NATURAL PHILOSOPHY. 193 



be increased, proportionally, with any increase in the length of the projec- 

 tile; and, secondly, that it is not advisable that the projectile should, in 

 length, exceed the triple of its diameter. 



Another question of great importance to be satisfactorily determined is, 

 whether the varying size of the projectile produces a similar effect upon the 

 turn of the grooves of a rifled cannon. Now, this cannot be ascertained 

 beforehand by theory, but depends entirely upon experiment, and on experi- 

 ment alone. It is true, that where the shot differ only in their diameters, a 

 law may be readily laid down; where, however, they differ loth in form and 

 weight, a laborious series of experiments must be made before any definite 

 principles can be enounced. Here, as in so many branches of the great sub- 

 ject of gunnery, no clear views appear hitherto to have been held or pub- 

 lished; hence, Mr. Thomas has been compelled to examine into and to 

 state all the bearings of the case with great minuteness. It would be impos. 

 sible, in this place, to follow him through all his reasonings; but we may 

 notice, generally, that the retarding effect of the air, for shot differing in 

 size, but of the same form and density, would appear to be nearly as the square 

 roots of their diameters. 



Having discussed these matters as fully as possible, Mr. Thomas goes on 

 to describe, with equal minuteness, the different forms of projectiles advisa- 

 ble under different circumstances. We cannot enter here into these details; 

 but we may state the general principles at which he has arrived, experimen- 

 tally. Thus, he states that the necessary qualifications for an elongated 

 projectile are: 1. That it should possess a certain definite density, so as to 

 insure the greatest possible range. 2. That it should completely fill the 

 bore, so that its axis should coincide exar-tly with it. 3. That its centre of 

 gravity should be thrown well forward, in order that the axis on which it 

 rotates should be, practically, a tangent to the line of its flight. And, 4. 

 That its form should be such as to expose it to the least possible resistance 

 from the air. Lastly, he shows that solid iron unexpanding shot can never 

 really produce the results attainable by compound shot, because in their 

 case space must always be allowed for their windage, for the fouling of the 

 bore, and for the contraction of the gun itself, when heated by repeated 

 firing. 



Mr. Thomas remarks, that it has generally been hitherto held that, on 

 firing, the whole of the powder is at once converted into an elastic fluid, and 

 that the ball is expelled by the gradual expansion of this fluid. The result, 

 however, of many interesting experiments made by him, appear to show that, 

 besides the ordinary explosive property of gunpowder, there resides in it a pe- 

 culiar force, which (for want of a better name) Mr. Thomas has termed impul- 

 sive; and that owing to this, large guns are much more liable to burst than 

 smaller ones. It is no less certain that, with a finely granulated powder, a 

 comparatively short gun may be safely used, such tribes as the Afghans, 

 on the other hand, who manufacture a powder much inferior to ours, being 

 compelled to use guns of a length apparently altogether disproportionate, 

 with the simple object of completely igniting the powder. 



WIND OF A SHOT. 



M. Pelikan, a Russian, has made some curious observations upon the con- 

 tusions supposed to be produced by the wind of passing balls. Mr. P. ap- 

 plied to the committee on artillerv of St. Petersburg, and having obtained- 



" 



