TERRESTRIAL REFRACTION AND MIRAGE. DS 
value of about 24", and a maximum value of about 8”; and taking a 
range of 3 miles these values of the refraction would become 12", about 
8, and abont 24" respectively ; and very exceptionally the refraction 
would amount to about 1’ 18". The meaning of this is that if we had 
a range of 3 miles (we will take the exceptional case of refraction of 
1’ 18") and if we were to lay tangent sights upon an object at that 
distance, a straight line joining the sights would not pass through the 
point aimed at but would have to be depressed through an angle of 
1’ 18" in order to do so. The curvature of the ray in fact would result 
in the straight line joining the sights passing over the object, and we 
would have to depress the gun in order to counteract the effect of re- 
fraction. As this correction only amounts to lL’ 18” it is not a matter 
of great importance. 
There is of course another question besides that of laying a gun by 
means of tangent sights upon an object; I refer to the use of the de- 
pression range-finder. If we take the range of an object by means of 
this instrument the result will be that the apparent angle of depression 
will be less than the real angle of depression, and accordingly the de- 
pression range-finder would lead to a slight over estimation of the 
range, and the range as found would have to be diminished in order to 
get the true range. Of course in the present case with which we are 
concerned, namely a range of three miles with 1’ 18" of refraction, that 
would not be a matter of great importance; but if it were necessary 
absolutely to eliminate refraction for all practical purposes (and it might 
be necessary out of Great Britain in tropical climates where the refrac 
tion was very much greater than that I have specified) the 
proper plan would seem “to be to employ not a vertical base range- 
finder, but one with a horizontal base, because lateral refraction | 18 
almost non- existent ; there are one or two examples of lateral refraction 
that I will mention presently, but itis almost non-existent. If we were 
to take our range by means of an horizontal base range-finder, and 
then to lay the gun not by means of tangent sights but by means of 
straight-edged sights and quadrant elevation, then, for all practical 
purposes, we would I think eliminate refraction even if the refraction 
were very much greater than in any case have mentioned ; and it is 
for that reason that I do not think this subject of refraction need cause 
us very much apprehension. ‘The general effect of refraction in Great 
Britain is to cause a slight over estimation of the range, and, in the case 
of taking the angle of elevation of the object, is to cause the object to 
appear rather higher than it really is; but that is not a matter which 
very much affects us in artillery fire. 
One effect of refraction which I will just mention (it does not concern 
us in artillery fire) is the recession of the visible horizon. Taking a 
point 4 on the surface of the earth (see Fig. 4) just outside the surface, 
Fie. 4. 
