[214 3 



be intimately connected likewise -with the motion of 

 the sap upwards. This is shewn by several experi- 

 ments of Dr. Hales. 



A branch from an appple tree was separated and 

 introduced into water, and connected with a mercurial 

 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 sycamore, which 

 have all soft and unvarnished leaves, raised the mer- 

 cury under favourable circumstances 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 var- 

 nished leaves, scarcely at all affected it ; particularly 

 the laurel and the laurustinus. 



It will be proper to mention the facts which 

 shew, that in many cases fluids descend through the 

 bark ; they are not of the same unequivocal nature 

 as those which demonstrate the ascent of the sap 

 through the alburnum ; yet many of them are satis- 

 factory. 



