moderate breezes. This led the author to contrive a levelling staff, obviating the objections here pointed out, It consists of two pieces, which are joined together by a peculiar sort of hinge, not very unlike a carpenter's rule. When closed, the graduated face is concealed, and therefore protected from injury or accident incidental to coach travelling. When unfolded, its face presents one uniform surface throughout, which makes it as easy to read the smallest subdivision at the bending joint as at any other part of the staff, which is not the case when the graduated face is not in the same plane. As one part does not slide in a groove within another, like those above described, it is of uniform strength, capable of resisting the pressure of the atmosphere in tolerably moderate weather, and free from the oscillating motion resulting from the weakness of the upper pieces, which must necessarily be very thin, so as to fit within the lower ones, in order to render them portable, which no doubt, the inventors had in view in their peculiar construction.

Unlike those, the author's staff is not affected by damp weather or dust, which renders it difficult to move the inner pieces up and down the grooves in which they fit. His is unfolded for use and closed for carriage in an instant, by merely touching a single bolt. The assistant holds it upright by means of a plumb, which is inserted in its side. It turns round freely upon an iron plate attached to its lower end, by which the error very often committed by the assistant, in turning the graduated face from the last forward station to become the next back-one, is prevented. Without such a plate the most careful and experienced assistant might be

liable to commit an error of small amount, especially in wet weather and soft soil, into which the staff might be apt to sink after having been reversed: or in lifting it up, a clod might adhere to its lower end, that would be apt to be a source of error in reversing it. In putting down the staff, it should be pressed against the ground, so that the spot should not yield on turning it round.

Much praise is due to Mr. Gravett for his improvement, not only in the levelling-staff, but also in the levelling instrument which is called after him.

Those gentlemen who have made further improvements in the staff, deserve the thanks of the profession. The improved telescope attached to Mr. Gravett's levelling instrument, and the peculiar graduation of the staff which is used in connection with it, enables the observer to read, and note down in his field-book, the several observed heights, by which accuracy is insured, and much time saved. The improved telescope inverts the figures on the staff, which led Mr. Brough to contrive one with the figures turned upside down, so as that when viewed through the telescope they may appear upright.

This is, however, a matter of little consequence, as after a little practice, the figures can be as easily read in one position as another. Care must be taken not to mistake the figure 6 for 9, one answering for the other when inverted. This is a mistake which cannot be committed in using the author's staff, as the figure 6 is exactly at the hinge, by which it is distinguished from the figure 9.

ON LEVELLING OPERATIONS IN THE FIELD.

It will appear, at once, that to obtain the difference of level between any number of places, and from thence to delineate the contour of the surface, some reference must be made to a point from which to commence the operation. In maritime countries the several heights are counted from a point which has reference to the surface of the sea; but in situations very remote from the sea, some visible permanent mark is assumed at either end of the line, from which the several heights along the undulating surface of the ground are reckoned. A line drawn from this point, or from a point below it, called the datum line, is that upon which the several levels are based, and by means of which the section of the line is drawn. The direction of this is parallel to the horizon. The point through which the datum line is drawn, has been differently assumed by different engineers; some making it to coincide with high-water mark, spring-tides; while others assume for its position low-water mark, spring-tides. The late Mr. Nimo, whose friendship the author long enjoyed, assumed highwater, ordinary spring-tides, as his datum line, in laying down the Dublin and Kingstown Railway. When a section of a line is represented on paper, particular care must be taken to note the point or datum line from which the levels were reckoned, that reference may be made to it whenever a case, making such reference necessary, may arise.

This indeed is a legislative enactment, the necessity of which was forcibly exemplified in the memorable action brought against the Dublin and Kingstown Railway Company, by certain land owners along the line,

for damages sustained by them from the overflowing of the river Dodder, which was alleged to arise from an alteration in the construction of the line and bridge; lowering the carriage-way many feet below the level shown in the original section deposited in the Clerk of the Crown's office, and thereby diminishing the waterway under the bridge. The case in dispute, (in which the author was employed, on the part of the Hon. Sydney Herbert,) could not have been well decided without a fixed datum line, which in this case passed through high-water mark, ordinary spring-tides, as shown in the original section.

In assuming a datum line, its position might be taken below the lowest point on the line. This may be effected by ascertaining how many feet below the commencement of the line, the lowest point in it is; then assuming any number greater than this for the distance of the datum line below the commencing point, will leave the contour of the surface wholly above the datum line; which we should always recommend.

As we are upon this subject, it may be well to mention, that the high water-line is far from being fixed or permanent, even in the same locality, owing to the variety of local and inconstant causes which conspire to raise or depress it above or below the exact point prescribed by the abiding governing laws of matter. This remark applies in a special manner to the tides in rivers, channels, bays, and estuaries, which rise at different times to different heights, just according to the particular cause then operating. With respect to the low-water line, it is found sometimes to be above the natural surface of the ocean. This may occur in narrow channels,