To prove the truth of his theory, Mr. 
Robins made the following experiments, be- 
fore several members of the Royal Society: 
“The first experiment, exhibited on this 
occasion, was to evince, that Re whirling 
IllFLE. 
the right or left of the vertical plane, in 
which it began to move. For if it appears at 
any time, that the bullet has shifted from that 
vertical plane, in which its motion began, 
^ this will be an incontestable confirmation of 
motion of a ball, combining with irt progres- j what we asserted. Since no other powei but 
give motion, would produce such an oblique that unequal resistance, which we here in- 
resistance and deflective power, as is herein sist on, can occasion a body in motion to 
mentioned. For this purpose, a wooden j deviate from the vertical plane, in which it 
ball, 4\ inches diameter, was suspended by j has once moved. 
a double string about eight or nine feet long, j “ Now by means of screens of exceedingly 
Now by turning round the ball, and twisting j thin paper, placed parallel to each other at 
the double string, the ball, when left to 1 proper distances, this deflection in question 
itself, would have a revolving motion given j m ay be many ways investigated. For by 
it from the untwisting of the string again. | firing bullets which shall traverse these 
And if, when the string was twisted, the ball ' screens, the flight of the bullet may be traced 
was drawn a considerable distance from the ou t ; and it may easily appear, whether they 
perpendicular, and there let go ; it would at ! do or do not keep invariably to one vertical 
first, before it had acquired its revolving mo- i plane. This examination may proceed on 
tion, vibrate steadily enough in the same ; three different principles, which I shall here 
vertical plane in which it first began to j separately explain. 
move; but when, by the untwisting of the , - << For first, an exact vertical plane may be 
string, it had acquired a sufficient degree of t race d out upon all these screens, by which 
its whirling motion, it constantly deflected deviation ot any single bullet may be 
on the right or left of its first track, and some- more rea dily investigated; only by measuring 
times proceeded so far, as to have its direc- q ie horizontal distance of its trace from the 
dion at right angles to that in whicn it began ve! tical plane thus delineated, and by this 
its motion ; and this deviation was not pro- meaus the absolute quantity of its aberration 
duced by the action of the string itself, but mav known, 
appeared to be entirely owing to the resist- y Qr jf 
, . . - , r ,. Q 01 ji the description of such a vertical 
ance being gieaei on ^ one p„ l plane should be esteemed a matter of diffi- 
leadmg suriace o l _ . cl , , culty and nicety, a second method may be 
other. For the deviation continued *hen fol - we(] . whi< £ is that 0 f vesting the piece 
fhe stung was o a V 11,1 ,'\ IS e ’ a ‘, e __ in some fixed notch or socket, so that though 
durug the time u u s i ng, * the piece may have some little play to the 
tion the globe had received, was pasting the ^ ^ left j yet a]1 the lines , in which the 
s “ directed. 
bullet can be directed, shall intersect each 
other in the centre of that fixed socket ; by 
this means, if two different shot are fired from 
the piece thus situated, the horizontal dis- 
tances of the traces made by the two bul- 
lets on any two screens, ought to be in the 
during the time that the string, by the mo- 
' ... lc j revived, was twisting the 
contrary way. And it was always easy to 
predict' before the ball was let go, which way 
it would deflect, only by considering on 
which side the whirl would be combined with 
the progressive motion, for on that side 
always the deflecting power acted ; as the 
resistance wa, g ieatei heic, than on dm shL game p r0 p 0r ti 0 n to each other as the respec- 
where the whirl and progressive motion weie ^ d ^ ta j lces of these screeM from the sock- 
opposed to each ol ler. j n which the piece was laid. And if 
“This experiment is an incontestable proof, these horizontal distances differ from that 
that, if any bullet, besides its progressive mo- proportion, then it is certain, that one of 
tion, hath a whirl round its axis, it will be these shot at lea t hath deviated from a ver- 
deflected in the manner here described, tical plane, although the absolute quantity of 
And .as it is scarcely possible to suppose, but that deviation cannot be hence assigned ; be- 
that every bullet, discharged from the pieces cause it cannot be known, what part of it is 
now in common use, must receive such a to be imputed to one bullet, and what to the 
whirl from its friction against the sides of the other, 
piece, the pr< position might perhaps be 
safely rested on this single experiment. But 
not to leave any thing doubtful in a subject 
liable to so much contestation, I undertook 
to evince, by an ocular proof, the reality of 
this deflection in musquet-bullets even in so 
short an interval as a hundred yards. And 
these experiments having succeeded to the 
general satisfaction of those who honoured 
me with their company, I shall here de- 
scribe, as briefly as I can, the manner in 
which they were tried, and the conclusions 
resulting from them. 
5S§ 
found the success agreeable to my expec- 
tation. But what 1 thought the most eli- 
gible for the experiments, which 1 proposed 
to shew to the society, was a compound of 
the two last, and the apparatus was as fol- 
lows : 
“ On ■, being the first day appointed 
for these trials, the weather was unfavourable, 
and the experiments on that account more 
confused than coukl have been wished, 
though they were far from inconclusive. 
But on the next Thursday two screens were 
set up in the large walk in the Charter-house 
garden ; the first of them at 2 30 feet distance 
from the wall (which wall was to serve for 
a third screen), and the second two hundred 
feet from the same wall. And at fifty feet 
before the first screen, or at 300 feet from, 
the wall, there was placed a large block, 
weighing about 200lb. weight, and having 
fixed into it an iron bar with the socket at 
its extremity, in which the piece was to be 
laid. The piece itself was of a common 
length, and was bored for an ounce ball. It 
was each time loaded with a ball of 17 to the 
pound (so that the windage was extremely 
small) and w ith a quarter of an ounce of good 
powder. The screens were made of the 
thinnest tissue-paper ; and the resistance 
they gave to the bullet (and consequently 
their probability of deflecting if) was so small, 
that a bullet lighting one time near tire ex- 
tremity of one of the screens, left a fine thin 
fragment of it towards the edge entire, which 
was so very weak, that it appeared difficult 
to handle it without breaking. These things 
thus prepared, five shot were made with the 
piece rested in the notch described above ; 
and the horizontal distances between the first 
shot, which w as taken as a standard, and the 
tour succeeding ones, both on the first and 
second screen, and on the wall, measured iu 
inches, were as follows : 
“ But if the constant and invariable po- 
sition of the notch or socket, in which the 
piece was placed, is thought loo hard an 
hypothesis in this very nice affair ; the third 
method, and which is the simplest of all, re- 
quires no more than, that two shot be fired 
through three screens, without any regard 
to the position of the piece each time. For, 
in this case, if the shots diverge from each 
other, and both keep to a vertical plane, then 
if the horizontal distances of their traces on 
the first screen be taken from the like hori- 
zontal distances on the second and third, the 
“As all projectiles in their flight are acted two remainders will be in the same propor- 
1 to 2 
3 
4 
1st screen 
1,75 R 
10, L 
1,25 L 
2,15 L 
2d screen 
3,15 11 
1 5,6 L 
4,5 L 
5,1 L 
wall 
16,7 R 
69,25 L 
15.0 L 
19.0 L 
on by the power of gravity, the deflection of 
a bullet from its primary direction, supposes 
that def action to be upwards or downwards 
in a vertical plane; because, in tue vertical 
plane, the action of gravity is compounded 
and entangled with the deflective force. 
And for this reason, my experiments have 
been principally directed to the examination 
tion with the distances of the second and 
third screen from the first. And it they are 
not in this proportion, then it will be certain, 
that one ol them at least hath been deflected 
from the vertical plane; though here, in 
the last instance, the quantity of that deflec- 
tion in 'each will not be know n. 
All these three methods I have myself 
tpf that deflection, which carries the bullet to 1 made use ol at different tunes, and hav e e\ ei 
“ Here the letters R and L denote, that 
(he shot in question went either to the right 
or left of the first. 
“ If the position of the socket in which the 
piece was placed, be supposed fixed (and I 
presume no person then present conceived, 
during these trials, that it could possibly vary 
the tenth of an inch from its first situation), 
then the horizontal distances, measured 
above on the first and second screen, and on 
the wail, ought to be in the proportion of 
the distances of die 1st screen, the 2d screen, 
and the wall, from the socket. But, by only 
looking over these numbers, it appears, that- 
none of them are in that proportion ; the 
horizontal distance of the 1st and 3d (for in- 
stance) on the wall being above nine inches 
more than it should be by this analogy. 
“If without supposing the invariable po- 
sition of the socket, we examine the compa- 
rative horizontal distances according to the 
third method described above, we shall in 
this case discover divarications still more 
extraordinary. For by the numbers set 
down it appears, that the horizontal distances 
of the 2d and 3d shot on the two screens, and 
ori the wall, are as under : 
