gent of an angle of 200 mils (11 degrees, 15 minutes) is 199/1000. No greater error is made if we take 200/1000. The angle of site could similarly be easily put in tangent form by dividing the mils by 1000. The angle (b), being an angle of slope, could easily be measured in tangent form, as, for example, a slope of 1 on 20 from the guns to the top of the mask. Thus the three angles involved are readily put in the form of tangents, with a common denominator of 1000. If the reconnaissance officer carried a clinometer graduated in mils the problem would be simplified, and he would, moreover, be able to read angles of site far more readily and accurately than by any instrument at present issued.

How would this method of determining the practicability of a mask work out in the field? Let us imagine the typical case of an officer sent out to find a defiladed position to fire on a given target. He finds several positions, each more or less fulfilling the requirements of defilade, good range, accessibility, etc. He decides to examine them in detail, beginning with the one that seems the most promising. This position offers a flash defilade on the reverse slope of a slightly convex crest. He wishes to determine whether or not the guns can clear the crest. He carries a clinometer graduated in mils, and, on the case of the clinometer, a table showing the elevations in mils due to range. He first goes to the top of the crest and there verifies his first estimate of the range (he would have to do this in any case). Looking at his table he finds the angle 130 mils opposite his determined range of 4000 yards. With his clinometer he measures the angle of site to the target-minus 5 mils. He corrects this for the position of the guns 4 yards below and about 30 yards to the rear-say minus 4 mils. Going to the proposed position of the guns he holds his clinometer at the height of a gun above the ground and measures the angle to the top of the crest-120 mils. Then

130/1000-4/1000 = 126/1000, and is greater than 120/1000. He sees that the guns would just clear the crest

for safety they had better be moved a little nearer.

He also is able to give the battery or battalion commander the correct range and angle of site.

It will be observed that unless the angle of site of the target is greatly above or below the normal the officer will make no appreciable mistake if he does not stop to correct the angle of site measured from the top of the crest.

Dead Space. It must not be thought that the sole consideration in taking up a covered position is whether or not the projectile will clear the mask. The flat trajectory of a modern field gun imposes certain limitations on it, which must be borne in mind when selecting a covered position. The angle of elevation of the 3" gun is only about 3° for 2,000 yards and 5° for 3,000 yards. Therefore, if the covered position be such as to require 3 degrees of elevation to clear the mask, the whole of the foreground within 2,000 yards of the guns will be dead space. On the defensive, then, the minimum of cover necessary to screen the guns from hostile view will have to suffice, since the guns exist to repel the enemy and not primarily to be hidden by cover.

The sector of fire assigned the guns, and the character of the included terrain, will generally determine whether a position on the rear crest of the mask or one well back therefrom will be taken up. If the position of the enemy is near the forward face of the mask the position of the attacking guns will necessarily be well away from the mask. But if the position to be attacked is well in advance of the mask it can be reached by the fire of guns either just in rear of the mask or well back therefrom. The former is generally the better position since the range is shorter, the fire therefore more effective, and since the guns can be run up to the "rear crest" position if direct fire becomes necessary. Again, all short shots are ineffective as well as the "overs," whereas if the second position is taken the "shorts" may have a demoralizing effect since the majority of them will be visible.

The principal disadvantage of the more advanced position is that the enemy will be more apt to search the reverse slope of a likely mask than the terrain well back therefrom of which they can see no part, and without making a reconnaissance know nothing of, unless a detailed map of the country is at hand.

Danger Angle.-When the rear position is taken the crest of the mask must be kept clear for a distance equal to the

front of the battery, plus a distance on either side equal to that of the battery from the crest. This is necessary in order not to infringe on the "danger" angle of 45° from the muzzles of the flank guns with the true direction of fire; that is, if the battery is 100 yards in rear of the crest, it will require the crest to be kept clear of other troops and other guns for 300 yards; if 200 yards in rear, 500 yards of crest must be unoccupied by friendly troops. Hence, the battery commander cannot use the logical position from which to make his observations without moving far to the flank; the best station being, of course, on the nearest crest from which the enemy is visible beyond the mask.

The entire danger angle is 90° or 1,600 mils of crest. It is found in practice that wild shots will frequently burst in this sector. The width of the sector is determined by errors in laying, and its depth by premature bursts due to error of fuze. In other words, a man standing anywhere in the angle AGB (Figure 8) is apt to be killed by the fire from his own guns at G, although the target is at T.