HALES EXPERIMENTS ON THE JIOTION OF THE .SAP. 359 



very little power to protrude sap out at their extremities, and 

 make it rise in a tube fixed to tbem. 



The motion of the sap, Hales concludes, is to be attributed 

 to the exhalation from the surface alone, and he proves that it 

 proceeds in an equal degree from the trunk, branches, leaves, 

 flower and fruit, and that the effect of the exhalation bears 

 a certain definite ratio to the temperature and moisture of the 

 air. WTien the atmosphere was charged with humidity little 

 water was imbibed, and on rainy days the absorption was barely 

 perceptible. Hales opens this second chapter of his statics 

 Avith the following introductory remarks : — 



' Having in the first chapter seen many proofs of the great 

 quantity of liquid imbibed and perspired by vegetables, I pro- 

 pose in this to inquire with what force they do imbibe moisture. 



* Though vegetables (which are inanimate) have not an 

 engine which by its alternate dilatations and contractions does in 

 animals forcibly drive the blood through the arteries and veins, 

 yet has nature wonderfully contrived other means, most power- 

 fully to raise and keep in motion the sap.' 



In his twenty-first experiment he laid bare one of the chief 

 roots of a thriving pear-tree at a depth of 2^ feet, cut off the 

 end of the root, and connected the remaining stump with a 

 glass tube filled with water and confined by mercury. This 

 glass tube represents the root lengihened. 



By the perspiration from the surface of the tree, the root 

 imbibed the water in the tube with such vigour that in six 

 minutes the mercury had risen in the tube as high as 8 inches, 

 which corresponds to a column of water 9 feet in height. 



This force is very nearly equal to that with which the blood 

 moves in the great crural artery of a horse. *I found,' says 

 Hales, in his thirty-sixth experiment, * the force of the blood 

 of several animals, by tying them down alive upon their backs, 

 then laying open the great crural artery where it first enters 

 the thigh, and fixing to it, by means of two brass pipes running 

 one into the other, a glass tube above ten feet long and one- 

 eighth of an inch in diameter. In this tube the blood of one 

 horse rose eight feet three inches, and the blood of another 

 horse eight feet nine inches ; the blood of a little dog, six 

 feet and a half.' 



