Page images
PDF
EPUB

pass through q, because the angle GAI, formed by the tangent AI and AG, is equal to the angle Arq, which will therefore stand on the same arc aq.

=

180. Corol. 2. If there be given the range AI and the velocity, or the impetus, the direction will hence be easily found thus: Take ak A1, draw kq perp. to AH, meeting the circle described with the radius ao in two points q and q; then ag or rq will be the direction of the piece. And hence it appears that there are two directions, which, with the same impetus, give the very same range AI. And these two directions make equal angles with AI and AP, because the arc pq is equal the arc aq. They also make equal angles with a line drawn from a through s, because the arc sq is equal the arc sq.

181. Corol. 3. Or, if there be given the range AI, and the direction aq; to find the velocity or impetus. Take sk= AI, and erect kq perp. to AH, meeting the line of direction in q; then draw qp making the Aqp Akq; so shall AP be the impetus, or the altitude due to the projectile velocity.

[ocr errors]

182. Corol. 4. The range on an oblique plane, with a given elevation, is directly proportional to the rectangle of the cosine of the direction of the piece above the horizon, and the sine of the direction above the oblique plane, and reciprocally to the square of the cosine of the angle of the plane above or , below the horizon.

sin.

For, put s

c = cos.

[ocr errors]

qai or arq,

4qAH or sin, paq,

IAH or sin. akd or akq or aq?.

Then in the triangle Arq, cs :: AP: Aq;

and in the triangle akq,

theref. by composition,

ccaq ak;

c2; cs: AP : AK = AI.

So that the oblique range a1 =

CS

X 4AP. c

183. The range is the greatest when ak is the greatest; that is, when kq touches the circle in the middle point s; and then the line of direction passes through s, and bisects the angle formed by the oblique plane and the vertex. Also, the ranges are equal at equal angles above and below this direc tion for the maximum.

184. Corol. 5. The greatest height cv or kq of the projectile, above the plane, is equal to

[ocr errors]

X AP. And therefore

[ocr errors]

it is as the impetus and square of the sine of direction above

the plane directly, and square of the cosine of the plane's inclination reciprocally.

For c (sin. Aqp): s (sin. Arq) :: ap: aq,

[ocr errors]

and c (sin. akq) s (sin. kaq) :: aq : kq,

theref. by comp. c: s:: AP: kq.

28 AP

C

185. Corol. 6. The time of flight in the curve Avi is = where g= 16 feet. And therefore it is as the velocity and sine of direction above the plane directly, and cosine of the plane's inclination reciprocally. For the time of describing the curve, is equal to the time of falling freely

[ocr errors][merged small][ocr errors][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small]

186. From the foregoing corollaries may be collected the following set of theorems, relating to projects made on any given inclined planes, either above or below the horizontal plane. In which the letters denote as before, namely,

c = cos. of direction above the horizon,

c = cos. of inclination of the plane,

s = sin. of direction above the plane, Rthe range on the oblique plane,

T

the time of flight,

V the projectile velocity,

H the greatest height above the plane,

a the impetus, or alt. due to the velocity v, g= 32 feet. Then,

[merged small][merged small][merged small][ocr errors][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][ocr errors][ocr errors][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][ocr errors][merged small][merged small]

And from any of these, the angle of direction may be found.

187. Geometrical constructions of the principal cases in projectiles in a non-resisting medium, flow readily from the properties of the parabola; and in many cases those con structions suggest simple modes of computation. The fol. lowing problems will serve by way of exercise.

1. Given the impetus and elevation; to find, by construc tion, the range, on a horizontal plane, the greatest height, and thence the time of flight.

2. Given the impetus, and the range, on a horizontal plane; to find, by construction, the elevation, and the greatest height.

3. Given the elevation, and the range on a horizontal plane; to find, by construction, the impetus, the greatest height, and thence by computation, the time.

4. Given the impetus, the point and direction of projection, to find the place where the ball will fall upon any plane given in position.

5. Given the impetus and the point of projection, to find the elevation necessary to hit any given point; and to show the limits of possibility. Both construction and mode of computation are required.

PRACTICAL GUNNERY.

188. We have now given the whole theory of projectiles, with theorems for all the cases, regularly arranged for use, both for oblique and horizontal planes. But, before they can be applied in resolving the several cases in the practice of gunnery, it is necessary that some more data be laid down, as derived from good experiments made with balls or shells discharged from cannon or mortars, by gunpowder, under different circumstances. For, without such experiments and data, those theorems can be of very little utility in real practice, on account of the imperfections and irregularities in the firing of gunpowder, and the expulsion of balls from guns, but more especially on account of the enormous resistance of the air to all projectiles made with any velocities that are considerable. As to the cases in which projectiles are made with small velocities, or such as do not exceed 200, or 300, or 400 feet per second of time, they may be resolved tolerably near the truth, especially for the larger shells, by the parabolic theory, laid down above. But, in cases of great projectile velocities, that theory is quite inVOL. II.

29

adequate, without the aid of several data drawn from many and good experiments. For so great is the effect of the resistance of the air to projectiles of considerable velocity, that some of those which in the air range only between 2 and 3 miles at the most, would in vacuo range aboul ten times as far, or between 20 and 30 miles.

The effects of this resistance are also various, according to the velocity, the diameter, and the weight of the projectile. So that the experiments made with one size of ball or shell, will not serve for another size, though the velocity should be the same; neither will the experiments made with one velocity, serve for other velocities, though the ball be the same. And therefore it is plain that, to form proper rules for prac. tical gunnery, we ought to have good experiments made with each size of mortar, and with every variety of charge, from the least to the greatest. And not only so, but these ought also to be repeated at many different angles of elevation, namely, for every single degree between 300 and 60° elevation, and at intervals of 5° above 60° and below 30°, from the vertical direction to point blank. By such a course of experi ments it will be found, that the greatest range, instead of being constantly that at an elevation of 45°, as in the parabolic theory, will be at all intermediate degrees between 45 and 30, being more or less, both according to the velocity and the weight of the projectile; the smaller velocities and larger shells ranging farthest when projected almost at an elevation of 45°; while the greatest velocities, especially with the smaller shells, range farthest with an elevation of about 30°, or little more.

189. There have, at different times, been made certain small parts of such a course of experiments as is hinted at above. Such as the experiments or practice carried on in the year 1773, on Woolwich Common; in which all the sizes of mortars were used, and a variety of small charges of powder. But they were all at the elevation of 45°; consequently these are defective in the higher charges, and in all the other angles of elevation.

Other experiments were also carried on in the same place in the years 1784, and 1786, with various angles of elevation indeed, but with only one size of mortar, and only one charge of powder, and that but a small one too; so that all those nearly agree with the parabolic theory. Other experi. ments have also been carried on with the ballistic pendulum, at different times; from which have been obtained some of the laws for the quantity of powder, the weight and velocity of the ball, the length of the gun, &c. Namely, that the velocity of the ball varies as the square root of the charge direct.

ly, and as the square root of the weight of ball reciprocally; and that, some rounds being fired with a medium length of one-pounder gun, at 15° and 45° elevation, and with 2, 4, 8, and 15 ounces of powder, gave nearly the velocities, ranges, and times of flight, as they are here set down in the following table. But good experiments are wanted with large balls and shells.

[blocks in formation]

190. But as we are not yet provided with a sufficient number and variety of experiments, on which to establish true rules for practical gunnery, independent of the parabolic theory, we must at present content ourselves with the data of some one certain experimental range and time of flight, at a given angle of elevation; and then, by help of these, and the rules in the parabolic theory, determine the like circumstances for other elevations that are not greatly different from the former, assisted by the following practical rules.—

191. SOME PRACTICAL RULES IN GUNNERY.

I. To find the Velocity of any Shot or Shell.

RULE. DIVIDE double the weight of the charge of powder by the weight of the shot, both in lbs. Extract the square root of the quotient. Multiply that root by 1600, and the product will be the velocity in feet, or the number of feet the shot passes over per second, nearly.

Or say-As the root of the weight of the shot, is to the root of double the weight of the powder, so is 1600 feet, to the velocity*.

*In more recent experiments carried on at Woolwich, by the Editor of the present edition, in conjunction with the select committee of artillery officers, it has been found that a charge of a third of the weight of the ball, gives, at a medium, a velocity of 1600 feet; gunpowder being much improved in its manufacture since the time when

« PreviousContinue »