MECHANICS AND USEFUL ARTS. 67 



KEVT MICROMETER FOR MEASURING LARGE DISTANCES. 



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At a recent meeting of the Royal Astronomical Society, England, Mr. Alvan 

 Clark, of Boston, Mass., exhibited a micrometer, invented by himself, which 

 is capable of measuring with accuracy any distance up to about one degree. 

 It is also furnished with a position-circle. Its character is essentially the 

 same as that of the parallel-wire micrometer; but it has sonic peculiarities, 

 not, it is believed, previously introduced, and on which its wide range de- 

 pends. The most remarkable of these peculiarities consists in its being fur- 

 nished with two eye-pieces, composed of small single lenses, mounted in sepa- 

 rate frames, which slide in a groove, and can be separated to the required 

 distance. A frame carrying two parallel spider-lines, each mounted sepa- 

 rately with its own micrometer screw, slides in a dovetailed groove in front of 

 the eye-pieces; and, by a free motion in this frame, each web can be brought 

 opposite to its own eye-lens. In using this micrometer, the first step is to set 

 the position-vernier to the approximate position of the objects to be meas- 

 ured. Then the eye-lenses are separated till each is opposite to its own ob- 

 ject. The frame containing the webs and their micrometer screws is then 

 slidden into its place; and, the webs having been separated nearly to their 

 proper distance by their free motion in the frame, they are placed precisely 

 on the objects by their fine screws, the observer's eye being carried rapidly 

 from one eye-lens to the other a few times, till he is satisfied of the bisection 

 of each of the objects by its own web. The frame is then removed for read- 

 ing off the measure by means of an achromatic microscope, on the stage of 

 which it is placed. One of the webs is brought to the intersection of cross- 

 wires in the eye-piece of the microscope; and by turning a screw, the revo- 

 lutions of which are counted, the frame travels before the microscope, and 

 the other web is brought to the intersection of the cross-wires. The parts of 

 a revolution are read off by a vernier from a large divided circle attached to 

 the screw. The advantages arising from the peculiar construction of this 

 micrometer are the following: ] . Distances can be observed with great ac- 

 curacy up to about one degree, and the angles of position also. 2. The Avebs, 

 being in the same plane, are free from parallax, and arc both equally distinct, 

 however high the magnifying power may be. 3. The webs are also free from 

 distortion and from color. 4. A different magnifying power may be used on 

 cadi of the objects; which may be advantageous in comparing a faint comet 

 with a star. 



DRAWINQ FROM RELIEF MODELS. 



At the last meeting of the British Association, Captain Cybulz, of the 

 Austrian army, called attention to a set of models intended to facilitate 

 instruction in the manner of delineating the features of the ground on topo- 

 graphical maps, and lately introduced into the technical schools of Austria. 

 It is the first aim of the author to lead the pupil, by means of these models, 

 to a correct understanding or appreciation of form, as the only way of pro- 

 ducing a first-rate topographical draftsman. Instead, therefore, of setting 

 him to imitate drawings from paper, his studies and copies will be made 

 from models, and, at a more advanced stage, from nature itself. These 

 models represent, firstly, inclined planes or slopes, separate, in combination, 

 or inseeting each other. It is from these the pupil acquires the first idea 

 of the principle upon which depends a correct delineation of the ground. 



