TERRESTRIAL REFRACTION AND MIRAGE. 285: 
remarkable: a ship actually coming up on one bank of the lake, and 
being accompanied by a spectre ship on the other bank. 
In conclusion I will only say that I think the result of my investiga- 
tion into this matter as affecting artillery fire is distinctly reassuring. 
I think that under normal conditions we have nothing to fear, and I 
think that under abnormal conditions when we have to fire from the 
top of a cliff and are uncertain altogether of the distribution of density 
of the air between the cliff and the object, the difficulties can be 
entirely overcome by having horizontal instead of vertical base instru- 
ments, and by laying the guns with straight-edged sights and quadrant 
elevation instead of tangent sights. 
DISCUSSION. 
Tue CHatrman—lI suppose we all know more about this subject now than we 
did an hour ago, and I am sure we are very grateful to Major MacMahon for what 
he has told us and the way he has told it. I dare say some of us have experiences 
to bring forward, but I think it is very gratifying to know that the Battery Com- 
mander of either Field or Garrison Artillery has not to make yet another correction 
to those he is already saddled with, and that we need not as artillerymen look 
upon refraction as a serious complication. I do not know whether any gentleman 
present will give us the benefit of their experiehces in the same line. Perhaps, 
Professor Boys whom we have the pleasure of welcoming will kindly do so. 
Proressor C. V. Boys, F.R.8.—Colonel Trench and gentlemen, I need hardly 
say how much pleasure it has given me to hear this brilliant lecture by Major 
MacMahon. ‘The first part of the lecture which dealt with the theoretical branch 
was given with a degree of clearness and precision which I wish we could more 
often meet with in London (applause). 
Major MacMahon was good enough to mention my name in connection with a 
mirage phenomenon which was observed in England. I think he made a slight 
mistake however; for he was told of it in my house the other day by a fellow 
guest, not by myself. The phenomenon was this: Some people were in a boat at 
Oxford on the Cherwell on a very hot day Jast year or the year before, when the 
grass in the flat meadowland was exceptionally heated and was producing the 
familiar rippling appearance. A lady in the boat, in the ordinary way seeing 
nothing peculiar, noticed that on dipping her head almost to the level of the grass 
—within an inch or two—the cows in the field appeared to rise up in the air 
while their legs drew out to an enormous length so as to reach the ground. The 
rest of the party saw the same thing when they put their eyes down within a few 
inches of the ground, but on raising them the cows became ordinary cows again. 
The phenomenon to which Major MacMahon referred as seen over the Maxim 
gun—the apparent cutting out of the top of the target—was described to me some 
time ago I think by Professor Greenhill, who as he is here, will correct me if I 
am wrong. ‘The question arose whether it was not possibly due to those conical” 
shells of compressed air which surround the bullets when their speed is greater 
than that of sound—photographs of which I had the honour of showing at this 
Institution some time ago. 1 do not quite see how those conical shells of air 
could produce the effect; but it must be remembered that if the target were 
a considerable distance away, and the gun were firing at the highest rate at which 
it can fire, some ten shots a second say, there might be several bullets between the 
observer and the target at the same time, and the eye being probably just about 
level with the top of the trajectory would be in such a position in fact as to cause 
the stream of bullets to produce the greatest possible effect, 
