416 
MINUTES OF PROCEEDINGS OF 
Hence this moment varies directly as the square of the breadth of the 
tire, directly as the tangent of the hollow of the arm, and inversely as the 
height of the wheel. 
It produces a continuous pressure on the back of the axletree-arm at the 
shoulder, and on its front near the point; these pressures being applied at 
the ends of the pipe-box. They vary directly as the moment, and their 
effect is to retard the movement of the carriage and to wear down the pipe- 
box or axletree-arm. Their amounts in each particular case can be readily 
found from the values of % and n, already given, provided [x be first ascer¬ 
tained. This can be done by trial, if not already known. 
If, then, on grounds of strength, a certain hollowing of the axletree-arm 
be unavoidable, we see at a glance, from the terms in which the value of the 
moment is expressed, how essential it is to confine the width of the tire 
within the lowest allowable limits; also that, besides the ordinary conditions 
of traction, showing clearly, as they do, the advantages conferred by high 
wheels as compared with low, an additional reason for their use is found 
when the axletree-arm is hollowed. With field artillery carriages, for¬ 
tunately, great height of wheel is admissible. Not so, however, by any 
means generally among the transport carriages of the service. And here, 
accordingly, it becomes an object of importance to reduce the breadth of the 
tire as far as other considerations will permit, in order to neutralise the 
excessive action of the grind. 
When, however, small wheels are connected with the fore-carriages of 
wagons having hind wheels of large diameter, other important considerations 
—such as the expediency of securing a uniform width of tire to make the 
track, and to simplify, as far as may be, the dimensions of material required 
for repairs—render it obligatory to make the width of the tire of the front, 
equal to that of the hind wheel. 
Now, it is clearly unnecessary to construct the front wheel of such a form 
as to withstand greater strains than those which the hind wheel is computed 
to bear effectively; because the chief portion of the load is always borne by 
the hind wheels in properly designed wagons of the type referred to here, 
and all the strains on the wheels are proportional to the weights they 
sustain. 
If, then, the front wheels are designed to bear equal strains with the 
hind wheels, they will be well within the limits of safety. 
Fig. 3. 
A brief inspection of Fig. 3 will show that the strength— i.e. } the capa¬ 
bility of resisting lateral thrusts—possessed by a dished wheel shod with 
