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E. R. Watts & Son Ltd., London, England



Serial Number



Telescope length 570 mm, Height 185 mm, Base width 220 mm


Tilting Level by E. R. Watts & Son


This tilting level by E. R. Watts & Son Ltd. is a fairly large and heavy instrument featuring a relatively long (570 mm) telescope with a three foot screw levelling system.

The telescope is supported at its centre by a pivot and may be tilted (up and down) by turning the "tilting screw" which is located on the stage component of the level between the telescope and the base. This tilting screw enables the telescope to be precisely levelled before each staff reading, without altering the height of the telescope. The screw carries a graduated drum working against an index, and indicating different gradients.

The telescope (and the main tubular level with it) can be rotated by 180° about the optical axis, it the plane parallel plate micrometer is not attached. The instrument is thus a "reversible" spirit level, like the ones shown in 306 and in 307. As noted below, the optical axis does not change more than 1.5" between the two telescope (and bubble) positions. By taking measurement with" bubble right" and "bubble left", the collimation error can be eliminated on the spot.

A tubular bubble (Patent 207871) with two reflecting prisms at each end, is attached to the side of the telescope and two small tubular levels (1 div = 90") at right angle are attached to the base. When a sight is taken, the level must be approximately levelled by the foot screws with the aid of the two small plate levels, and the main bubble brought to the exact centre of its run by means of the tilting screw. The two ends of the bubble can be viewed from the eyepiece end through the two respective 90 degree prisms. The illumination windows have small covers (white painted on the bubble side) that have to be opened during measurements. The instrument 306 shows a similar arrangement.

This levelling instrument is discussed below in more detail. The instrument at hand features only one eyepiece. Most of the parallel plate micrometer attachment (shown attached to the instrument below) is missing. Only the telescope attachment with the micrometer scale (graduated from 0 to 20, 1 div = 0.001 feet) is present, however without the plane parallel plate. The adjustment screw at the eyepiece end and the connecting mechanism to the parallel plate at the objective is also missing.

The transport box contains a few labels. One label lists "Instrument 10114 (15.11.1924), (Stadia) Constant 08 feet, Stadia 1 in 200". Another one "E.R. Watts & Co, Engineers & Surveyors, Instruments & Supplies, 123 Camberwell Road, London, England".

The box also includes a test report on the temperature stability of the main tubular level by the National Physical Laboratory (NPL) in Teddington, England (Level Tube 308, 15 Oct 1924): "The result of the test showed that, taking 65°F (18.3°C) as normal, for an increase of 65°F in (18.3°C to 36.6°C) temperature the length of the bubble decreased by 0.01 inches and for a decrease of 65°F (18.3°C to 0°C) the length of the bubble increased by 0.00 inches".

This test proves the manufacturer's claim of a "constant level tube" in which the bubble remains of constant length throughout a temperature range of 130 F (36.6 C) or more (see Clark, D., Clendinning, J. 1948. Plane and Geodetic Surveying for Engineers, Volume One Plane Surveying, reprint of 4th edition, Constable & Company Ltd, London, Page 36). The cross section of this special level tube is a ellipse with the major axis in the horizontal.

History & comments

D. Clark & J. Clendinning ("Plane and Geodetic Surveying for Engineers", Vol. 2 "Higher Surveying", 3rd edition revised and enlarged, Constable & Company Ltd, London, 1944) discuss this Watts precise level (on Pages 368 and 369). The level can be read with bubble left or right since the telescope can be rotated 180 degrees about is longitudinal axis. This 21 inch (533 mm) "self-adjusting level is rather heavy for ordinary day-to-day work and is better suited for levelling across wide gaps (e.g. rivers) or for setting out work where the greatest precision is required". The authors also note that "one of these instruments, for instance, was used in the laying out and construction of (the) Sydney Harbour Bridge and the makers claim for this particular model that, to the best of their knowledge and belief, it is the most accurate levelling apparatus of its kind that has yet been made."

"The telescope is of 21 inch (533 mm) focal length with a clear aperture of 2.5 inch (63.5 mm), and is fitted with two eyepieces which give magnifications of 45 and 65 diameters respectively. With this instrument, 0.01 foot (3 mm) can be read at a distance of 1600 feet (488 m) or more, according to atmospheric conditions."

"The main bubble is accurately reversible to 1.5 seconds of arc and its sensitiveness is 6-7 seconds of arc per 2 mm. The final setting of the level before reading is made by means of a patent slow motion differential screw and a special handle is provided for reversing the telescope and bubble about the longitudinal axis of the former."

This precise level by Watts can be supplied with a parallel plate micrometer, as shown in the black-and-white figure. The readings of the micrometer are taken on a graduated arc fitted at the objective end of the telescope. The graduations are in thousands of a foot (0.3 mm) . Estimation to 0.0001 foot (0.03 mm) is possible,


In a wooden box


Good, well maintained


  • Patent No: 207871 (on level tube block)
  • The level tube was calibrated by the 'National Physical Laboratory (NPL), Teddington.'
  • Box marked "L2, Harbour Bridge". It is likely that the instrument was used during the construction of the Sydney Harbour Bridge.
  • Catalogued by T. Ko
  • Updated by F. Pall
  • Updated by J. M. Rüeger in 2015


Manufactured on 15 November 1924. Catalogued in 1997 and 2015.

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