A LETTER ON TERRESTRIAL REFRACTION AND MIRAGE. 499 
Take another range, say 4000 yards— 
the old formula gives 11°46 minutes deduction 
» lew oy) ”? 12°43 »” oe) 
————— 
a difference of °97 minutes. 
This difference, although small, is a correction, and as one formula is 
worked as easily as another, the modification is probably worth con- 
sideration. 
The D.R.F. is corrected for this curvature, and therefore it would 
appear that the quadrant elevation of the gun should also be subjected 
to a similar correction, that it has been ignored i is Shewn by the manner 
the formula (1) is deduced :— 
Fic. 2. 
io) Sane [RB ATHTICR Yee com? Mn ts Lee ee, 
A eee ay i 
Wao \ 
ee 
Wy = angle of depression in minutes. 
h = height of battery in feet. 
t= range in yards. 
h 
then 3: 2aR :: Y : 360,60 (approximately). 
__ 360 Xx a xh 3600 4 
Dar lb Tita) el 
3600 
Bw yas, 1146 
{ = amount to be deducted in minutes from he range table elevation to ) 
(give quadrant elevation, as per formula—(1). 
‘This proves, conclusively, that this formula does not consider cur- 
vature. 
In Fig. 8 the effect of curvature is shewn; instead, therefore, of yy 
Fie. 3. 
being substracted from the range table elevation, the larger angle 8 
