80 EXPERIMENTS OF HALES ON THE 



tables (which are inanimate) have not an engine, 

 which by its alternate dilations and contractions, 

 does in animals, forcibly drive the blood through the 

 arteries and veins ; yet has nature wonderfully con- 

 trived other means, most powerfully to raise and 

 keep in motion the sap." 



Great force In his experiment XXI., he exposed one of the 

 the sap chief roots of a pear-tree in full growth at a depth 

 of 2^ feet, cut off the point of it, and connected 

 the part of the root left in connection with the 

 stem with a tube, which he filled with water, and 

 closed with mercury. 



In consequence of the evaporation from the sur- 

 face of the tree, the root absorbed the water in the 

 tube with such a force, that in six minutes the 

 mercury rose to 8 inches in the tube. This corre- 

 sponds to a column of water 9 feet high, 

 it is nearly This force is nearly equal to that with which the 



equal tOfc 



that of the blood moves in the great femoral artery of the 



circulation . . 



in animals, horse. HALES, in his experiment XXXVI., found 

 the force of the blood in various animals : " By 

 tying those several animals down alive upon their 

 backs, and then laying open the great left crural 

 artery, where it first enters the thigh, I fixed to it 

 (by means of two brass pipes which run one into 

 the other) a glass tube of above 10 feet long, and 

 ^th of an inch in diameter in bore. In which tube 

 the blood of one horse rose 8 feet 3 inches, and the 

 blood of another horse 8 feet 9 inches. The blood 

 of a little dog 6^ feet high." 



