498 A LETTER ON TERRESTRIAL REFRACTION AND MIRAGE. 
above the mean water level at battery, to give its height above an 
object three miles off; this is seen from Fig. “l, 
Fig. 1. 
BATTERY 
Ava ras = = 2, Ea 
j cA WA 
>] 
O:rWw 
tho t < |" 
ale S55 ( 
bl eal 
Git E|= 
| < { | EIN 
=|a 3 MILES \ mi: 
5 ——— —— = = {|b 
| wig 
U = i 3 o } >) 
ihe le 
[eo aT 
Pe 2 ale 
pANeO US 
= = 
<= 
Ee 
In the new calculation the dip due to the extreme fighting range 
of the gun could be allowed for, and the range would, of cour se, vary 
with the gun used, the limits being probably between one and threo 
miles; the amount to be added to the height of the battery would then 
be approximately as follows :— 
Extreme fighting range Feet to be added to 
of gun in miles. height of battery. 
il 65 
2 . 2°60 
Dae dee ete nem hen slot steetieatcostntnowces 5°85 
4, 10°40 
Dio ano a oadees Horloss Sepang anon sop mnneD 16°25 
The formula would then be— 
h+aA 
6=o— (7) 1146.. Facet stdatiadco de sasnese pee rome ce) 
Where 4 = additional height to be added to the height of the nate and 
corresponding to the dip at the extreme fighting range of ¢ gun. 
This formula, although probably better than the one at present in 
use, is not absolutely correct, because “4A” is taken as a constant, 
whereas it is really a variable varying with the range “ R.” 
As the varying dip for any range can easily be calculated and 
embodied in a general formula, I would therefore suggest the follow- 
ing :—No. 3 as the most satisfactory of the three, because the variation 
of dip due to any range, ‘‘ R” is taken into account. 
he 65 R 
— (G+ aT DM Bn Alene pens AO) 
6, @ and / are in the same vans as tani (QQ) 
this is equal to— 
6=$— (i + -0000002098 rn) MB. onan 
Working out the examples given on p. 181, Part III., Garrison 
Artillery, according to this formula, the amount to be deducted is 23:4 
minutes, or ‘48 minutes more than given by the old formula (the book 
says 22 minutes deduction, this should obviously be 22°92 minutes). 
