78 EXPERIMENTS OF HALES ON THE 



the fact that they date from the age of NEWTON. 

 They ought to be familiar to every vegetable 

 physiologist. 



In the beginning of his work, HALES describes 

 the experiments which he made on the motion of 

 the sap in plants in consequence of their evapora- 

 tion in branches covered with foliage, in cut plants 

 as well as in those still provided with roots, 

 influenceof He shews by the following experiment the in- 



a column of 



water, with- fluence of the mechanical pressure of a column of 



out evapo- 

 ration ; water, with and without the help of evaporation. 



To a branch of an apple-tree bearing its twigs 

 and leaves, HALES fastened, air-tight, a tube 7 feet 

 long. He kept the branch with its twigs and leaves 

 immersed in a large vessel of water, and filled the 

 tube with water. By the pressure of the column 

 of water, water was forced into the branch, and in 

 2 days the water in the tube had sunk 14^ inches. 



On the third day, he took the branch out of the 

 water, and exposed it to free evaporation in the air. 

 The water in the tube fell, in 12 hours, 27 

 inches. 



and with To compare the force with which water is 

 evapora driven through the vessels of the wood, by pres- 

 sure alone, with that produced by pressure and 

 evaporation, he joined an apple branch, 6 feet long, 

 with leaves, and exposed to the air, with a tube 9 

 feet long, which was filled with water. 



From the pressure caused by the column of 

 water, and by the evaporation going on at the sur- 



