AUTOMATIC SIGHTING. 
271 
At 50 feet the depression angle for 1,000 yards is 57* -3. minutes. 
„ „ „ J.025 „ 55-9. 
„ „ „ 5,000 „ 11-46. 
„ ), is 5,025 „ 11’40. 3} 
That is to say to observe to an accuracy of 25 yards an angle of 1*4 
minutes must be taken at 1,000 yards, but at 5,000 yards an angle 
observation of 0*16 minutes is required. 
At 200 feet the depression angle for 1,000 yards is 8° 49. minutes. 
„ „ „ 1,025 „ 3° 43 
,, „ „ 5,000 „ 0°. 45-84 
,, „ „ 5,025 „ 0°. 45-61 ,, 
That is a difference of 6 minutes, only alters the range 25 
yards at 1,000 yards, but at 5,000 yards an error of 023 minutes, 
will cause a same alteration of range. 
These examples show what small angles have to be observed when 
the height of the battery is small, and that far more accuracy is 
required at long ranges than short ones. A good rough rule for 
obtaining the errors at various ranges, is to multiply the error at 
1,000 yards by the square of the range. Thus, supposing the error 
due to laying be 20 yards at 1,000 yards, the error from the same 
cause at 2,000 yards would be 20 x 4 = 80 yards, at 3,000 yards it 
would be 20 x 9 = 180 yards and so on. The error due to height 
for any given range is very nearly inversely proportional to the 
height. Thus, for example, if an error of 20 yards is obtained at 
150 feet height, the error from the same cause would be three times 
twenty at a height of 50 feet. 
The errors we have been discussing are those due to actual laying, 
the important question then is with what accuracy can the human eye 
observe an angle. I have had considerable experience in this matter, 
and I feel convinced that the limit for deliberate laying is one minute, 
and a greater angle if the eyesight is not good, or properly trained. 
But where as in automatic sighting the observer has to follow a 
rapidly moving object, this limit must be at least doubled. 
Granting this, it is easy to lay down the error that will be obtained 
with an automatic sight, from this cause, at any given range and 
height. The following is a table assuming the error to be two 
minutes:— 
Range. Yards. 
Height. 
Beet. 
1000 
2000 
3000 
4000 
5000 
50 
36 
149 
351 
651 
1058 
100 
18 
72 
160 
305 
478 
150 
12 
47 
109 
196 
308 
