ENGINEERING INSTRUMENTS. To attempt a definition of an engineering ment is hardly practicable. as the wide range of departments into which the profession is now divided demands so many special appliances for their requirements that no one description is possible and an extended catalogue is inad missible within the limits of this article. The earliest known engineering instrument was the Diopter of Hero of Alexandria, 130 B.C., al though rude appliances must have been used long before that time by the ancient engineers in the construction of . the public works of Chaldas and Egypt, the ruing of which even now awaken our admiration and wonder. It was not, however, until the beginning of the 19th century that the great impulse to the con struction and use of engineers' instruments was and lightness of construction combined with great strength and an adaptability of parts for the special service required. It is not the purpose of this article to attempt a description of the various instru ments used by engineers — this may be found in the article to give the reader a general idea of their construction.
The metals used in the construction of engi neers' instruments are principally the alloys of copper and tin with small quantities of silver, aluminum and German silver. Great care must be constantly exercised that these substances be free from iron or other materials which would given by the advance of civilization and com merce incident to the application of steam as a motive power on sea and land. Since that time great advances have been made not only in the design and accuracy of engineering in struments but also in the invention of new in struments for the many purposes required by engineers in the construction of railroads, canals, bridges, harbors, etc.
The characteristics of engineers' instruments differ in the various nations as the requirements of engineering practice and thus American en gineers' instruments possess a distinct char acter of their own as compared with other nations, having as a rule few parts affect the magnetic needle. In the construction
of an instrument such a distribution of the metals is aimed at that the greatest strength consistent with light weight may be obtained and that the metals coming into contact at the bearing surface may be of such varying compo sition as to cause the least friction.
Take, for the purpose of better illustration, an American transit, illustrated herewith, as typical, as far as the construction is concerned, of nearly all engineering instruments. The plate of the instrument on which the magnetic needle is mounted, or as it is termed, the compass circle, is turned with great care so that the surface may be absolutely true and is gradu ated usually into 720 spaces, each representing one-half of a degree.
Compass circles are usually figured in quad rants of a circle, that is, from 0 at the point marked uN" or °North" to 90 and back again, while the figuring of the limb varies with the custom of the maker or the requirements of the engineer.
In engineers' instruments, however, the angu lar measurements are made usually without the use of the needle, by a telescope so mounted as to revolve in a vertical or a horizontal plane. The angular measurement of its movement is indicated on circles divided into fractional spaces of a degree and read for convenience to finer spaces by one or more verniers. Accuracy of graduation of the compass circle, and especially of the limb, is essential to the perfection of the and great pains are taken by manufacturer- in perfecting and improving engines for Iraduating. The best machines are automatic in action and the spaces are so accurately laid off that there is no appreciable error in the finished work. The instrument rests on the socket or bearing surface to which the compass plate and limb are rays of light entering the object glass may be properly refracted and concentrated at a point called the focus.