124 



A DISCOURSE 



I?OOK II. iintliriving' till removed ; such as that of the Oak and ]\Iast-holm, Wal- 

 -"'^'y^^ nut, Tine, Fir, &c. the thickness of the leaves intercepting the sun 



and rain ; whilst that of other trees is good, as the Ehn, and several 



others. 



Secondly, Suckers should be duly eradicated, and with a sharp spade 



" the leaden plate, and then covered the stump well with cement ; and upon weighing, 

 " found there perspired through the unglazed porous pot two ounces every twelve hours' 

 " day : which being allowed in the daily weighing of the plant and pot, I found the 

 " greatest perspiration of twelve hours in a very warm dry day, to be one pound fourteen 

 " ounces ; the middle rate of perspiration, one pound four ounces. The perspiration of a 

 " dry warm night, without any sensible dew, was about three ounces ; but when any sen- 

 " sible, though small dew, then the perspiration was nothing ; and when a large dew, or 

 " some little rain in the night, the plant and pot was increased in weight two or three 

 " ounces. [^I used avoirdupois weights.] I cut off all the leaves of this plant, and laid 

 " them in five several parcels, according to their several sizes, and then measured the 

 " surface of a leaf of each parcel, by laying over it a large lattice made with threads, in 

 " which the little squares were a quarter of an inch each ; by numbering of which I had 

 " the surface of the leaves in square inches, which, multiplied by the number of the leaves 

 " in the corresponding parcels, gave me the area of all the leaves ; by which means I found 

 "the surface of the whole plant, above ground, to be equal to 56l6 square inches, 

 " or 39 square feet. I dug up another sun-flower, nearly of the same size, which had 

 " eight main roots, reaching fifteen inches deep and sideways from the stem : it had 

 " besides a very thick bush of lateral roots from the eight main roots, which extended 

 " every way in a hemisphere, about nine inches from the stem and main roots. In order 

 " to get an estimate of the length of all the roots, I took one of the main roots with its 

 " laterals, and measured and weighed them ; and then weighed the other seven roots, 

 with their laterals ; by which means, I found the sum of the length of all the roots to be 

 " no less than 1448 feet. And supposing the periphery of these roots, at a medium, to 

 " be 0.131 of an inch, then their surface will be 2276 squai-e inches, or 15.S square feet; 

 " that is equal to 0.4 of surface of the plant above ground. If, as above, twenty ounces 

 "of water, at a medium, perspired in twelve hours' day, (i.e.) thirty-four cubic inches 

 " of water, (a cubic inch of water weighing 254 grains,) then the thirty-four cubic inches 

 " divided by the surface of all the roots, is = 2286 square inches (i. e.) ^f Jij is = ; this 

 " gives the depth of water imbibed by the whole surface of the roots, viz. ^'^ part of an 

 f< inch. — And the surface of the plant above ground being 56l6 square inches, by which 

 " dividing the thirty-four cubic inches, viz. ygig = ^^j, this gives the depth perspired by 

 " the whole surface of the plant above ground, viz. part of an inch. Hence the 

 " velocity with which water enters the surface of the roots to supply the expense of per- 

 " spiration, is to the velocity with which their sap perspires, as l65 ,* 67, or as 5'- ; -^^j, or 

 " nearly as 5 .* 2. The area of the transverse cut of the middle of the stem is a square 

 " inch ; therefore, the areas on the surface of the leaves, the roots, and stem, are 



