3*8 Professor Playfair's Descri'ption 
great, and engage us in attempts that prove abortive, and are 
followed by disappointments and even ruin. In the present 
case, the experiments on the small scale represent the thing as 
more difficult than when tried on a great one it is found to be, 
and would lead us, by an error, the direct opposite of the last, 
to conclude things to be impracticable that may be carried into 
effect with ease. Had the ingenious inventor of the slide at 
Alpnach been better acquainted with the received theories of 
friction, or the experiments on which they are founded, even 
those that are the best, and on the greatest scale,' such as those 
of another most skilful engineer, M. Coulomb, or had he placed 
more faith in them, he would never have attempted the great 
work in which he has"* so eminently succeeded.” 
Observations on the preceding Paper. 
It is much to be regretted that the editor of Professor Play’^- 
fair's works did not terminate the preceding interesting and 
valuable paper with the second paragraph of p. 345. In the 
succeeding pages, the distinguished author maintains, 1. That 
the phenomena of the slide are incompatible with the recent 
theories of friction ; %. That Mr Rupp, the engineer, would 
never have executed that great work, had he been acquainted 
either with these theories, or with the experiments of Cou- 
lomb. 3. That heavy bodies are less retarded by friction on an 
inclined plane than lighter bodies ; and, 4. That he strongly 
suspects that friction diniinishes with the velocity of the moving 
or sliding body. The two first of these conclusions are evident- 
ly incorrect, and founded on an oversight of the author; and 
the two last have long formed a part of the received theories on 
friction, and have been deduced from actual experiment. 
On the evening upon which Professor Playfair read this paper 
to the Royal Society, the writer of this note reminded him of 
CoulomVs beautiful discovery, that when the touching surfaces 
were small compared with the pressure, (which was obviously 
the case with the trees of Alpnach,) the friction diminished as 
the velocity increased ; and in our brief notice of the paper in 
Vol. I. p. 193. of this Journal, published during Mr Playfair’s 
life, we remarked, “ that the very singular phenomena described 
in Mr Playfair’s paper, arose from the diminution of friction 
