netic north nearly, screwing it fast by means of the hut H, and also the head of the instrument by means of the pin L; I make the north point in the compass box coincide with the needle very exactly, by turning the screw K; both telescopes being then in the magnetic meridian, I look through the lower one I, and notice what distinct object it points to.

"Then unscrewing the moveable index by the aforesaid nut H, the first assistant having been pre viously sent to the station point marked N, I turn about the telescope A, or moveable index, by means of the nut G, till it takes up the pole now placed at N, raising or depressing the telescope 'by means of the nut E, till the cross hairs or wires cut the pole near the ground. This done, I look through the lower telescope to see that it points to the same. object it did at first; if so, the bearing or angle is truly taken, and reading it upon the limb of the instrument F, find it to be 40° 55′ S. W. I then take my sheet of paper, and placing the horn protractor upon the point A, plate 21, along the meridian line, passing through it, I prick off the same angle, and with the first ruler draw a faint line with the pen, and by the scale set off the length of the line, which I find to be, by my first sketch, plate 20, 7690 links; and writing both bearing and distance down, as in plate 21, and again reading off the angle to compare it with what I have wrote down, I then make a signal to the first assistant to come forward with his pole; in the mean time I turn the moveable index about, till the hair or wire cuts the pole which the second assistant holds up at B, and looking through the lower telescope, to see that it points on the same object as at first, I read the angle upon the limb of the theodolite, 48° 55′ S. E. and plotting it off upon the sketch with the horn protractor, draw a straight line, and prick off by the scale the length to B, 817 links; writing the

same down on the field sketch, and again reading the angle, to see that I have written it down right, I screw fast the moveable index to the limb, by means of the screw H, and making a signal to the second assistant to proceed with his pole, and plant it at the third station C, while I with the theodolite proceed to B, leaving the first assistant with his pole where the instrument stood at A.

"Planting the instrument by means of a plumbline over the hole, which the pole made in the ground at the second station, and holding the moveable index at 48° 55′ as before, and the limb levelled, I turn the limb till the vertical wire in the telescope cuts the pole at A nearly, and there screw it fast. I then make the vertical wire cut the pole very nicely by means of the screw K; then looking through the lower telescope to see and remark what distant objects it points to, I loosen the index screw, and turn the moveable index till the vertical hair in the telescope cuts the pole at C nearly, and, making the-limb fast, I make it do so very nicely by means of the screw K, and then fix the index. Reading the degrees and minutes which the index now points to on the limb, I plot it in my field sketch as before, the bearing or angle being 48° 30′, and the length 885 links. Sending the theodolite to the next station C, the same operation is repeated; coming to D, besides taking the bearing of the line DE, I take the depression from D, to the foot of the declivity 600 links below it, which I perform thus: Having the instrument and the double quadrant level, I try what part of the body of the assistant, who accompanies me for carrying the instrument, the telescope is against, and then send him to stand at the mark at the bottom of the declivity, and making the cross wires of the telescope cut the same part of his body equal to the height of the instrument, I find the depression pointed out upon the quadrant to be of a link upon each chain,

which upon six chains is four links to be subtracted from 1388 links, the measured length of the line, leaving 1384 links for the horizontal length, which I mark down in my field sketch, plate 2, in the manner there written. Also from the high ground at D, seeing the temple O, I take a bearing to it and plotting off the same in my field sketch, draw a straight line; the same temple being seen from station L, and station M, I take bearings to it, from each; and from the extension of these three bearings intersecting each other in the point O, is a proof that the lines have been truly measured, and the angles right taken.

"Again, from station E to station F, the ground rises considerably, therefore, besides the bearing, I take the altitude in the same manner I did the depression; and do the same with every considerable rise and fall round the circuit.

66

Coming to station N, and having the moveable index at 61° 15′ N. W. taking up the pole at M, and making fast the limb, I loosen the index screw, and turn it till I take up the pole at the first station A. This bearing ought to be 40° 55′ N. E. being the same number of degrees and minutes it bore from A south-west, and if it turn out so, or within a minute or two, we call it a good closure of the eircuit, and is a proof that the angles have been -accurately taken.

"But if a greater error appears, I rectify the moveable index to 40° 55', and taking up the pole at A, make fast the limb, and take the bearing to M, and so return back upon the circuit again, till I find out the error. But an error will seldom or never happen with such an instrument as here described, if attention is paid to the lower telescope; and besides, the needle will settle at the same degree (minutes by it cannot be counted) in the box, as the moveable index will point out on the limb, provided it is a calm day, and no extraneous matter to attract

it. If it does not, I then suspect some error has been committed, and return to the last station to

prove it, before Í go farther.

"With regard to plotting off the angles with the horn protractor in the field, much accuracy is not necessary; the use of it being only to keep the field sketch regular, and to preserve the figure of the ground nearly in its just proportion.

"Coming home, I transfer from the first field sketch, plate 20, all the intermediate distances, offsets, and objects, into the second, plate 21, nearly to their just proportions; and then I am ready to proceed upon surveying the interior parts of the circuit. For doing this, the little light theodolite, represented at fig. 4, plate 15, or the more complete one, fig. 7, plate 14, will be sufficiently accurate for common surveying. It is unnecessary for me to describe farther the taking the angles and mensurations of the interior subdivisions of the circuit, but in doing of which, the young surveyor had best make use of the horn protractor and scale; because, if he mistakes a chain, or takes an angle wrong, he will be soon sensible thereof by the lines not closing as he goes along; the more experienced will be able to fill in the work sufficiently clear and distinct without them. The field sketch will then be such as is given in plate 22, which indeed appears rather confused, owing to the smallness of the scale made use of to bring it within compass; but if this figure was enlarged four times, so as to fill a sheet of paper, there would then be room for entering all the figures and lines very distinctly.

"Having finished this first circuit; before I begin to measure another, I examine the chain I have been using, by another that has not been in use, and find that it has lengthened more or less, as the ground it has gone through was rugged or smooth, or as the wire of which it is made is thick or small;

the thick or great wire drawing or lengthening more than the small.

"The careful surveyor, if he has more than two or three days chain work to do, will take care to have a spare chain, so that he may every now and then correct the one by the other. The offset staff is inadequate for this purpose, being too short. For field surveying, when no uncommon accuracy is required, cutting a bit off any link, on one side of 50 links, and as much on the other side, answers the purpose better than taking away the rings.

"The above method is what I reckon the best for taking surveys to the extent of 100,000 acres; be-yond this, an error from small beginnings in the mensuration becomes very sensible, notwithstanding the utmost care; therefore, surveying larger tracts of country, as counties or kingdoms, requires a different process. Surveys of this kind are made from a judicious series of triangles, proceeding from a base line, in length not less than three miles, measured upon an horizontal plane with the greatest possible accuracy. Thomas Milne."

OF

PLOTTING,

OR MAKING A DRAUGHT OF THE LAND FROM THE FIELD NOTES.

OF PLOTTING, AND OF THE INSTRUMENTS USED IN PLOTTING.

By plotting, we mean the making a draught of the land from the field notes. As the instruments