very small force, provided its pressure be taken off 
by continued divisions in the column of fluid. This 
principle, there is great reason to suppose, must 
operate in assisting the ascent of the sap in the 
cells and vessels of plants which have no rectilineal 
communication, and which every where oppose 
obstacles to the perpendicular pressure of the sap. 
The changes taking place in the leaves and buds, 
and the degree of their power of transpiration, must 
be intimately connected likewise with the motion 
of the sap upwards. This is shown by several ex- 
periments of Dr. Hales. 
A branch from an apple-tree was separated and 
introduced into water, and connected with a mer- 
curial gage. When the leaves were upon it, it 
raised the mercury by the force of the ascending 
juices to four inches ; but a similar branch, from 
which the leaves were removed, scarcely raised it a 
quarter of an inch. 
Those trees, likewise, whose leaves are soft and 
of a spongy texture, and porous at their upper sur- 
faces, displayed by far the greatest powers with 
regard to the elevation of the sap. 
The same accurate philosopher whom I have 
just quoted, found that the pear, quince, cherry, 
walnut, peach, gooseberry, water-elder, and syca- 
more, which have all soft and unvarnished leaves, 
raised the mercury under favourable circum- 
stances from three to six inches. Whereas the elm, 
oak, chesnut, hazel, sallow, and ash, which have 
firmer and more glossy leaves, raised the mercury 
only from one to two inches. And the evergreens, 
and trees bearing varnished leaves, scarcely at all 
