226

OF PROJECTILES.

4. As a difference in the quality of the powder,in the moistuie and temperature of the atmosphere,considerably affects the ranges, they should be taughtto make allowance for these circumstances, by ele-vating or depressing the mortars ; by increasing or de -creasing the charge.

5. Thev should fire at a butt with the degree ofelevation that gives the longest range, and increase ordiminish the charge till they strike the object : this willconvince them that less irregularity ensues from alter-ing the charge, than from altering the elevation; forthe more the elevation deviates from that which givesthe longest range, generally between 40° and 50%the more irregular are the ranges.

6. When the object is to fire at troops or enfiladeworks, it is better to lay the mortars at small elevations,that the shells may not bury themselves.

7. When the plane of the mortar battery is belowthe plane of the object, it is much easier to project theshells with justness, than when they are both on thesame, or the mortar on the higher plane.

234. The second case (232) viz. to break through case-mated buildings, requires much theoretical knowledge in theofficer charged with the execution of this piece of service,in order to determine the situation of the mortar, its pro-per charge and elevation ; that the shell may impinge onthe object with the greatest possible force. Suppose a shellprojected from the point A, in the direction A P (PI. 6, Fig.22) has described a curve A F N B L of the fourth kind ;in order to determine the force that the projectile has ineach point of this curve, the direction and quantity of com-pound velocity at each point must be found. For this pur-pose it is necessary to have a scale of the spaces pasted throughin times of unequal movement (170, 172) from whencemay be deduced the scale of corresponding velocities : then,to ascertain the direction and quantity of compound velocityat the point B, in the perpendicular line B P, make B E equalto the corresponding velocity at this point, of a movementB P unequally accelerated by gravity ; draw E H parallel toA P, and equal to the velocity which at the same point Bcorresponds to the retarded movement of impulsion AP ;then the right line H B will express the direction and quan-tity