“Backsights.” The difference between the sums of the numbers in these two columns will be equal to the height of one extremity (_a_) of the line, above the other (_d_). When a number of levels have to be taken in succession, it will be found a saving of time to use the surveyor’s level or theodolite instead of the ordinary spirit-level illustrated. With regard to the use of the instrument as shown in Fig. 1, it may be necessary to state that the height of the eyesight _b_ from the ground must be deducted from the point observed. As an example: if the object-pole or staff be marked in feet or inches, and the hair wire in taking a sight strike the same at, say, 8 ft., then, if the eyesight be 4 ft. from the ground, the difference of level between the two stations (instrument and station) will be 4 ft., that is, there will be 4 ft. of a fall from the spirit-level station to that of the station where the staff was placed. On the other hand, should the hair wire strike the object-pole at, say, 2 ft. from the ground, these 2 ft. must be deducted from the height of the eyesight, which, as we have said, was 4 ft., then the ground at the station-pole must be 2 ft. higher than that at the spirit-level. =Clearing the Ground.=—Coarse-growing herbage, which often includes heath, gorse, and rough grasses, that would interfere with planting operations, should be removed, but not indiscriminately, as it may prove of inestimable value under certain conditions. On exposed and high-lying ground, the heath and grasses will prove of great advantage to the young trees, and usually they are not too luxuriant or apt to cause damage at high altitudes. Where, however, the growth of such shrubs or grasses would be detrimental to the young trees, by all means have them removed. The best way is either to cut them over or grub out by the root, and burn on the ground. During a continuance of dry weather it may be possible to burn without either cutting or uprooting, but, generally speaking, removal of the roots is to be encouraged. =Pitting.=—The advantages of pitting over any method of planting cannot be questioned, and this is particularly the case with ground that has hitherto been uncultivated. Compared with notch planting, this system is, no doubt, more expensive, but that the future benefits to the plants are greatly enhanced is admitted by all practical arboriculturists. It is not, however, to be inferred that, though pitting is preferable, notching is to be abandoned, for there are many precipitous, rocky places where it would be the only practice feasible, and plants so inserted have often succeeded admirably.