130 TRANSACTIONS OF THE ROYAL CANADIAN INSTITUTE  [VOL. XI. 
tubes were introduced of simple construction, invented by Marié Davy 
in 1854 and gradually perfected. In its simplest form the optical tube 
had a mirror at each end inclined at 45° to the axis. The tube, being 
fitted watertight in the top of the boat, projected a foot or two above 
water when the boat was immersed to a certain depth and thus a view 
of a part of the horizon might be obtained, but the angle of vision was 
only one or two degrees, and the image was very imperfect. The mirrors 
were replaced by prisms, lenses were introduced and during the “‘nineties’’ 
several improvements were made, but not till about ten years ago 
was any serious progress made. Then in a few years the optical tube, 
or the ‘‘periscope’’ as it is now usually called, was developed to a high 
degree of perfection, enabling the submarine boat to perform attacks 
without showing anything but the top of the periscope occasionally 
above water, at the same time obtaining a perfect view of the enemy. 
The periscope was the last link in the chain of inventions which were 
needed to give to the submarine boat positive military value. The 
improvements in the periscope comprise a larger field of vision spanning 
an arc of more than 50°, as large as or greater than that of the human eye, 
convenience of observation, and the addition of means for measuring 
distances and indicating directions. The magnification of the object 
is only about 1.5, which is found to give to the observer the same im- 
pression as when using the naked eye. By using the utmost refine- 
ments of optical art and science a perfect image of unsurpassed clearness 
and distinction is obtained. Mechanical power is used for handling 
the tube, enabling it to be pushed up and down readily and quickly 
and to be turned round its axis. The length of modern tubes is up to 
25 ft. with a diameter of about 6 in. The head of the tube projects 
from 15 to 22 ft. above the superstructure. Difficulties still exist due 
to the vibration of the boat and due to spray on the front glass, but 
they are of secondary importance. Instruments have been constructed 
by which an all-round view of the horizon can be obtained without 
turning the tube, but have not proved quite satisfactory. 
VARIATIONS IN BUOYANCY. 
In order to go from the light to the submerged condition and vice 
versa it is necessary to admit or to discharge water. Main tanks of great 
capacity must therefore be found provided with large sea-valves and 
powerful pumps. Means are provided also by which the water may 
be forced out of the tanks by admitting compressed air. Ordinarily 
the air is not admitted till the boat is on the surface, where only low 
pressure air is required, but a sufficient store of highly compressed air 
is carried by means of which the tanks can be emptied very quickly 
