324 MESSRS. R. H. FOWLER, E. G. GALLOP, C. N. H. LOCK AND H. W. RICHMOND: 
type I., the value of B is 145*7 lb. (inch) 3 . Neglecting the effect of the wood, suppose 
that the lead plug is x inches further forward. In such a case 
(2.21) B = 145*7 + 1*9£C 2 . 
If, moreover, l is the distance of the centre of gravity from the base in inches, then 
(2.22) l = 4*727 + 0*117;c. 
The altered position of the centre of gravity can therefore be recovered by 
calculation, if the altered value of B can be deduced from the observations. This is, 
in fact, the case (see § 4.1), so that even for shells of type II. the dynamical constants 
of the shells after firing are satisfactorily certain. 
§ 2.3. Possible Disturbing Factors. 
There are two further possible causes of error which we have not yet mentioned. 
These are (l) the wind, and (2) the impulsive action between the shell and the card. 
2.31. The Effect of Wind. —-Since we are studying the motion of the shell under the 
force system impressed by the air, we are concerned solely with the motion of the 
projectile relative to the air, but we can only observe, by means of jump cards, the 
motion of the projectile relative to the ground. 
If the strength and the direction of the wind are known, it is an easy matter to 
convert the observed values of the size and orientation of the yaw, and the observed 
motion of the centre of gravity, into the corresponding quantities for the motion 
relative to the air. It is, however, very difficult to determine what is the strength of 
the wind, at the moment of firing, only a few feet above the ground. It is, therefore, 
necessary to carry out jump card trials in calm weather. During the experiments 
the wind exceeded 10 f.s., only at the moments of firing three rounds, and was usually 
only 5 or 6 f.s. at 20 feet above the ground. Its strength near the ground will have 
been still less, and its effects may therefore be neglected. 
2.32. The Impulsive Action between the Shell and the Card. —When the experiments 
were started it was not expected that the effect of the cards would be decidedly 
bigger than the probable random errors of the results. This, however, appears to be 
the case. A limited amount of evidence, for determining the necessary correction, is 
supplied by the few comparative rounds fired without cards on the nearer screens. 
Such comparative rounds would have been included in all, or at least the majority, of 
the groups, if their importance had been realised earlier. The evidence supplied by 
the comparative rounds was carefully analysed, and was supplemented, after the 
conclusion of the trial, by determination of the magnitude of the impulse between 
the cards and the shells by observation of the extra loss of velocity so caused. The 
