342 Dr Davy's Agricultural Discourse. 



soft porous freestone on which the scanty soil of that island rests, 

 from time to time the industrious natives bare the rock of its soil, 

 and make grooves in it, penetrating through the hard incrustation, 

 so as to admit the passage of rain-water into the rock, and its exha- 

 lation to the soil during the dry season. The qualities of soil likely 

 to be benefited by thorough-draining, are the stift* clay soils, or the 

 lighter and more porous soils, with a substratum of such clays on 

 which, after heavy rain, water rests in a state of stagnation. Should 

 it, as regards any soil, be a doubtful question whether it requires or 

 not the process of thorough-draining, a simple experiment may be 

 made, which may help to remove the doubt : it is by taking a por- 

 tion of the soi!, and subjecting it to the action of water in a tube, or 

 a piece of bamboo covered at its lower end by linen, which will sup- 

 port the soil, and allow water to pass. If the soil, when compressed, 

 acts in the manner of stiff clay, and does not allow the water to drop 

 — to flow through it, it is a criterion of the propriety of draining ; 

 and the contrary, if it permit the passage of water. Trials of this 

 kind, I apprehend, may be advantageously made to test the proper- 

 ties of soils, as to their retentive powers ; which are graduated, in a 

 great measure, by the proportion of alumine present, and the pro- 

 portion of the other earths, in a finely-divided state, — any earth, if 

 finely-divided, tending to have the same effect as alumine, in retard- 

 ing or preventing the descent of water. The results of such trials, 

 moreovei', may indicate whether clay should be added to the soil to 

 increase, or sand or lime to diminish, its power of retaining watei". 



I have spoken of Dr Skier's experiment on thorough-draining; 

 let me add, what I should have done before, that you will find the 

 particulars of it clearly detailed in his published report on thorough- 

 draining, a report n)ost highly creditable to him as a scientific agri- 

 cultural chemist, and as a scientific inquirer, and which, for the va- 

 luable and new information it contains on the subject of which it 

 treats, is particularly deserving the attention of all tropical agricul- 

 turists who wish to enter into the minute details of the operation. 

 It affoi'ds a happy example of science and practice combined. 



I remarked in commencing this discourse, that irrigation and tho- 

 rough-draining arc allied: they are so, not only inasmuch as water 

 is mainly concerned in both, but also as to the manner in which it 

 is concerned. Thorough-draining may be viewed as a slow and deep 

 irrigation, the water descending from the surface to the drains or 

 channels conveying away what is superfluous ; whilst irrigation is the 

 conducting of water over the surface of land in constant slow flow, so 

 as to aftord nourishment to the growing crop, which all experience 

 proves it to do with wonderful effect. At the same time, it is to bo 

 kept in mind, that the slowly flowing water does not act merely super- 

 ficially, but that it penetrates deeply ; and not only promotes vege- 

 tation, but likewise, when properly managed, has a tendency to en- 

 rich the soil, either by what it deposits, or by its action, through the 



