GENEEAL PBEPAEATION OE THE SOIL. 169 



is not great enough for there to be any very appreciable difference 

 of level between its lowest and highest portions, this end is accom- 

 pUshed simply by means of the ploughing operations, followed by 

 a few sufficiently heavy falls of rain. The deeper depressions 

 must, however, be specially filled up, and this is generally best 

 done by bringing in good topsoil from outside the nursery. 



If, all other circumstances being the same, the difference of 

 level is marked, exceeds say 1 in 10, the area must be divided off 

 into horizontal terraces, the edges of the terraces being protected, 

 if necessary, against slipping or erosion by means of retaining 

 walls of dry masonary, which, besides being cheap and effective, 

 permit of a sufficiently free drainage. 



The amount of terracing will be directly proportional to the 

 steepness of the slope, and the width of the terraces will vary 

 jointly with the angle of slope and the depth and richness of the 

 topsoil, for the subsoil should nowhere, as far as can be helped, be 

 exposed or even brought too near the surface. The terraces should 

 be so laid out that the cutting shall, as far as practicable, be equal 

 to the quantity of embankment, in order thereby to minimise both 

 the labour of catting and the distance of lead for the earth cut, and 

 to utiHse every bit of this latter. 



The use of a levelling instrument can never be dispensed with, 

 as the best work with the eye cannot but fail at many points, and 

 cause hereafter much trouble, serious loss of good planting material, 

 and heavy expenditure on frequent repairs of broken terraces and 

 eroded soil. 



Under any circumstances, we can seldom get rid of the necessity 

 of protecting the soil against erosion. During heavy showers a 

 large proportion of the rain, not being able to enter the soil as fast 

 as it falls, rushes over the surface, cutting channels in it, and 

 carrying away a considerable portion of the top-soil, especially if 

 this has been loosened by cultivation. 



Whatever the character of the ground, the following grand rule 

 should be universally observed : — The rain falling on higher land 

 outside the nursery should never he allowed to run down into the 

 nursery, except it he along a natural, well-defined drainage channel 

 passing through it, which it has not heen found possible to close ; nor 

 ought any portion of the fall within the limits of the nursery to he 

 permitted to he carried off outside hy surface dt'ainage. 



In the plains, the mound system of fencing already described 

 on page 167 answers perfectly as a protective work. The mound 

 prevents a rush of water both into and out of the nursery ; while 



