in Fluids. 265 



continue to be the case as long as any degree of fluidity 

 remains ? 



As the bodies and branches of trees are not covered in 

 winter by the snow which protects their roots from the 

 cold atmosphere, it is evident that extraordinary meas- 

 ures were necessary to prevent their being frozen. The 

 bark of all such trees as are designed by nature to sup- 

 port great degrees of cold forms a very warm covering ; 

 but this precaution alone would certainly not have been 

 sufficient for their protection. The sap in all trees 

 which are capable of supporting a long continuance of 

 frost grows thick and viscous on the approach of winter. 

 What more important purpose could this change answer 

 than that here indicated ? And it would be more than 

 folly to pretend that it answers no useful purpose at all. 



We have seen by the results of the foregoing experi- 

 ments how much the simple embarrassment of liquids 

 in their internal motions tends to retard the propagation 

 of Heat in them, and consequently its passage out of 

 them ; and when we consider the extreme smallness of 

 the vessels in which the sap moves in vegetables and par- 

 ticularly in large trees ; when we recollect that the sub- 

 stance of which these small tubes are formed is one of the 

 best non-conductors of Heat known ; * and when we ad- 



* I lately, by accident, had occasion to observe a very striking proof of the ex- 

 treme difficulty with which Heat passes in wood. Being present at the foundery at 

 Munich when cannons were casting, I observed that the founder used a wooden in- 

 strument for stirring the melted metal. It was a piece of oak plank, green or unsea- 

 soned, about ten inches square and two inches thick, with a long wooden handle 

 which was fitted into a hole in the middle of it. As this instrument was frequently 

 used, and sometimes remained a considerable time in the furnace, in which the Heat 

 was most intense, I was surprised to find that it was not consumed ; but I was still 

 more surprised, on examining the part of the plank which had been immersed in the 

 melted metal, to find that the Heat had penetrated it to so inconsiderable a depth, 

 that, at the distance of one twentieth of an inch below its surface, the wood did not 

 seem to have been in the least affected by it. The colour of the wood remained un- 

 changed, and it did not appear to have lost even its moisture. 



