12 



THE GARDENER'S ASSISTANT. 



the surface shall have been levelled, is necessary 

 to be ascertained. If this be done with suffi- 

 cient accuracy, a proper datum-line, or start- 

 ing-point for the height of walls and other 

 structures, the surface of the walks, the depth 

 of the drains, &c, will have been obtained. 

 Here it is evident that some knowledge of 

 levelling is necessary; and as much expense has 

 been too frequently incurred for want of such a 

 knowledge, some plain directions on the subject 

 may prove useful, and lead to better results, 

 with less expense than is usually the case 

 when the operation of levelling the ground is 

 commenced in hap-hazard fashion, and the result 

 left to chance. 



Supposing a plumb-line to be suspended from 

 p (fig. 795), it is evident that it would hang in 





1 



::::::::;;"!"" n 



o 



J 



6 a 





\ 



> \ 



/ /^\ 





' \\ 











Fig. 795. — Levelling. 



the direction of the line pc, or towards the centre 

 of the earth. Any line crossing this at right 

 angles, as del, is generally termed the horizontal 

 line, the line of sight, or the apparent level. 



It is scarcely to be supposed that a gardener 

 can command a spirit-level so nicely adjusted 

 that the line of sight will be perfectly horizontal 

 when the bubble is in the middle of the tube. 

 Its line of sight may direct to a point too high 

 or too low, but by placing the instrument exactly 

 in the middle, as at p (fig. 795), any error aris- 

 ing from the above cause may be avoided. If 

 the instrument at p, when the bubble is in the 

 middle of the spirit-tube, has its sights directed 

 to I, say on the right, it is evident that that 

 point is below the horizontal line; and when 

 the instrument is turned round, it will point as 

 much below the horizontal line at I on the left. 

 These two points / I are both in the same hori- 

 zontal line, although not in that of the instru- 

 ment ; for the distance being equal, the error is 

 equal on both sides. It increases in proportion 

 to the distance; therefore, if the instrument 

 were not placed exactly in the middle, the two 

 points 11 would not be equidistant from mm, and 

 consequently the straight line between the two 

 former would not be parallel to the horizontal 



line of sight. In all cases, therefore, where the 

 instrument is not of the most perfect descrip- 

 tion, and particularly well adjusted, it should 

 be placed half-way between the staves, on which 

 points in a horizontal line are to be marked. It 

 may be observed, that on viewing from / to /, 

 the error of the instrument will be seen to be 

 pn; by measuring that distance from I to m, and 

 then adjusting the sights of the instrument so 

 that they will direct to m whilst the bubble is 

 in the middle of the spirit- tube, a level may then 

 be taken for a short distance without risk of 

 any material error, even although the instru- 

 ment should not be placed midway, which may 

 not be possible in some cases. 



To level a line, as, for example, that intended 

 for the edging of a walk, prepare a lozenge-shaped 

 piece of wood (fig. 796) about 6 inches broad; 

 paint it white, with the exception of an inch all 

 round the margin, which should be black; also, 

 a strong black line across from angle to angle. 

 A square hole cut on the upper side of the cross 

 line admits of anything against which it is placed 

 being marked exactly at the height of that line. 

 If the length of the edging intended to be 

 levelled do not exceed 600 feet, let a rod be 

 placed at each end, and the instrument half-wa) r 

 between these. Let an assistant hold the lozenge- 

 shaped mark against the rod at one extremity 

 of the line, whilst the person at the instrument 

 directs him to slide it up or down, till the line 

 across its centre coincides with the line of sight 

 from the instrument when 

 js=. the bubble is in the middle 



jg^^Sk of the spirit-tube. Mark 



jy i^^k, tne r °d at tne height of 



^ m I ^i^ the cross-line; and in the 



^lk ^^ same way the rod at the 



^^ ^m other end of the walk. The 



^HypF two points so marked on 



^35? the rods at each extremity 



are in the same horizontal 



Fig. 796. 



line, like the points mm, 

 or 11, in fig. 795. The instrument may now 

 be removed, and a rod put in its place. By 

 placing the cross-line of the lozenge-slide on 

 one of the points to which the level was directed, 

 and then viewing from the point at the other 

 extremity, the rod placed in the middle can be 

 marked at a point which will be in a horizontal 

 line with the other two. There will then be 

 three ascertained points on the same level ; and 

 by viewing between any two, as many more 

 may be marked along the line as may be found 

 necessary. Thus, on the rods placed between 

 the two extremities, a series of points may be 



