OF FOREST-TREES. 



127 



Fourthly, Over much wet is to be drained by trenches, where it infests CHAP. VI L 

 the roots of such kinds as require drier ground : but if a drip do fret into '^-^'v-*^ 

 the body of a tree by the head, which will certainly decay it, cutting first 

 the place smooth, stop and cover it with loam and hay, or cerecloth, tiU. 

 a new bark succeed. But not only the wet, which is to be diverted by 

 trenching the ground, is exitial to many trees, but their repletion of too 



" arising from the heat of his blood, could carry off above half the fluid which was neces- 



sary to be discharged every twenty-four hours, there was a necessity of providing the 



''kidneys to percolate the other half through. And whereas it is found that 17 times more 



" enters, bulk for bulk, into the sap-vessels of the plant, than into the veins of a man, 



" and goes off in twenty-four hours ; one reason of this greater plenty of fresh fluid in the 



"vegetable than the animal body, may be, because the fluid which is filtrated through 



" the roots immediately from the earth, is not near so full freighted v/ith nutritive particles 



^' as the chyle which enters the lacteals of animals ; which defect it was necessary to 



" supply by the entrance of a much greater quantity of fluid. And the motion of the sap 



" is thereby much accelerated, which in the heartless vegetable would otherwise be very 



" slow ; it having, probably, only a progressive, and not a circulating motion, as in animals. 



" Since, then, a plentiful perspiration is found so necessary for the health of a plant or tree, 



it is probable that many of their distempers are owing to a stoppage of this perspiration by 



" inclement air. The perspiration in men is often stopped to a fatal degree ; not only by 



" the inclemency of the air, but by intemperance, and violent heats and colds. But in the 



" more temperate vegetable, perspiration can be stopped only by inclement air ; unless by 



an unkindly soil, or want of genial moisture, it is depi'ived of proper or sufficient nourish- 



ment. As Dr. Keill observed in himself a considerable latitude of degrees of healthy per- 



" spiration, from a pound and a half to 3 pounds ; I have also observed a healthy latitude 



" of perspiration in this sun-flower, from l6 to 28 ounces, in twelve hours' day. The more 



it was watered, the more plentifully it perspired, ( cceteris paribus,) and with scanty 



" watering the perspiration much abated." 



From these accurate experiments, it is evident that vegetables inspire and expire. Pure 



air is necessary for animals. Vegetables require the same. When obliged to breathe 



their own vapours, they become unhealthy. For that reason, corn is seldom good in small 



inclosures ; neither are trees healthy when much crowded. The superior goodness of the 



grain produced by the drill and alternate husbandry *, evinces the necessity of a free cir- • yj^^ 



culation of air. There is a certain height to which the soil ought to raise the eai-s of gical Essays, 



p. 87 



corn. When, from too much closeness, they are elevated beyond that pitch, the real nou- 

 rishment that should go to the grain, is spent upon the strav/. The stems, also, that should 

 have been hardened by the air, become weak, and unable to stand against moderate storms 

 of wind and rain. The culture of beans shews the truth of this observation. When sown 

 too thick, they push themselves upward with seeming vigour ; and the crop has the appear- 

 ance of being a good one. But when examined, we find the pods small, and few in num- 

 ber. On the contrary, when sown in drills, with proper intervals, the straw is shorter^ 

 and the pods much larger, and more numerous. But to return to our philosophical argu- 

 ment. The analogy that subsists between plants and animals, has induced some very emi- 



R2 



