TRENCHING. 



515 



TRENCHING. 



inches ; and in Fig 2 a longitudinal section 

 of the ground, showing it divided into 

 layers 8 inches in depth, and into trenches 

 12 inches wide, so that the trenching is 

 carried to the depth of 2 feet. The opera- 

 tion of trenching is commenced by taking 

 out the top spit from the trench A and 

 wheeling the soil to the other end of the 

 piece of land to be trenched, placing it at 

 H in a long row, the length of the row 

 being equal to the width oi the land to be 

 trenched, and at a sufficient distance from 

 the space to be occupied by the last trench 

 to allow the second spit from A to be placed 

 at K, alongside of the mould at H, and 

 nearer the last trench, G, than the mould 

 at H. The top spit from the second trench, 



is then to be wheeled to L. The dispo- 



FIG. 2. SECTIONAL DIAGRAM SHOWING TRANSFER OF EARTH 

 FROM TRENCH TO TRENCH IN TRENCHING. 



sition of the first and second spits from A 

 and the top spit from B is thus made to 

 bring the mould into a convenient position 

 for filling in the trenches F and G at the 

 completion of the trenching ; for as the 

 second spit in G is thrown into F to form 

 the second spit of that trench, the top spits 

 from A and B being thrown into F and G to 

 form the top spits of those trenches. It 

 will manifestly be easier to throw the 

 mould of the second spit from A into G 

 from K than if it had been placed to the 

 rear of the top spits from A and B, and 

 when this mould has been put in its place, 

 the mould of the top spits from A and B 

 may be thrown over the second spits in F 

 and G just as the mould comes to hand. 

 In the sectional view in Fig 2, the transfer 

 -tf the spits of earth from trench to trench 



is shown by the arrows. No furlher men- 

 tion need be made of the temporary 

 removal of the first and second spits in A 

 and the top spit in B to the rear of the 

 ground to be trenched. In the diagram, 

 the third spits are distinguished by crossed 

 irregular lines. This spit in each trench is 

 broken up into rough lumps when the spits 

 above it have been removed. The second 

 spits are shown by diagonal lines, and the 

 transfer of the spits from B to A, from C to 

 B, &c.,is shown by the short arrows. The 

 top spits are shown by dotted spaces, and 

 the transfer .of the spits from C to A, from 

 D to B, &c., is shown by longer arrows. 

 When the second spit in H has been 

 removed to a similar position in G, and the 

 top spits in G and H to similar positions in 

 E and F respectively, the 

 vacant spits which are left 

 white in the diagram are 

 filled in with the mould 

 previously removed from the 

 trenches A and B in the 

 manner and order already 

 explained. 



Action of Air on Soil. 

 Thus, while the entire soil in the ground 

 that has been trenched has been stirred 

 to the depth of two feet, it still retains 



that 

 was 



the position, in point of depth', 

 it held in the ground before it 

 trenched ; the surface being left exposed 

 for a time in rough unbroken lumps, 

 till it crumbles naturally under the dis- 

 integrating influence of the atmosphere. 

 Oxygen enters into combination with all 

 soils ; and it can be demonstrated that 

 about one-half of the materials of the 

 globe's crust, including its animal and 

 vegetable products, is composed of oxygen. 

 The chemist can only obtain it as a gas ; 

 but when combined, it assumes divers 

 forms, and occurs as liquids and solids, as 

 well as a gas. About a half by weight of 

 flint, rock-crystal, and other forms of silica, 



