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LEVELLING INSTRUMENTS.

It is essential to the good execution of work, that the surveyor should possess instruments most proper for the purpose, and of the best construction. Upon the subject of instruments, we shall generally refer the reader to a cheap work, entitled, "A Treatise on the principal Mathematical Instruments employed in Surveying, Levelling, and Astronomy, explaining their construction, adjustments, and use ;" where the various kinds of spirit-levels, and levelling staves, together with the method of performing their several adjustments, &c., are minutely detailed, and represented by engravings and as the work alluded to contains also a similar account of the most important instruments used in surveying and astronomy, and has had an extensive sale, we presume it to be in the hands of most beginners in the profession; we shall, however, give some particulars in this place, and annex a description of the cause of, and a remedy for, the parallax between the wires of a levelling telescope, and the levelling staves, which is the cause of much annoyance to observers.

SPIRIT LEVELS.

The Y level, so called from the supports in which the telescope rests, resembling in shape the letter Y, is the

* Also a work published by Mr. Weale, on Drawing Instruments, with Instructions for Field Work, in 12mo, price 3s. 6d.

oldest construction of the spirit-level now in use; its adjustments are convenient to be performed, but, on the other hand, this kind of instrument seldom retains its adjustments perfect for any length of time; besides, there are conditions in its construction which are assumed to be perfect, but which practical men know to present difficulties in the manufacture. The use of this instrument is now very much superseded by those of modern construction.

Troughton's Improved Level. This instrument has been a very general favorite among engineers for a length of time; its construction renders its adjustments much more permanent than those of the Y level, and it is altogether a more stable instrument. The telescope, which, in the former instrument, is capable of reversion on its supports, for the adjustment of the line of collimation, is, in Troughton's construction, firmly fixed in its place, as is also the glass tube of the spirit bubble. The verification and correction of the adjustments are performed very differently, and may at first appear more complex and difficult than those of the other; yet when a person has once mastered and become familiar with his instrument, these apparent difficulties vanish.

The Dumpy Level. This modification of the spiritlevel has but recently been in troduced by William Gravatt, Esq., and bids fair to become the favorite instrument among civil engineers. In its general figure it does not differ very essentially from the level last spoken of, but it possesses many decided advantages.

The aperture of the object glass is much larger for the same length of telescope; consequently more rays of light are admitted to the eye, producing the advantages of greater distinctness. We lately tried a fourteeninch level, constructed upon Mr. Gravatt's principle, and found that we could distinctly read the levellingstaff at twenty chains (a quarter of a mile) distant, which was the utmost we could do with a twenty-inch level upon the old construction; we have, therefore, the advantage of a more portable instrument, fourteen inches in length, capable of performing the same work as a more cumbersome one of twenty inches. Besides this advantage, the instrument in question is more complete in its details. It possesses a cross level, placed at right angles to the principal level, which affords very great facility in setting up the instrument, and adjusting for observation, as will be hereafter described; it likewise has a reflecting mirror, mounted with a hinge joint, and capable of being placed on the principal level tube and adjusted, to show the observer if the instrument shifts from its horizontality whilst he is noting the observation; it also possesses other important though minor additions, all of which, in fact, could be applied by the maker to the other kind of instruments, if ordered, and for the particulars of which we refer to the work before alluded to.

From the large aperture and short focal length of the telescope, the instrument has altogether a dumpy appearance, and hence it is generally known by the cog

nomen of "Gravatt's Dumpy Level;" usually of nine or fourteen inches. We have seen some beautiful specimens of this kind of levelling instrument constructed for I. K. Brunel, Esq.

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LEVELLING STAVES.

In the Treatise on Mathematical Instruments will be found a description of the different kinds of levelling staves in use. The former construction, even as improved by Troughton, was decidedly defective in practice, inasmuch as the staff had to be read off by the assistant, who had then to communicate the result to the observer; or, if he was not sufficiently intelligent to be intrusted with so responsible a duty, he was obliged, after the observation was made, to carry the staff to the observer, or wait for him to come and read off the height of the vane, and register it in his fieldbook. This occasioned great loss of time and uncertainty in the results, for the vane on the staff might possibly be shifted in the mean time.. We remember an instance of an ignorant attendant holding the staff upside down, which at once introduced an error of several feet in the result. To obviate this, a new staff has been contrived, originally, we believe, by Mr. Gravatt, and subsequently by Mr. Hennett, Mr. Bramah, Mr. Sopwith, &c., each varying the mechanical arrangements, but all agreeing in retaining the main advantage, viz., a sufficiently distinct graduated face for the observer to read off the quantities himself through

the telescope of his instrument; the sliding vane is therefore dispensed with, and the only dependence to be placed on the staff-holder is, that he may hold it perpendicularly. To assist him in this, a small plummet is suspended in a groove cut out in the side of the staff, by which its verticality can be determined in one direction, and the observer himself can detect if it be held aslant in the other direction, as may be understood from the diagram at page 26, which represents the staff e as it appears in the field of the telescope, which shows objects inverted. If the staff be held perpendicularly, it will appear between and equally distant from each of the two vertical wires c d, fixed in the telescope; consequently, if it be held aslant, it will cross the wires obliquely, and any want of verticality in the staff will be immediately detected, and the observer must signal to the staff-man accordingly. The advantages from the use of the modern staves, over those of the old construction, are so great, especially in saving of time, that we have no doubt of their general adoption.

THE IRON TRIPOD.

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