1868.] On the Resistance of the Air to Rifled Projectiles. 321 



presented by the substance even when in very thm plates. Besides this, 

 contrary to what is found in Harmotome, while the dispersion belonging 

 to the optic axes is very distinct, the horizontal and twisted dispersions, 

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

 on the contrary, but slightly evident. However, on examining in oil some 

 thin plates placed so as to have the plane of their optic axes horizontal and 

 perpendicular to the plane of polarization, the author observed in the plates 

 normal to the obtuse, positive bisectrix, some faint blue and red fringes, 

 dispersed in contrary directions 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 dispersion 

 being thus feebly indicated. 



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

 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, parallel to its plane of sym- 

 metry, and cleavages which are more difficult in the directions of the lateral 

 faces m and of the plane A' which is parallel to the horizontal diagonals 

 of the base. 



The author then describes in detail the crystalline form of Wohlerite as 

 thus corrected. 



III. " On the Law of the Resistance of the Air to Rifled Projec- 

 tiles.^" By Charles W. Merrifield^ F.R.S.^ Principal of the 

 Royal School of Naval Architecture. Received March 19^ 

 1868. 



(Abstract.) 



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 eleva- 

 tions required for the ranges 100, 200, &c. up to 1100 yards, each deter- 

 mination being derived from a very large number 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 elevation amounting 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 efl'ect 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 



