MASS OF THE EARTH. 45 



That this power is great, has been confirmed 

 by various curious experiments, especially by 

 those of Hales. He measured the force with 

 which the stems and branches of trees draw the 

 fluid from below, and push it upwards. He 

 found, for instance, that a vine in the bleeding 

 season could push up its sap in a glass tube to 

 the height of twenty -one feet above the stump of 

 an amputated branch. 



The force which produces this effect is part of 

 the economy of the vegetable world ; and it is 

 clear that the due operation of the force depends 

 upon its being rightly proportioned to the force 

 of gravity. The weight of the fluid must be 

 counterbalanced, and an access of force must 

 exist to produce the motion upwards. In the 

 common course of vegetable life, the rate of 

 ascent of the sap is regulated, on the one hand, 

 by the upward pressure of the vegetable power, 

 and on the other, by the amount of the gravity 

 of the fluid, along with the other resistances, 

 which are to be overcome. If, therefore, we 

 suppose gravity to increase, the rapidity of this 

 vegetable circulation will diminish, and the rate 

 at which this function proceeds, will not corres- 

 pond either to the course of the seasons, or the 

 other physiological processes with which this 

 has to co-operate. We might easily conceive 

 such an increase of gravity as would stop the 

 vital movements of the plant in a very short 



