14 



Irrigation. 



Vol. XIT. 



to the soil and climate are produced. Nei- 

 ther does the fact of the deposition of mud, 

 or other fertilizing sediment, explain the 

 phenomenon; for however such depositions 

 may increase the effect, it is likewise found 

 that water, without the least perceptible 

 sediment, may be employed with success. 

 It has been supposed that the water acts 

 beneficially by maintaining the soil at a 

 higher temperature. Water, at a tempera- 

 ture of 40°, is of greater specific gravity 

 than at a lower temperature; and hence as 

 the water tends to the freezing point, the 

 warmer portion of it is next the ground. 

 Much, however, cannot be ascribed to this 

 cause, in a current so shallow and constant 

 as that which passes over the watered mea- 

 dow. It is probable, therefore, that the 

 main effect is produced by a mechanical ac- 

 tion of the water, acting upon and bringing 

 nourishment to the fibrous roots of the 

 plants." 



Although it may be very true, as Mr. 

 Stephens observes, that " however authors 

 may disagree on this interesting subject — 

 the theory of irrigation, — I believe all ex- 

 perimentalists acknowledge that early win- 

 ter watering is necessary to produce early 

 and abundant vegetation ; in what way this 

 operates is, as to practical purposes, less ma- 

 terial;" yet it is always satisfactory to man 

 to be able to give a reason for what he does. 

 To this view I shall add another theory of 

 irrigation, that has been suggested by Pro- 

 fessor Rennie, late of King's College, Lon- 

 don. It is believed by some vegetable phy- 

 siologists that plants excrete certain matter 

 from their roots, which proves inimical to 

 the health of other plants of the same kind. 

 Hence it is concluded that grasses do not 

 continue permanently in a healthy state in 

 the same site, because they are in time in- 

 juriously affected by their own excretions, 

 which, encouraging the gfowth of plants of 

 a different nature, such as mosses, they 

 spring up and extirpate the grasses. It is 

 supposed to be probable that every species 

 of grass is not alike affected by its own, or 

 the excrementilious matter from other grass- 

 es, and therefore some species withstand the 

 poison longer than others. Now the water 

 of irrigation, in its descent through the soil 

 and subsoil, washes away or carries off in 

 solution the injurious excrementitious mat- 

 ter exuded by the grasses, and thereby 

 cleanses the soil in which they are growing, 

 free of it. Hence the perennial verdure of 

 irrigated meadows. 



In order to arrive at a satisfactory expla- 

 nation of this subject, and believing that 

 both this theory, as well as the one given 

 by Sir Humphrey Davy, referred to by Pro- 



fessor Low, contains truth, I proposed some 

 time ago a conjunction of the two theories; 

 and the compound theory certainly explains 

 the four great points of irrigation, namely, 

 that it supplies moisture to the soil in dry 

 seasons and in tropical climates; it affords 

 protection to plants against the extremes of 

 heat and cold ; it disseminates manure in 

 the most minute manner to plants ; and it 

 washes away injurious matter from the roots 

 of plants, i'he benefits derived from irriga- 

 tion I therefore maintained are purely me- 

 chanical, and doubted the correctness of Sir 

 Humphrey Davy's opinion, when he says 

 that "in the artificial watering of meadows 

 the beneficial effects depend upon many dif- 

 ferent causes, some chemical, some mechan- 

 ical," because chemical action only com- 

 mences c/ier the act of irrigation has ceased, 

 as the nature of the following particulars at- 

 tending irrigation will show. No doubt, the 

 effects of the substances, whatever they may 

 be, which are deposited by the water of irri- 

 gation, may be chemical, as well as those of 

 manure applied to grass by the hand of man. 

 But the act of the water in depositing fertil- 

 izing materials, can be no more chemical 

 than that of the instruments used in spread- 

 ing dung upon the soil. The truth is, that 

 whenever the water of irrigation, or the 

 subt^tances contained in it, act chemically 

 upon the grass or soil, while subjected to the 

 process, that moment irrigation proves inju- 

 rious to the plants. The chemical action 

 and the injury are both evinced by the same 

 phenomenon, namely, the existence of white 

 scum floating on the water. " If the wea- 

 ther should be mild," observes Mr. Stephens, 

 "and you suffer the water to run over the 

 meadow too long without intermission, a 

 while scum is generated, which is very de- 

 structive to the tender grass^ The par- 

 ticulars of irrigation I referred to are these: 

 " The operation of water bringing matter 

 into minute subdivision; the sediment w-hich 

 it contains when used in irrigation being 

 minutely distributed around the stems of the 

 plants; water protecting plants in irrigation 

 against the extremes of heat and cold, by 

 completely covering and embracing every 

 stem and leaf; and the supplying of moisture 

 to the soil and washing excrementitious mat- 

 ter out of it, are all purely mechanical ope- 

 rations." For, ^^ could the hand of man dis- 

 tribute manure around the roots and stems 

 of grass as minutely and as incessantly as 

 turbid water; could it place a covering of 

 woollen texture upon each blade and around 

 each stem of grass, as completely as water 

 can embrace each plant and keep it warm; 

 could it water the grass as quietly and con- 

 stantly as the slow current of irrigation; and 



