and on Wheel- Carriages. 199 
It has been frequently alleged, that the attraction of cohesion 
contributes to increase friction. But when both surfaces are of 
the same sort of metal, it is highly probable that particles or 
-crystals of the one body may often come into that particular po- 
sition relatively to those of the other body, as to enable them 
firmly to lay hold of each other; and on the event of the surfaces 
heing again separated by moving along, there may possibly be 
found some particles which have ‘deserted the surface to which 
they originally belonged, and gone over as it were to the enemy’s 
standard. The rapidity with which two pieces of iron mutually 
wear or rather grind each other, particularly after getting hot and 
free of grease, furnishes a strong presumption in favour of this 
hypothesis : possibly, even magnetism may claim some little share 
in the case referred to. The little coliesion which subsists be- 
tween the surfaces of different metals, is undoubtedly one reason 
for their having less friction than if the metals were of one sort. 
It is truly surprising, after all that has been said against the 
unfortunate and unscientific contrivance of bending down the ex- 
tremities of the axles of carriages, that this lamentable practice, 
so destructive of human lives, should still be continued, even in 
an age boasting so much of perfection. If, as some suppose, 
this favourite absurdity took its origin from the legislature’s li- 
miting the distance of the wheels below ; yet surely contracting 
the wheels beneath was only a mock obedience to the letter of the 
law, with a real and substantial breach of it, in spreading them 
above. For although a carriage had only one wheel in the mid- 
dle, if it occupied the same extent above as another carriage, 
where could be the mighty difference in making room on the 
road? 
The numerous disadvantages attending this long-censured con- 
trivance have all been more or less exposed ; and so I would just 
barely mention a few of the most prominent: viz. that such car- 
riages are extremely liable to be overturned, when one wheel goes 
into a rut or low side of the road; and that ‘this overloaded wheel, 
sustaining the whole carriage (if a two-wheeled one) in a diree- 
tion much inclined to the plane of its rim, is apt to be crushed 
altogether ; besides, a wheel constrained to move in such an awk- 
ward position experiences an enormous friction, both at the 
centre, and also where it grinds and twists in contact with the 
road. 
Many of the broad conical wheels of waggons are nothing short 
of a disgrace to the nation, There is a most provoking perse- 
verance in absurdity exhibited in their perverse construction. 
Some of these wheels are a foot broad on the edge ; and the dia- 
meter of one side a tenth part at least greater than that of the 
other; but still all pomts of the wheel must obviously revolve in 
N4 : the 
