496 FLIGHT? AND FLYING MACHINES. 
and it might therefore betier be called a“ revolver.” Now, I feel very 
difident in saying anything about guns before the members of the 
Royal Artillery Institution, but I cannot help thinking, as the result of 
personal experience, that this photographic gun would be an excellent 
thing to put in the hands of our volunteers for class-firing, instead of the 
Lee-Metford rifle. If every volunteer, in order to fire his third-class, 
were required to take a certain number of “snap-shots” at the targets 
with this harmless camera, and if these photographs were properly 
developed under official supervision, a smaller number of deaths would 
annually occur from stray rifle-bullets, and a smaller number of 
volunteers would succeed in firing their third-class! M. Marey gives 
an excellent photograph of the path of a projectile going through the 
air, which well illustrates the advantages of instantaneous photography, 
You will see, that in order to illustrate the well-known propositions of 
our text-books, that the path of a projectile is a parabola, and so forth. 
the simplest way is to make a number of successive exposures on the 
same plate. ‘I'his photograph will explain the whole thing on a single 
diagram ; if carefully examined, it will show the horizontal distances 
between the successive positions are equal, and hence that the hori- 
zontal velocity remains practically constant. Thus the whole of what 
takes a considerable space to prove theoretically in our text-books may 
be very readily verified graphically. 
We will next examine what is the effect of taking a large number of 
successive images of a gull on the same plate. If the exposures are 
made at very short intervals, the result is a blurred jumble giving the 
appearance of a number of gulls all eating each other up. In order to 
separate the images, various methods have been suggested. One 
involves the use of a revolving mirror by which each image is reflected 
on to the plate a little in front of the preceding one, so that they no 
longer overlap. Ina photograph of the same gull, taken by a revolving 
mirror, the images are completely separated, and can be examined 
individually. In both this and the aforementioned blurred photograph 
there are 50 images per second. There is another method called the 
stroboscopic method, which I should like to mention, because it some- 
what resembles a method used by Professor Boys, in his highly success- 
ful lecture on ripples, to show waves travelling along the surface of 
liquid. If we consider the action of a wing in going down and up as 
constituting one complete stroke, we take the first photograph of the 
bird when the wing is (say) at the highest point ; we take the second 
photograph when the wing has made a little more than one complete 
stroke, i.e., when it has gone down and up, and has again gone down a 
little below the highest point; we take a third when the wing has 
made a little more than one complete stroke after the second, and has 
got a little further down still in its path and so forth, and when these 
_ photographs are developed we find that the succession of images 
simply represents the various phases in one complete stroke of the 
wing. Moreover, the images no longer overlap, because the bird has 
moved through the distance traversed in a complete stroke and rather 
more in the interval between two consecutive exposures. <A similar 
method might be applied to examine the motion of waves or ripples on 
