THE EOYAL AETILLEEY INSTITUTION. 
258 
common centre A, and conversely if tlie arcs be moved in tlie direction of 
the circumferences of the two concentric circles, the point A will remain 
fixed. I will endeavour to illustrate this by means of a cardboard template 
I have had prepared of a 68-pr. gun. The axis of the piece coincides with 
the line AB, the point A being in the plane of the muzzle, and the lines on 
the arcs attached to the gun coinciding with the circumferences of the con¬ 
centric circles. Now, if I put a pivot in the point A and move the gun 
through several degrees of elevation and depression, you will observe that 
the arcs attached to the gun invariably move in the direction of the 
circumferences of the concentric circles. It therefore struck me, that if I 
forced these arcs to move in the direction of the circumferences of circles 
due to their common centre by means of the concentric guide-pieces here 
shown, I should be able to dispense with the pivot at A, and the point A 
would remain fixed, and so the gun is pivoted at the muzzle. That, 
gentlemen, is the theory of my principle of muzzle-pivoting. Now to apply 
it to practice. 
The woodwork of the carriage is put together in the usual way (see 
Tig. 6). The brackets are of 7 ft. teak, 8 ft. 6 in. high, 7 ft. 2 in. long, with 
a distance between them of 2 ft. 7 in. The transom is teak and the blocks 
sabicu. On reference to the diagram of the inside of the bracket (see 
Tig. 7), it will be seen that the concentric circles are represented by gun- 
metal sockets and the arcs are wrought-iron, 6 in. wide and 2 in. thick, 
toothed in front and made accurately to fit the sockets; of course the centre 
from which these arcs and sockets are described is at the muzzle of the gun. 
The sockets are cast with a pinion box and bearing for a shaft to go through 
to the outside of the bracket. 
As the front set of arcs have a smaller radius than the hind set, they 
would have to move through a shorter space than the latter when the gun 
is raised or lowered. This proportion is regulated by the diameter and 
number of teeth in pinions which work the arcs. In this case the proportion 
is as 5 to 8; consequently the front set of pinions have a diameter of 5 in. 
and 10 teeth, and the hind set have a diameter of 8 in. and 16 teeth. The 
front set of pinions are keyed on a shaft, which goes through both brackets 
in front of the transom. On the outer ends of this shaft are worm-wheels 
16 in. in diameter; the hind set of pinions are not connected by a shaft, but 
are each attached to a worm-wheel outside the bracket by a short shaft that 
works in the gun-metal bearing before alluded to. 
The worm-wheels are worked by a shaft passing under the hind wheels 
and over the fore. This shaft has a right-handed screw worm for the hind 
wheel, and a corresponding left-hand screw worm for the front wheel. 
These shafts are turned by a cast-iron hand wheel at each end two feet in 
diameter. 
The whole of this machinery is geared together by the shaft that connects 
the two front pinions. A compressor is fitted to each arc, which, when set 
up, takes the strain off the pinions. The degrees of elevation are marked 
on one of the worm-wheels, and the degrees of depression on one of the 
hind arcs. 
With a power of 30 lbs. applied to each hand wheel, a weight of about 
22,400 lbs. can be raised ; of course we must deduct the friction, which in 
