464 On the Theory and Practice of Water -Meadows. 



that the main principle of irrigation is the warmth produced by 

 the water trickling over the surface. Warmth is a prime agent 

 in vegetation, and a slight difference in warmth has a marked 

 effect in hastening or retarding the growth of plants. On hills of 

 a very moderate height — as the chalk range in Berkshire — the 

 harvest is sometimes a fortnight later than in the vale at their feet. 

 The warmth of the London air opens the buds earlier. A south- 

 ern wall, by reflecting heat, hastens the growth of vegetables near 

 its foot. The warm spring too or river does not merely flow over 

 the surface, but sinks largely into the land if it be at all porous, 

 and such land is most benefited by irrigation ; some of the best 

 water-meadows being mere gravels, almost bare of soil. Thus the 

 roots of the grass are kept in a state of genial warmth. But the 

 conclusive argument, as appears to me, may be drawn from a very 

 curious operation called Gurneyism, of which an account was 

 given in the seventh volume of this Journal. 



Mr. Gurney having observed, what many may have remarked, that 

 wherever any loose object, a bare branch, or an old gate, lies on a mea- 

 dow in March, the grass grows exuberantly beneath it, conceived the 

 idea of spreading a field with straw, at the rate of about a ton to 

 the acre, and thus promoting the growth of the grass. The scheme 

 succeeded so well that it was adopted by many neighbouring 

 farmers in Cornwall ; and thus, curiously enough, a thin coat of 

 dry straw produced the same effect which had hitherto been 

 obtained only by a thin sheet of moving water. How, it may be 

 asked, did the straw produce its effect? I can see but one way. 

 Gardeners, it is well known, spread light nets over their young 

 crops in order to protect them from morning frosts in the spring. 

 This effect is clearly due to the interception of the radiation of 

 heat. The earth is constantly sending forth, in a perpendicular 

 direction upwards into empty space, especially when the sky is 

 clear, its warmth derived from the sun, just as a stove darts its 

 heat around it ; but a very slight interruption, such as the gar- 

 dener's net, is found to check the passage of the heat, and thus to 

 prevent that morning frost on the surface so much dreaded by gar- 

 deners. Gurneyism must act in a like manner, though on a larger 

 scale, by preventing the escape of the natural warmth from the 

 soil of a meadow. Irrigation, we have had reason to conclude, 

 acts by imparting to the meadow the superior warmth of the 

 stream or the spring. The effect is the same : the mode of 

 action is the same nearly. The difference is as between covering 

 up a sick man with blankets, or placing him in a warm bath. 

 The one is stronger treatment than the other, and so also irriga- 

 tion acts in winter — Gurneyism only in spring. It may even 

 be questioned, I think, whether irrigation do not also act in 

 some degree by intercepting the radiation of heat. Mr. Gurney 



