IS (h-posikd Jming JVniier. z\\ 



TiiG shortness of the trunks of ^the svcaniore laecs, which 

 •tvcre the subjects of my experiments, did not permit me to 

 extract the s.ip at a greater elevation than seven feet, except 

 in one instance ; and in that, at twelve feel from the ground, 

 I obtained a very suect fluid, whose speciiic gravity was 

 1.012. 



I conceived it probable, that if the sap in the preceding 

 cases derived any considerable portion of its increased spe- 

 cific gravity from matter previously existing in the alburnum, 

 I should lind some diminution of its weight, when it had 

 continued to flow some davs from the same incision, because 

 the alburnum in the vicinity of that incision would, under 

 such circumstances, have become in some degree exhaustwi: 

 and on comparing the specific gravity of the sap which had 

 fiowed from a recent and an. old incision, I Ibund that from 

 the old to be reduced to 1 .002, and that from the recent one 

 to remain 1.004, as in the preceding eases, the incision 

 being made ciose to the ground. >Vhereyer extractcdy 

 whether close to the ground, or at some distance from it, 

 the sap always appeared to contain a large portion of air. 



In the experiments to discover the variation in the spe- 

 cific gravity of the alburjium of trees at different seasons, 

 some obstacles to the attainn>ent of any very accurate re- 

 sults presented themselves. The wood of different trees of 

 the same species, and growing in the same soil, or that 

 taken from different parts of the same tree, possesses 

 different degrees of solidity; and the weight of every part 

 of the alburnum appears to increase with its age, the ex- 

 ternal layers being the lightest. The solidity of wood varies 

 also with the greater or less rapidity of its growth. These 

 sources of error might apparently have been avoided by 

 cutting ofT^ at different seasons, portions of the sam.e trunk 

 or branch : but the wound thus made might,- in some degree, 

 have impeded the due progres of the sap in its ascent, and 

 the part below might have been made heavier by the stagna- 

 tion of the sap, and that above lighter by privation of its 

 proper quantity of nutriment. The most eligible method 

 therefore, which occurred to me, was to sclect^and mark in 

 the winter some of the poles of an oak coppice, where ail 

 are of equal age, and where many, of the same size and 

 growing with equal vigour, spring from the same stool. 

 One half of ihc poles which f marked and numbered were 

 tut on the 31st of December, 1S0.3> and the remainder on 

 the I jth of tiic following May, when the leaves were nearly 

 half grown. Proper marks were put to distinguish the 

 wmter-felled from the summer-felled poles, Ithe bark 



bcina; 



