200 On Friction in Machinery, tc. 
the same time; and of course, while one side alone could roll 
over ten miles, the other would roll over. eleven: from which it 
is clear, that while one part actually rolls, another must slide. 
Not satisfied, however, with this degree of absurdity, recourse is 
had to a most distinguished contrivance—that of fixing nails with 
gigantic heads projecting beyond the sliding surface; as if there 
were really a possibility of preventing the sliding altogether. 
Such a contrivance offers an affront to common sense and an in- 
sult to sober reason. yen in the case of narrow-edged wheels, 
these horrid nail-heads must be viewed as a great incumbrance. 
They might well be denominated an invention as a universal sub- 
stitute for rough roads. 
It has been justly observed by Mr. Wingrove, that there is no 
need for continuing in force the act limiting the under distance of 
wheels. But although this desideratum were denied, wheels need 
not be spread above, as long as carriages and roads are not li- 
mited in length. Granting, however, that the legislature were to 
repeal this unnecessary restriction, it is not probable that any re- 
formation would soon take place in the construction of carriages ; 
for it is firmly believed with the great bulk of the people, that 
carriages, but especially the wheels, are at present formed on the 
most improved and learned principles. 
We may therefore reasonably despair of success, in using any 
remonstrance with the public, so long as the legislature does not 
again interfere. Ido not pretend to dictate in this case ; but 
perhaps something like double toll-duty, when the wheels are 
more than an inch closer below than above, might in time have 
its effect. This would be a considerable advantage, not only to 
the road, but even to the owner of the carriage himself, though 
against his will ; for it has been concluded from experiment, that 
four horses would do as much with straight axles, as-five can 
with those in common use. I would, however, still approve of 
having an axle bent just so much that the weight of the load might 
bring it to the straight. A similar allowance should also be made 
for the axle bending backward at the extremities, in consequence 
of the resistance the wheels encounter on the road. For a like 
reason, it is probable that the wheels of a heavy carriage, whose 
axle turns with them, are a little wider below than above, and 
before than behind, But this is certainly nothing worse than the 
tottering of wheels that turn loosely on a fixed axle. 
It has been sometimes thought best to make the axle fast to 
the carriage, without turning round; as the force to turn the 
axle is thereby saved. But it should also be recollected, that the 
weight of a fixed axle adds to the friction at the axle; whereas 
the weight of an axle turning round does not affect this friction at 
all, Besides, the force to turn the axle is only needed when the 
carriage 
