THE ROYAL ARTILLERY INSTITUTION. 
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ordinary cases affect the traction, providing that the relative position 
of the centre of gravity of the load as to the axles remains unaltered. 
2. Stability .—“ Mobility ” in a field artillery carriage would not, so 
to speak, be perfect, unless accompanied by “ stability,” which point 
has, therefore, next to be considered. Stability is influenced by the 
number of points on which the carriage rests, and by the vertical and 
horizontal position of the centre of gravity with regard to those points. 
The carriage being supposed on the level, the first mentioned distance 
should be as small as possible, and the second as great as possible 
(which latter, with convenience of width, governs the track), to yield 
maximum stability. This will be readily understood by supposing a 
carriage placed upon the level, and a vertical drawn through the centre 
of gravity, then the carriage tilted over through a certain angle; when 
it will be seen that the higher the position of the centre of gravity, the 
greater the distance through which it has moved, and therefore the 
more nearly will the vertical then drawn through it be to falling outside 
the bearings on the ground. Again, with regard to the horizontal 
position of the centre of gravity from the bearing points, it is evident 
that the further it is from them, the greater moment the weight, acting 
at that centre, has to resist any force tending to overturn the carriage. 
Stability in wheeling or reversing, will further be influenced (setting 
aside the weight of the carriage, already fixed at a minimum by the 
consideration of mobility) by the height and mode of connection of the 
fore and hind carriages, as well as by the height of the attachment of 
the traces to the fore carriage. 
That the carriage may be stable in any position of rest or of motion 
to the front, it is simply necessary that the vertical through the centre 
of gravity should fall within the figure formed by joining with straight 
lines the points upon which it rests. In field artillery carriages, the 
stability is considered sufficient when the upsetting angle for the carriage, 
packed, is about 35°; that is to say, the vertical falls outside the points 
on which the wheels rest, and the carriage overturns, when it stands 
upon a side incline exceeding 35°. 
3. Strength andDurability .—The next points to be taken into 
consideration in the building of the carriage, are “ strength” and 
“ durability.” The material used should be the strongest consistent 
with lightness; .the scantling of each particular part being, in the same 
view, at a minimum consistent with the stress which the part may be 
called upon to bear. The material should also be such as will stand well 
the effect of shot striking it, the action of climate, &c., and should not 
be liable to deteriorate when kept in store. 
Of late years, wrought-iron has very much superseded wood as a 
material for carriages, being much more durable. For instance, a shot 
striking a bracket of an iron gun-carriage will make a clean hole 
through it, without splintering and damaging the adjacent parts; 
whereas a shot striking the bracket or trail of a wooden gun-carriage 
will splinter it, more or less. Notwithstanding, comparing iron with 
English oak, it is but slightly heavier for the same strength; because, 
from its nature, its mass can be better disposed to withstand any given 
