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187.—Circular Levels have been made tentatively for a long period. They consist of a worked concave lens fitted into a brass cell with indiarubber seating, the glass being secured by burnishing over a bezel. This construction answers very well when new, but the spirit the level contains is certain to evaporate slowly under every possible care. Mr. J. J. Hicks has patented a hermetically sealed circular level, in which he has succeeded in working the upper surface of the glass to curvature. These levels, of course, are not subject to evaporation, and are very useful and portable for approximate levelling—as for plane tables, cameras, etc.
Fig. 53.—Hicks' patent circular level.
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188.—Surveyors' Levels, of which there are many forms, consist essentially of a telescope with a diaphragm at the mutual foci of the objective and the eye-piece, the axis of the telescope being placed in a direction truly parallel with the crown of a sensitive level tube. The telescope with its level is mounted upon metal frame-work, carried up from a vertical axis upon which the telescope rests. The vertical axis is adjustable in relation to the axis of the telescope, so that they may be brought perfectly perpendicular, the one to the other. The whole instrument is also adjustable to a position of verticality of its central axis, and the horizontality of the telescope in relation to the surface of the earth in what is termed the setting-up of the instrument; so that when it is set up in this position levels may be taken from it in any horizontal direction from one point of observation, by rotation of the telescope about the vertical axis. Having these essential objects in view in the construction of the level, the form of the instrument may be varied as to details according to the mechanical skill and taste of the maker and the special demands of the civil engineer.
189.—In the design of a surveyor's level very important considerations are:—That the metal should be so distributed that every part is as light as possible, consistently with sufficient solidity to take a moderate amount of accidental rough usage, and ensure freedom from vibration; that the whole structure should be in equilibrium about its vertical axis when the telescope is extended at mean range, that is, at about the focus of three chains—this is a quality often neglected; that there should be sufficient light in the telescope, and that it should possess a firm and durable stand. Every form of level should embrace these qualities.
190.—The Oldest Form of Surveyor's Level is that termed the Y-level, so named from the telescope being supported in Y-formed bearings. This instrument was originally invented by Jonathan Sisson, a leading instrument maker of the 18th century. It was much improved and brought nearly to its present state of perfection by Ramsden, to whom practical opticians owe so much for many advancements of their science, and to his liberal publication thereof. This instrument is now very little used in Great Britain; but it still maintains its original position, to a certain extent, on the Continent and in America. In the eyes of the optician it is still the most perfect level, possessing all the instrumental refinements of adjustment he can desire. The reasons for its partial abandonment by the profession will be discussed further on.
191.—The Y-Level in a modern form is represented in the engraving below, Fig. 54. The Y's are shown at YY″ edgewise. They are supported by standards SR upon the limb L. The telescope is surrounded by two collars which are soldered upon it at positions exactly corresponding with the Y's. The collars are turned perfectly cylindrical and parallel on the surface with the axis of the telescope, and ground in a gauge-plate to exact size so that the telescope may be turned end for end in the Y's without altering the linear direction of its axis in reversing it. The telescope is held from shifting longitudinally in its Y's by a pair of flanges placed on the inside of the collar pieces.