On Shooting Stars. 201 
carriage begins to move ; but in the other case, the increased 
friction continues during the whole motion. Some have alleged 
that the improvement of roads, by cutting hills and raising hol- 
lows, has sometimes been carried to excess: because, say they, a 
horse is sooner worn out by constant pulling, though lightly, than 
if he had now and then to exert himself going up hill, and after- 
ward rested his shoulder going down. But if there is any force 
in this allegation, it is certainly not of general application; for 
where two or more animals work together, they might draw and 
rest by turns :—even a single horse could move rather quicker for 
a few minutes, and then stand alternately. However, J rather 
think the main fault of a very level road, is its retaining the water 
longer in its ruts, so as seldom to be clean and smooth like a 
hilly one. 
Many vulgar notions containing their own refutation might be 
mentioned. But it deserves also to be noticed, that from the sub- 
ject’s often appearing much simpler than it really is, there is great 
danger of falling into mistake. A single instance will be sufficient 
to show the truth of this remark: It has been asserted by high 
authority, that the use of wheels is only to overcome the inequa- 
lities of the road; and, that if the bare axle were to slide on two 
polished railings of metal, the carriage would be as easily drawn 
along as if with wheels in the usual way (abstracting from the 
inequalities of the road). Now were this really the case, even 
friction-wheels themselves would be altogether useless, or rather 
a pompous incumbrance. Admitting, however, that friction is 
not affected by varying the velocity, it would obviously follow, 
that the friction on the axle is the same in both cases. But still 
it should never be forgotten that friction and the momentum of 
friction are very different things. In the case of the bare axle 
sliding along, we have the whole friction to contend with ; but 
when wheels are added, we have only to overcome its momentum ; 
which evidently is to the friction, as the radius of the axle to that 
of the wheel. This being agreeable to what is contained in most 
books on the subject, it need not be commented on here. 
Some further observations may possibly be added at another 
period. _| am, sir, your most obedient servant, 
Henry MEIKLE. 
XXXI. On Shooting Stars. 
To Mr. Tilloch. 
Sir, — Your correspondent Mr, Farey* appears to take it for 
granted that his theory, or rather hypothesis, respecting the na- 
* Phil. Mag. No, 250, p. 116. 
ture 
