Gentilli's Tacheometer

Abstract

AMONG the instruments exhibited in the South Kensington Loan Collection is one likely to prove of great use in survey-making; it is the invention of M. Gentilli, an Austrian engineer, and its main purpose is to accomplish rapid surveys (hence its name) of difficult country. Not only does it survey the ground, but it gives the height and distance of every point surveyed. The instrument itself differs little from an ordinary surveying telescope. A vertical lever, A, is attached to the axis of the telescope by means of a screw, C (in figure); this lever moves the axis through a given angle, which can be exactly adjusted by means of the two stops, B B′, opposite the free end of the lever. The points to be surveyed are marked by a surveying staff, on which are shown in a manner to be visible at a great distance, very minute divisions of a foot. The telescope is pointed to this distant staff of which it measures: (a) the horizontal angle of position, (b) the vertical angle of elevation, (c) the distance of the instrument from the staff. It is the accuracy with which this last datum can be read that is accomplished by the peculiar mechanism of Gentilli's instrument. As an example: Suppose the staff marked with divisions, to have a scale of 12 feet, on which feet, inches, and eighths of an inch are shown. The telescope is directed to the top of the scale, of which it gives the horizontal and the vertical angle. It is next directed downwards by the screw to a fixed stop, and there it reads on the staff, say 10 feet 5 inches below the former reading; that distance on the staff is 1,000 eighths of an inch, and tells us that the staff is 2,000 yards off. In short, the greater the angle through which the telescope is moved, the greater the distance and vice versâ, Gentilli's telescope reading the distance and giving it exactly as read, without calculation of any kind. The mechanism is so precise that the telescope can be moved through any given angle and restored to its original position with almost perfect accuracy. Practice has shown that the distances so measured by a small instrument of only 40 magnifying power are correct to within 1/2000 part. The instruments seems likely to be of the greatest use both to ordinary surveyors and to those who have to carry on extensive topographical operations.