute negative bisectrix, the transverse 

 bar of each system is bordered on one side by a very pale blue, and 

 on the opposite side by an equally pale yellow, the horizontal disper- 

 sion beiDg thus feebly indicated. 



The crystals of Wohlerite ought, then, to be referred to an oblique 

 rhombic prism, in which the plane of symmetry is normal to the 

 plane which contains their optic axes. The primitive form which it 

 seems most convenient to choose is a prism with an angle of very 

 nearly 90°, which presents a cleavage, easy though interrupted, pa- 

 rallel to its plane of symmetry, and cleavages which are more diffi- 

 cult, in the directions of the lateral faces m and of the plane h' } which 

 is parallel to the horizontal diagonals of the base. 



The author then describes in detail the crystalline form of Wohler- 

 ite as thus corrected,, 



" On the Law of the Resistance of the Air to Rifled Projectiles." 

 By Charles W. Merrineid, F.R.3., Principal of the Royal School of 

 Naval Architecture. 



At the beginning of this month Lieut. -Col. H. R. Halford applied 

 to the author to obtain for him the law of atmospheric resistance 

 resulting from his experiments in shooting with Metford's match-rifle, 

 a small-bore with increasing pitch. Col. Halford had determined by 

 experiment the elevations required for the ranges 100, 200, &c.upto 

 1100 yards, each determination being derived from a very large num- 

 ber of shots ; and the table of experimental elevations corresponding 

 to these various ranges formed the datum furnished to the author. 



As all the trajectories were very low, the greatest eleyation amount- 

 ing to only 2° 35' 30", the author assumed, as a sufficiently close 

 approximation, that the vertical motion was determined solely by the 

 force of gravity, and that the effect of the resistance of the air on 

 the velocity was the same as if the projectile had moved strictly in a 

 horizontal line. Consequently the depression of the point in which 

 the target is struck, below the initial tangent to the path, becomes 

 a measure of the time of flight, according to the usual law of falling 

 bodies ; and the mean horizontal velocity being thus known for a 

 series of different ranges, we can calculate the mean velocity for every 

 100 yards of along range, and thence determine the resistance. 



The author commenced his calculations from an assumed velocity 

 of 1360 feet per second, in accordance with the results obtained at 

 short ranges, and assumed for trial a resistance varying as the square 



