THE ROYAL ARTILLERY INSTITUTION. 
83 
applicable, the trajectory would simply have to be raised by half the 
height of the 50 per cent, vertical spread in order to further utilise 
25 per cent, of the lower half of the set of trajectories, and the mean 
point of impact would then coincide with the intended one. In default 
of direct observation we must remain in ignorance as to whether- or 
not the elevation so arrived at is correct, or whether the charge calcu¬ 
lated according to part 7 should or should not be altered. 
The characteristic signs, as ascertained by experiment, which partly 
take the place of direct observation, and which render possible a correc¬ 
tion of the laying, are essentially as follows :— 
When a shell strikes sound masonry there appear, immediately after 
the impact, a radiating shower of stone or brick splinters and dust. 
The smoke usually rises in a spherical or wreathed form, and is, in 
general—especially when lighted up by the sun—a decided red if the 
scarp is of brick, and yellow or dark grey if it is built of stone. In 
damp weather or rain it is difficult to decide whether the rising dust is 
from earth or masonry. In the case of wet ditches the water that is 
thrown up shows that the surface has been struck. 
The smoke appears above the intervening crest very soon after the 
explosion. We can nevertheless distinguish, by the greater or less 
interval which elapses* before the cloud rises, whether the shot has struck 
high up or near the foot. 
When masonry is struck the sound of the explosion is clear and hard. 
If the shots impinge on brickwork that has been already damaged, the 
cloud of smoke is as before coloured red, and the shower of brick 
splinters may be looked for; but the smoke appears later, and rises 
more slowly and in a less compact shape. The deeper the scarp gets 
cut into the more clearly can the above-mentioned signs be recognised, 
and when it has been quite broken through the smoke appears a dark 
grey colour. 
These signs make their appearance in various degrees, and are often 
difficult to distinguish. It is therefore, in every case, very desirable 
to obtain direct observation of the effects of the firing from the very 
first. This is, moreover, absolutely necessary in order to be able to 
judge with sufficient certainty of the instant when the horizontal and 
vertical cuts have respectively penetrated right through the scarp, and, 
afterwards, when the breach is practicable. Lastly, direct observation 
is indispensable for the completion of the breach when there are 
counter-forts or counter-arches. 
Direct observation may be dispensed with when demolishing reduits , 
cavonnieres, batardeaux, &c., of which the profiles are known. 
If, in general, a tolerably practicable breach has to be effected, a 
favourable point of view somewhere near the breach must be made use 
of; and this presupposes that the glacis has already been crowned. 
Care must, however, be taken that the crowning does not extend to 
the part of the glacis which has to be fired over.* 
* At the breaching of Lunette No. 53, during the siege of Strasburg in 1870, it happened that 
on the night of the 14th-15th September, contrary to an agreement which had been made, the 
crowning of the glacis was extended to the part which had to be fired oyer, and the crest of the 
