458 
MINUTES OF PEOCEEDINGS OF 
caused by motion, or to tbe formation of infinitely small vacua, which depend 
on the velocity. Before I dismiss this topic I would insist that in future 
considerable attention should be paid to the surfaces of projectiles, as if 
rough and wet the friction, and consequent increase of resistance will be 
correspondingly great, while the contrary will be the case if they are smooth 
and oily. 
The particles of compressed air struck by the projectile ought by analogy 
to produce electricity and heat, and a portion of this heat light and sound. 
Of these the first, electricity, will be wasted in space ; light will never be 
manifest on the darkest night for want of solid particles to incandesce, and 
sound perhaps is a hum distinct from, but swallowed up by the noise of the 
projectile, but the remaining portion of heat is of great importance as 
influencing the velocity with which the compressed air will rush over the 
sides of the projectile to fill the vacuum behind. Now the thermal effect of 
the friction of the air appears, (vide Beport 1859, British Association), to be 
independent of the shape and length of the mobile, and increases moreover 
as the square of the velocity. Thus if a projectile moving at the rate of 
175 ft. per second were heated 1° Fahrenheit, when moving at ten times this 
velocity, it would be heated 100° Fahrenheit above its ordinary temperature. 
The heat thus caused by the projectile will increase the velocity of the 
compressed air in filling the void, and from a similar reason red hot shot 
ought to incur less resistance than cold shot of the same diameter, although 
they expose greater comparative section. A centre of expansion will always 
be generated exactly in front of the projectile. 
In establishing the forms of bodies having least resistance in air, as these 
might practically turn out also the most accurate, it appears evident that a 
curved line will offer less resistance than a straight one, for the curve may be 
so fashioned that the greater part of the resistances may be resolved tan¬ 
gentially. Sir Isaac Newton gives a conoidal form (generated by the motion 
of a line, partly on a fixed line, and partly on a conic-section) as the best, and 
for this in practice may be substituted a cylindro-ogival, as better suited for 
rotatory purposes. The length of the best theoretical projectile should be 
some function of the velocity, and the best form for the hinder part of the 
projectile should be the curve of least resistance to the motion of the 
compressed air, for the compressed air will fill the vacuum behind quicker 
than ordinary air. 
Air rushing in behind a spherical shot will produce an appearance like I 
have shown in the diagram (Fig. 4). By approximating to the directions of 
the ripples, a form analogous to the nautical wave line will be obtained, 
which produces no such appearance, and which will perhaps be a pear-shaped 
shot with a cylindro-ogival head. This form of projectile is evidently ill 
adapted to resist the force of the explosion, and sabots, however ingenious, 
will not counteract this defect, but we can approximate to some of its 
important features by the method I have shown in the companion figures 
(Fig. 5), viz. by shortening it, and rounding off the extremity. Piobert in his 
Corn's d’ Application d } Artillerie, gives the best length at five times the 
greatest breadth, and this breadth at two-fifths the length, and nearest the 
fore part of the projectile. 
It is necessary that the shot be of a certain density, in order to obtain a 
certain range and accuracy of fire, as this density of the shot will qualify the 
