TERRESTRIAL REFRACTION AND MIRAGE. 287 
Coast gunner as a rule, would fire percussion common, at a comparatively small 
target, and, being provided with a fixed platform and accurate instruments, every 
25 yards is worth considering. 
I think Sir we ought not to let this matter stay where it is, but that we should 
investigate, whether, in tropical places any correction is due for refraction or not. 
This, 1 think, could be done, by taking a certain number of observations, with 
the instruments that we have now, at the datum point; and if records were kept 
of these observations, we might form some sort of conclusion, as to the effect of 
refraction. I hope this effect is negligible, but it is no use blinking the matter, 
if it ought to be taken into account. 
With regard to what Major MacMahon said of the quadrant elevation, garrison 
artillery would always use it unless they were driven to use tangent sights. 
I should like to mention one case of refraction that was told us in the Channel 
Islands, in July last. There is a well defined rock, I regret I have forgot- 
ten its name, which was known to be in a certain direction. One day, as seen 
from Elizabeth Castle, it appeared to be, I am afraid to say, how many degrees to 
the right or left; [ would suggest that reference should be made to Major H. A. 
Scott, R.A., who observed the phenomenon, because, after the experiences that 
Major MacMahon has given us I think that that case, of lateral refraction, if it 
can be substantiated, might be very interesting to record. 
Ligvr.-Cotonni R. W. RatnsrorD-Hannay—lI have some diffidence in saying 
anything after the very scientific lecture that we have had, and after the remarks 
of Professor Boys; but it might interest you to know something of the practical 
experience of mirage at Lydd which has been mentioned once or twice. At Lydd 
there is at times a good deal of mirage. I tried the experiment of having a gun 
laid on a target at a time of day when the mirage was at its worst. The gun was 
laid by clinometer and the training and height of the tangent scales were marked. 
The gun was afterwards laid on the same target with the same elevation and 
training when there was no mirage and there was no apparent difference. ‘That is 
explained by the fact, as Major MacMahon told us, that the extent of refraction 
at home is not more than 1’ 16". But there are other things with regard to mir- 
age which make it exceedingly hard to lay when there is much of it. At Lydd 
last year three field batteries were practising ; they began at 6.30 in the morning, 
and the mirage increases gradually from morning to mid-day. The battery that 
shot best was the battery that shot when there was the least mirage, while the 
battery that shot worst was the battery that shot when there was the most mirage. 
Another thing about mirage is that the stronger the telescope the more exagger- 
ated is the mirage. The dummies appear like ghosts, a battery using Scott’s 
sights can scarcely see them at all, much less lay on them. 
With regard to the experience of Captain Osborn I was myself at Lydd at the 
tine. The range, so far as I recollect, was about 600 yards, and Captain Osborn 
was trying the Maxim gun and firing as fast as possible. I should say that the 
height of the eye above the ground was about 34 feet. 
Cotoner R. D. HE. Locknart—Colonel Trench and gentlemen, it must be very 
satisfactory to the artillery to know that they need not be anxious about the effect 
of mirage as regards the laying of field guns; but there is no doubt that it is 
disturbing as Colonel Rainsford-Hannay remarks, to see these images, and it 
disarranges the firing very much, But another thing which is a practical 
ditheulty is the rippling of the atmosphere, whether it is to us as gunners upon 
the plains of Hindostan, or as sportsmen on the highlands of Thibet, so that 
although there may not be any important obstacle before us there are undoubtedly 
difficulties that arise in connection with these atmospheric disturbances. 
Prorzssor A. G. GREENHILL, F.R.S.—I feel some diffidence in joining in the 
discussion on this interesting lecture, more especially as my experience of the 
subject arises only from the astronomical applications, and not from the point of 
