306 PHILOSOPHICAL TRANSACTIONS. [aNNO ] 669. 



or something like it, between the nature of wood and bark. The sap rises 

 within and without that superadded coat. 



Dr. B. says, if a circle be drawn round about any common English tree, as 

 oak, elm, poplar, &c. by incision to the timber, how thin soever the knife be, 

 so that no part of the rind or bark to the very solid timber be uncut, the tree 

 will die from that part upwards, except the ash. To get the gum of plum- 

 trees, I have sometimes wrenched the branch till the solid timber has cracked, 

 and the rind forced open in some parts ; so leaving it to grow, but forced to 

 continue in a posture somewhat wreathed, it yielded me store of gum next 

 summer. 



Dr. T. remarks that a branch, whose bark of the breadth of about 2 or 3 

 inches is taken off round towards the bottom, in some trees, and particularly 

 the lime-tree, will live and bear leaves for many years, and grow as other 

 branches by means of the sap ascending through all the pores of the inner 

 coats. 



He further observes that piths are of a very different nature and substance. 

 In the walnut it is a multitude of films manifestly distant from one another. 

 In others, as in elders and briars, it is a continued, soft, loose, dry substance. 



Also, that the points or ends of the roots being cut off, they will in propor- 

 tion bleed as copiously as the branches, and probably more ; certainly longer, 

 because there is greater plenty of juice ascended above them than the branches, 

 and consequently more will issue by them than by any part of the tree, higher 

 than they are. 



That from the latter end of January to the middle of May, trees will bleed. 

 Those that are said to run first, are the poplar, asp, abel, maple, sycamore ; 

 some, as willows and the birch, tried by myself, are best to tap about the middle 

 of the second season ; and the walnut towards the latter end of March. They 

 generally bleed a full month in the whole. Mr. Midford of Durham, a very 

 expert gatherer and preserver of saps, affirms that the saps of the poplar and 

 asp rise so briskly in January, that they will bleed before the end of that month. 

 The sycamore will run in hard frost, when the sap freezes as it drops. 



The best time of the day for tapping is about noon. In the latter season, 

 when sap is not very plenteous in trees, they will neither run morning nor even- 

 ing, nor probably at any time of the night ; but when they are very full of sap, 

 and emptied but by small vents, the sap may run night and day till exhausted ; 

 but never in large vents. And perhaps this observation may give light to that 

 opinion which holds that the ascending of the sap depends on the pressure or 

 pulsion of heat striking the earth, and thereby driving the moisture of the earth 

 into the root. 



