500 A LETTER ON TERRESTRIAL REFRACTION AND MIRAGE. 
should be deducted, this being the correction considered in formula (3). 
It is perhaps worthy of note that, with a range of three miles, the 
dip is approximately 5-85 feet, and this is equal to about 1! 17”, or say, 
within a second or two of the maximum refraction given by Major 
MacMahon. The total amount of depression at this range—for very 
low batteries—would therefore apppear to be— 
l’ 18" for refraction 
1’ 17” for curvature 
2’ 35" = additional depression required on the gun. 
for a range of three miles, if the present method of calculating 
quadrant elevation is continued. 
It will be probably considered that a dip of ‘65 feet for one mile is 
too great, I rather think it is, but in the absence of anything reliable— 
no two authorities absolutely agreeing—lI have considered ‘65 feet as 
sufficiently near for the purpose. 
