346 Professor Playfair’s Description 
-less than It follows, then, that the friction that trees suffer 
in the slide is less than one-fiftieth of their weight. 
Now, from what can we suppose the small proportion that 
friction in this instance, bears to the weight, to arise ? It is 
not that the surfaces have a great smoothness or a fine > polish. 
The logs that form the trough are coarsely dressed with the 
adze, and I observed that there was not even the precaution 
taken of making the grain of the wood lie downward, or toward 
the declivity. It was so in the tree, but not in the trees which 
composed the slide. It is not that any lubricating substance, 
oil, grease, soap, or black-lead, is interposed between their sur- 
faces. Water is the only substance of this kind that is applied. 
We have fir rubbing on fir, which is supposed a case remark- 
ably unfavourable to the diminution of friction. It can only 
arise, therefore, from a principle that some mechanical writers 
have suspected to exist, but which was never before, I think, 
proved by the direct evidence of facts, namely, that the force of 
friction does not increase in the proportion of the weight of the 
' rubbing body, so that heavy bodies are, in reality, less retarded 
in their motion on an inclined surface than lighter bodies. 
Thus, the whole of the phenomena I have been describing, 
tend to prove, especially the fact I mentioned, that heavy trees 
made their way more easily than light ones, and that a tree 
must be of a certain magnitude to make its way to the bottom. 
Friction, therefore, does not bear even in the same materials a 
given ratio to the weight, but a ratio that evidently decreases 
as the weight increases ; so that, in a fir of ordinary size it is 
or 5 ^ 5 , in one of 100 feet in length it is between and 
► According to what law this change takes place, it would be 
• most useful to investigate ; it is an inquiry for those engineers 
who have strong machinery and great power ready at com- 
mand. 
I must observe also, that I strongly suspect that friction 
diminishes with the velocity of the moving or sliding body. 
That it passes all at once when a body begins to move, to be 
only half of what it was when the body was at rest, is quite 
certain, and is proved by many experiments. It seems to me 
not unlikely that the same progress continues as the motion be- 
comes greater. Perhaps in as much as friction is concerned, 
