MECHANICS AND USEFUL ARTS. 119 



couple of pins. The pins rest against one of the circular edges above named, 

 and on being moved around against the circle, the blade of the square will 

 always indicate the direct line of perspective. 



CrandalTs Surveying Instrument. Mr. E. A. Crandall, of Friendship, 

 Alleghany Co., N. Y., has invented an instrument for indicating distances by 

 inspection, winch will, if completely successful, be of great value to surveyors 

 everywhere, and particularly in rough districts, where it is desirable to 

 measure across rocks and chasms, creeks and lakes. The principle is that of 

 triangulation a method always adopted in extensive and accurate measure- 

 ments, as in the Coast Survey, but which requires too much figuring for 

 ordinary field-work, and especially for the mass of excellent practical but 

 rather unlettered backwoods surveyors. Triangulation consists in taking two 

 observations of an object, either at the same instant or successively, from 

 stations a short distance apart, so that the lines in which the object is viewed 

 shall taper in a proportion corresponding according to a certain law with the 

 distance of the object. Mr. Crandall's instrument carries two small tele- 

 scopes, each provided with the usual cross wires for very accurate pointing. 

 One of these is firmly fixed to the instrument, but the other is free to swivel 

 horizontally, and is ingeniously provided with very delicate and accurate 

 verniers for reading off the angle. The distance apart of the two telescopes 

 is only one foot, but even with this narrow base the triangulation may be 

 made to indicate distances as great as a mile with a degree of accuracy 

 depending of course on the correctness of the instrument and the keen obser- 

 vation of the operator. To facilitate the measurement of considerable 

 distances, the extremely slight motion of the telescope is magnified as the 

 distance increases, by employing levers which voluntarily and successively 

 come into play, and make the very slight change of position for every ten or 

 one hundred feet very sensibly readable. The base of the triangle being 

 uniformly one foot, it follows that the instrument may be graduated not with 

 degrees, minutes, and seconds, but with the distances themselves, in plain 

 figures. The value of such an instrument, in denoting at once the distance 

 of any inaccessible as well as familiar object, can hardly be overrated. 



Proportional Dividers. An invention, by H. M. Parkhurst, of Perth 

 Amboy, N. J., consists in providing each of the legs of common dividers with 

 a short adjustable secondary leg, jointed at right angles to the middle of the 

 primary legs, and so arranged as to open and close parallel with the latter. 

 "When the dividers are opened or closed, the secondary legs will move, more 

 or less, proportionate to the distance of their points from the joint of the 

 original legs. If the points of the secondary legs are set at precise right 

 angles to the other legs, the secondary pointers will move just one half the 

 distance of the other points. The secondary legs can be set so as to exhibit any 

 desired proportion with the utmost exactness. There is a scale, set screw, &c., 

 for adjusting the angle of the secondary legs, wlu'ch facilitate accuracy. 



IMPROVEMENTS IN SAWS AND SAWMILLS. 



BarlovJs Improved Saw. Mr. Nelson Barlow is the inventor of a saw for 

 either cross cutting, splitting, or bevel cutting any kind of wood, with all the 



