early hour of that day, and he immediately commenced a series of observations on all the instruments, at short intervals. After a time, the disturbance became so small, that the extraordinary observations were discontinued; but it again increased, and observations were again immediately made with all the instruments, and repeated as rapidly as it was possible for one observer to do so.

But for the promptitude and judgment displayed by Mr. Glaisher on this occasion, the record of this disturbance, unprecedented in this magnetic latitude, would probably have been lost.

The following statement will give an idea of the magnitude of the disturbance-that within eight minutes of time, the declination needle changed its position more than 24 degrees, (having passed in both directions the range of the observing telescope, which includes that angle); that the vertical force was increased by more than 4th of its whole value, the instrument having then reached the extremity of its range; and that the horizontal force was increased aboutth of its whole value.

Mr. Glaisher observes, "that early in the morning of September 25, the needles were in an agitated state. During the appearance of an aurora, additional observations were taken, and the declination needle, in less than three hours, traversed an arc of 34 minutes. After this the needles were in a tolerably quiet state, and extraordinary observations were discontinued. The observation at 10h A.M. shewing a change of 17 minutes of an arc from the previous reading at 8h A.M. In 2s or 3 the needle frequently moved through many mi

nutes of an arc, and then suddenly became stationary again.

"At 3h 36m 20° P.M., a bold sweep carried the north end of the needle towards the west, 56 minutes of an arc, in one minute of time."

We think it unnecessary to trespass on the reader's time by detailing more of these extraordinary observations, the foregoing extracts being quite sufficient to show what little reliance should be placed on the common circumferentor, as a surveying instrument, by which angles are taken on the supposition that the needle always retains a parallel position, or is directed to the same point of the compass during the progress of the survey a thing that does not always happen, as is proved by the foregoing observations, as well as by similar ones made by others at different times and places.

Mr. Lecount; an eminent engineer, and author of a new work on railways, conducted a series of observations on the magnetic variation, when a midshipman in the Royal Navy, with a view to ascertain the law by which it is governed; and the result of his enquiries and observations on this subject led him to assert positively that the needle does not retain a parallel position. In his work on this subject, published in 1820, he says, "it has been long known that ship's compasses will point differently the instant after the ship has been tacked; that they will often agree in some positions of her head, and materially disagree in others; and placed in different parts of the ship, they will point differently.

"There did not remain a doubt in my mind, but that these variations in the compass were the causes of those

numerous errors into which ship's reckonings have been brought, and which, as Mr. Bair remarks, are too often charged to currents, &c. Mr. Lecount however attributes the changeable position of the needle to local causes, such as the particular position of the needle, in reference to bars of iron, guns, hoops, &c. variously posited. He found that a bar of iron, for instance, placed in one position, affected the needle differently from the same bar placed in a different position-a phenomenon known to every naval officer who pays attention to such subjects. In the same manner, the surveyor's circumferentor is deranged when in the vicinity of iron ore, or of a building containing iron. Indeed any perpendicular object in the immediate neighbourhood of the instrument is known to affect the parallelism of the needle, rendering the result of surveys effected by it, if not erroneous, at all events uncertain.

From the result of these observations, and that of others noticed in another part of this work, it is evident that the practice of surveying by taking the bearings of the different station lines, cannot be too soon abolished. Its use may be permitted in subterranean surveys, as also in the surveys of new countries, where land is of little or no value; but where land is valuable it should never be employed.

The theodolite is quite free from the errors entailed by the needle: in taking angles by this instrument, no dependence is placed in so unsteady an adjunct, but upon its own separate parts, which are admirably adapted for the purpose. And though the theodolite is always furnished with a compass, it is never used, except in giving the bearings of some of the lines to mark the

position of the map in reference to the meridian, and also to serve as a sort of presumptive proof of the accuracy of the angles. The theodolite has the advantage of a good telescope, to render objects at a distance distinctly visible. The circumferentor has no such help. Unassisted by the telescope, the distance at which an object can be seen distinctly must be comparatively limited. The horse-hair in the circumferentor is also a source of inaccuracy-from its magnitude and proximity to the eye, it subtends so large an angle as to completely cover an object at a distance.

To Find the Quantity of an Angle with the Circumferentor.

Α

Let the angle proposed be ABC; place the instrument at B, with the flower-de-luce of the card towards you; then direct the B sights to C, and observe what degrees are cut by the south end of the needle, which let be 259; then turning the

instrument about on its stand, direct the sights to A, noting again what degrees are cut by the south end of the needle, which let us suppose to be 210. Subtract the less from the greater, and the remainder, 49 degrees, is the measure of the angle ABC.

How to Survey and take Field-notes by the Circumferentor.

The following directions are taken from Gibson's Surveying :

Let your instrument be fixed at any angle, as A,

G7

A 1

B 2

your first station, and a person stand at the next angle B; or cause a staff with a flag to be set there; then having placed your instrument horizontally (which is easily done by turning the box, so that the ends of the needle may be equidistant from its bottom, and play freely); turn the

F6

D4

C 3

flower-de-luce or north part of the box to your eye, and looking through the small aperture, turn the index about, till you cut the object at the next angle B, with the horse-hair of the opposite sight; the degrees then cut by the south end of the needle give the number to be placed in the second column of your field-book, in a line with station No. 1, and express the number of degrees the stationary line is from the north, counting quite round the sun. Having taken the bearing of the first stationary line AB, let the line be measured, and the length thereof, in chains and links, be inserted in the third column of your field-book, opposite to station No. 1. Having done with your first station, set the instrument over the spot where your object stood, as at B, for your second station, and send your assistant forward to the next angle of the field, as at C; and having placed the instrument in a horizontal direction, with the sights directed to the object at C, and the north of the box next your eye, count your degrees to the south end of the needle, which register in your field-book, in the second column, opposite to station No. 2; then measure the stationary distance BC,