288 TERRESTRIAL REFRACTION AND MIRAGE. 
view of military requirements. For instance it was a novelty to me to find 
refraction measured as so many minutes per mile; but the Lecturer made it quite 
clear why that mode of measurement was requisite for artillery purposes. When 
we look at the setting sun, and see it just touching the horizon, the sun is really 
well below the horizon; so that the effect of refraction is to raise the sun up toa 
distance of something more than its angular diameter; but in that case it would 
never do to measure the refraction in the military manner by dividing that 
amount by some 90 millions of miles, the distance between us and the sun. 
I had forgotten the conversation that Professor Boys alluded to, but we are 
very much indebted to him for his beautiful photographs, and for directing our 
attention to the instantaneous mirage which accompanies the bullet in its flight. 
This can now be seen, I am informed, when the bullet is flying parallel to a line 
of palings or towards a clump of trees. It is very noticeable I am told when the 
bullet is watched through a telescope pointed on the target. 
Lreut.-Cotonen H. 8. 8. Warkin, C.B., writes—I much regret that I was 
unable to be present at the interesting lecture on refraction by Major MacMahon, 
so I should be glad to be allowed to add a few remarks on its bearing on practical 
artillery fire, more especially as regards my particular subject range-finding. 
The ordinary refraction, and even the extraordinary refraction mentioned by the 
lecturer, as far as artillery fire either on sea or land is concerned is quite 
insignificant ; the variations of powder, or even of cordite being so much greater. 
But I can quite imagine that over the heated plains of India, especially when 
there is a variation of surface, the refraction caused by the numerous heated 
layers of air will give trouble in laying. This effect can easily be seen on looking 
at an object through a telescope over a hot water pipe or gas jet. The image 
moves about and is distorted, I believe owing to the varying refracted rays 
following one another so rapidly, that the image is seen blurred. I have noticed 
this at Okehampton and the marshes here, objects seen perfectly clearly when the 
sun was obscured by a cloud, were quite unrecognisable when the sun shone out 
and heated the layers of air close to the surface of the ground. I do not believe 
this blurring will take place, at all events to anything to the same extent over the 
sea when the surface is uniform. As mentioned by Lieut.-Colonel Jocelyn garrison 
artillery are not affected, as elevation is almost entirely given by mechanical 
appliances. 
Now as regards the effect of refraction on depression range-finding ; it does 
not seem to be generally known that the D.R.I’. is corrected for normal refraction. 
‘The amount I took from the text-books on surveying at ,% that of curvature. 
That being the case the only errors arising from this cause, would be the difference 
between the refraction allowed for and the actual refraction at the time an obser- 
vation was taken. ‘This as seen from the figures given in the lecture would be 
very small. But abroad as hinted by Major MacMahon possibly this error might 
be of some consequence. Are the results obtained by the D.R.F. therefore to be 
considered unreliable? I think not, for this simple reason that every time the 
instrument is set up, this very error is discounted, by laying on the datum point. 
The datum post as you all know is placed at a considerable known distance from 
the battery ; now whatever effect refraction (normal or abnormal) has when taking 
observations on to the target, it will equally have when setting on the datum post. 
In this way I believe the effect of refraction is practically discounted. 
THe CuarrmMan—lIf there is no other gentleman who desires to join in the dis- 
cussion it only remains for me to thank Major MacMahon in your name for his very 
able lecture, and at the same time those gentlemen who have made remarks on the 
lecture which have been most interesting. As Colonel Jocelyn says, the subject 
must not be allowed to drop, but must be threshed out, and we shall have great 
pleasure to help in doing so. In your name I beg to thank Major MacMahon for 
his very brilliant interesting lecture (applause). 
