137 



C. L. Berger & Sons' Engineers' Precise Level. 



Patented. (.For cut see Page I3Q.} 



With micrometer screw for tfose setting the spirit level. 



For use in cities in establishing benches, etc., alno for all work requiring upeed and the highext 

 degree of accuracy in t>pirit leveling. 



It is a well-known fact that, satisfactory as it may be on account of its great sim- 

 plicity and compactness, the ordinary wye level (pp. 132, 133) will fail in degree of ac- 

 curacy or in rapidity of manipulation when the closest results are required. It often 

 happens when precise work is required, the time spent in leveling up and keeping the 

 level bubble of an ordinary good wye level in the center of its graduation by means of 

 the four leveling screws is often very considerable and, when the course is over 

 swampy or frozen ground, the vexation attending the work is apt to be great, and the 

 results vitiated l>y the numerous readjustments required to keep the bubble in its 

 place. This manipulating of the leveling screws is very apt to lead to a change in the 

 height of the telescope, varying in magnitude according to the style of the instrument. 

 (It is here to be noted that this change in the height of the telescope is less in our 

 levels, or transits with leveling attachments, than is the case with the instruments 

 of other makes). 



To aid the Engineer in the prosecution of exact work, avoiding the errors caused 

 by the readjustments above referred to, we have designed and are prepared to furnish 

 the instrument shown on page 139. 



By referring to the cuts it will be seen that this instrument is mounted on three 

 leveling screws, and that the center about which the instrument revolves is unusually 

 long and unyielding. Two small spirit levels attached to arms extending from what 

 we may call the cross-bar (since the center of the instrument is permanently secured 

 to it as in the ordinary style of levels) serve to put the center in a vertical position, 

 thus securing at once a nearly horizontal position to the cross-bar. These small levels 

 are adjusted the same as the ordinary plate levels of a transit. 



At the eye end this cross-bar carries a micrometer screw by which the telescope 

 and its level can be raised or lowered at will independently of the leveling screws. A 

 strong spiral spring on the same side holds the wye-bar down upon the micrometer 

 screw. This arrangement provides a most delicate motion up and down, and enables 

 one to set the bubble accurately at every sight and in a very much better manner than 

 can be done by the leveling screws alone. The head of the micrometer screws is di- 

 vided into one hundred parts, and as a rule its pitch will be such that 250 to 252 parts 

 of revolution of the screw will make a change of one foot in the reading of the rod held 

 at a point 100 feet away from the center of the instrument. It may be seen that the 

 instrument can be very advantageously used for making grade measurements. The 

 graduated disc, when reading zero on the index-bar, brings the instrument at once 

 within one or two divisions of its normal position. The disc can also be readily turned 

 on its hub by taking hold of the milled head (the disc is held on its arbor simply by 

 friction), so that, for convenience, a reading may always start from zero, though the 

 cross-bar be not leveled up. This instrument, as above stated, is provided with three 

 leveling screws, which give a firm support on the tripod, and allow a closer setting of 

 the bubble when the instrument is run as an ordinary wye level, without making use 

 of the micrometer. (Seep. 38.) 



The Chief Feature of the Instrument, however, consists in the fact that the 

 pivots * on which the wye bar can be raised or lowered, are in the middle of the instru- 

 ment and within a fraction of an inch of the plane of the line of collimation, thus se- 

 curing to the telescope a motion in altitude free from any change in height of the line 

 of collimation, though the telescope were to move throughout the entire range of the 

 micrometer screw during an extended leveling operation. As a rule, the working range 

 of the micrometer will be limited to a few revolutions each way from its normal posi- 

 tion in order to keep the instrument as compact as possible. The instrument is also 

 arranged so that, whenever desirable, it may be used as an ordinary wye level. For 

 this purpose, it is provided, at the object end of the cross-bar, opposite the micrometer 

 screw, with a milled-head screw and check nut, by means of which, and by the micro- 

 meter screw, when set at zero (see cul), the wye-bar may be set exactly at right angles 

 to the vertical center. However, for the fine settings of the bubble in bench leveling 

 or pointing of the telescope, etc., the micrometer screw should be used exclusively. 



A clamp and tangent screw motion is also provided and so arranged, that it can 

 be readily reached from the eye end of the telescope. The cross and wye-bars are 

 cast hollow and the former fits inside the latter. 



*NOTE. It will be noticed that in instruments of a similar character, having pivot screws acting in and below 

 the wye opposite the micrometer screw, as for instance, in the U. S. Coast Survey geodesic levels, designed 

 after Stampfer (see Report 1879), any motion of the telescope in altitude will also change its height. By an 

 injudicious use of the micrometer screw our own hydrographic wye level (see page 1043, catalogues 1888-1891). 

 partook of this same error, and this together with the marked wear on the collars due to this same motion led 

 us to the abandonment of it. We note, however, that other firms who are in the habit of copying our styles 

 and patterns have since brought it out as a detail of a precise leveL 



