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spirit-bubble at the same time, and then he can make his observation at the instant he sees the bubble in its proper position. The foregoing description of the method of taking levels is general, and applies equally to every kind of levelling operation, with whatever additional matters may require attending to, when taking levels for the formation of a section, &c., which we shall hereafter describe.

The first levels being completed, the surveyor must take up his instrument, and, passing the man who holds the forward staff, proceed to some convenient spot to set up the instrument a second time, which, as before remarked, should not be more than four or five chains distant; the other man, also, who held the staff at the back station, must likewise take up a new station still further onwards in the required direction, and as nearly as possible at the same distance from the instrument as the instrument is from the staff, which has now become the back station; it being in every case necessary, to insure correct work, that the instrument should occupy very nearly the middle point between the staves, for reasons which will be understood by those who have perused the former part of this book. Having set the instrument up, adjust it for observation as before, viz., see that the cross wires are distinct; turn the milled head by the side of the telescope till the graduation on the staff is quite distinct, and no parallax exists; and, lastly, set the spirit-bubble level in every direction of the telescope by the parallel plate screws; which done,

note the reading on the back staff, and enter it in the book; then examine the bubble, and again read the staff to insure accuracy; then turn the telescope about, and do the same for the forward station, which will complete the second level. As the third and fourth, and all the following levels are conducted in precisely the same manner, it will be unnecessary to repeat the instructions again.

The man holding the back staff should be instructed never to move it in the least from its position till the forward observation is completed, which he can always tell by seeing the surveyor carry his level onwards. It is sometimes the practice to use one staff only, and after taking the back observation, to cause the assistant to go on and take up a position suitable for a forward station; but besides the loss of time attendant upon such a process, if the instrument should in the interval get moved by accident, those two observations will be incorrect, unless the back sight be taken again, and this cannot be done unless the precise spot before occupied by the staff can be identified, which is sometimes uncertain. When this is the case, no alternative is left but to go back and renew the work at the last bench mark, or known station; and if none such exist, the whole operation will probably have to be gone over again, where great accuracy is required.

The iron tripod, described at page 20, should in all cases be placed on the ground by the staff-holder, to rest the staff upon, as it insures to the observer the

certainty of the staff keeping exactly the same spot when the face of it is presented to him in the two directions, forward and backward. The staff-holder should likewise be instructed to hold the staff perfectly upright, which he can himself determine, in one direction, by a little plumb-weight suspended in a groove in the staff; and as the observer can tell if he holds it upright in a lateral direction (as explained at page 20), he should frequently look to see if he signals for him to move the upper end of the staff to the right or left, taking care not to disturb its position on the iron tripod.

We have been supposing the use of the newly introduced staves, as we do not expect that those of the former construction will hold their ground against them, they having the advantage of providing to the observer the means of noting the reading of the staff himself. If, however, from habit or otherwise, the use of the staff with the sliding vane should be preferred, the foregoing instructions equally apply; the only dif ference in its use is, that the observer must signal to the staff-holder to move the vane up or down on the staff, till it appears bisected by the cross wires of his telescope; then the reading of the staff must be noted, and entered by the assistant in a temporary book carried by him for the purpose; or if he cannot be trusted to perform so important a part of the business, he must convey the staff to the observer, or wait for him to come and read it himself. It requires no comment to

show the uncertainty, and loss of time, in this method of proceeding compared with the use of the newlycontrived staves.

Having explained the method of taking observations for checking levels, we must refer to our example. The levels, as before stated, were taken along the public road shown by the dotted line, that being the most convenient route from the town of A to the town of B, avoiding the necessity of passing through private property; the strong black line on the plan shows where the original section was taken; the section itself is represented above the plan, and is drawn to two scales; the one giving the horizontal measure, is the same as that of the plan, viz., one inch to one mile ; and the vertical scale, inch to 100 feet; from this section it appears that the crown of the bridge at A is fourteen feet above the datum line D E of the section, and that the bench mark (a stone by the road side) at B is 111 feet above the same datum; therefore the difference of level between the two places is 111 14 = 97 feet. Now, by referring to our observation book, of which we have subjoined a copy, we make the difference of level to be 96.8 feet, differing from the original section no more than two-tenths of a foot, or 2.4 inches, a quantity that may be disregarded; the inference, therefore, to be drawn from such a coincidence in the two results is, that the whole of the section between the points in question is sufficiently correct.

Copy of Field-Book, for running or check levels.

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