497 
Je ie ASH AR! Ase) dew, 
ON 
TERRESTRIAL REFRACTION AND MIRAGE, 
BY 
LIEUTENANT F, BROWN, I.0.M., R.A. 
Tn lecture delivered by Major MacMahon, R.A., F.R.S. on “ Ter- 
restrial Refraction and Mirage” I have read with much pleasure, 
more particularly perhaps, as [ happened to be serving in Rangoon 
during the same period as Major Barlow, R.A. and, therefore, well 
remember the interest this question of refraction and mirage occasioned 
in that command. 
Since the object of the lecture was to investigate whether any 
correction in elevation should be given to guns to counteract the 
influences of refraction, and if so, how much, it has occurred to me 
that it might not be out of place for me to submit to the Institution a 
correction for quadrant elevation which appeared to me some few 
months ago as desirable. The correction is definite, though small, 
and possesses the merit of not fluctuating with the varying atmospheric 
conditions. 
It is as follows :— 
The formula at present in use for calculating the quadrant elevation 
of guns in elevated batteries is— 
i 
= ipl Cpeateatee meeeee 
where 6 = quadrant elevation in minutes. 
»» = angle of elevation in minutes as per range table, 
» = height of battery in feet. 
» =range in yards. 
Now this formula does not take into account the curvature of the 
earth, and moreover, the mean water level at the battery itself appears 
to be just the place where it is of the smallest importance. Of course 
this height must be known before the corrected height can be obtained, 
but it is not, in my opinion, the proper altitude to use in the calculation. 
That curvature should be taken into account must be admitted, be- 
cause the height of the gun above the object aimed at will vary with 
the range, or distance of the object. With a range of 5000 yards, or 
say three miles, the dip due to curvature amounts to about 70 inches, 
and this amount must obviously be added to the height of the gun 
IQ, WOlt, seguir, 
sects NS epee el 
