348 On Spontanems TnJtammatioTis, 



Changing the experiment, and rubbing a cylinder of onf, ■ 

 of the kinds of \vnod between two tablets of the other, a 

 cylinder of poplar for example between two tablets of mul- 

 berry wood, the increase of the rubbed surfaces which arc 

 in contact with the air produced a heat much more consi- 

 derable, and almost the whole of the kinds of wood above 

 enumerated took tire. 



The efteet of friction still varies according as the woods 

 employed, especially if they are of the same kind, are rub- 

 bed ill the direction of the fibres, or when the fibres cross 

 ^ach other. In the first case, the friction and heat are much 

 niore considerable than iu tiie second. 



In large machines where there is much friction, heating 

 may be prevented by continually directing a current of cold 

 water on the rubbing surfaces : in common machines, car- 

 riages, &c. it is diminished by co\-ering the surfaces with an 

 oily matter. There are many instances, durmg the great 

 heats in summer, of carriages and other machines exposed 

 to violent motion inllaming, because care has not been taken 

 to grease them. Grease, by hardening on the rubbing sur- 

 faces, instead of Ljssening the Iriction increases it j and as 

 this covering is highly combustible, it renders spontaneous 

 inflammation more easy. In many cases, therefore, it is 

 better to rub the machines with soap, talc, plumbago, or 



other 



