NATURAL PHILOSOPHY. 197 



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The arrows which point upward indicate circular motion in the direction 

 screw in, and the arrows which point downward, screw out; where the 

 motion is linear, the downward arrow indicates motion from right to left." 



CHAUVENET'S GREAT CIRCLE PROTRACTOR. 



One of the most valuable nautical instruments recently invented is the 

 " Great Circle Protractor," of Professor Chauvenet, of the United States Naval 

 Academy. It consists of a circular chart of the meridians and parallels of lat- 

 itude of the globe, 'in a form familiar to all navigators ; on which is centered a 

 thin transparent revolving-chart, representing to the eye the great circles 

 which pass through any given points on the lower chart. It gives by inspec- 

 tion, without the aid of scales, compasses, or other instruments, the great circle 

 or shortest route from place to place the distance as well as the course being 

 read off from the Chart or Protractor ; also the latitude and longitude of all 

 intermediate places. In sailing with head winds, the navigator can see at a 

 glance which is the best track to sail upon. The chart itself contains all 

 necessary directions, and is quite portable, being only 18 niches square. It 

 gives the azimuth and amplitude of the sun or a star with all necessary pre- 

 cision, and a sufficient approximation to the time and latitude to serve as a 

 check to the ordinary calculations. To get the latitude from a meridian alti- 

 tude of -the sun, the navigator has only to " set the point "W. on the declina- 

 tion of the sun, take the parallel of distance on the transparent chart, which 

 corresponds to the altitude of the sun, counting from the line of courses toward 

 W., follow this parallel to the edge of the fixed chart, and read off the latitude 

 of the ship." It is true that no saving of time is gained by this particular 

 process, but it is useful as a check to the usual calculations. Having the time 

 from noon by the chronometer or otherwise, and the sun's altitude, a simple 

 inspection of the chart shows the latitude. The time at the ship is also found 

 by inspection from the altitude. 



NEW SURVEYING INSTRUMENT. 



An apparatus for delineating sections of surveys for rail-roads, canals, etc., 

 and for computing the solid contents of cuttings and fillings, was exhibited by 

 M. Collin & "Wagner, at the Paris Exhibition. It consisted of a standard 

 three feet high, supported on a carriage having three wheels. From this 

 standard there was suspended a pendulum, the rod of which extended beyond 

 the suspension-point and there actuated a series of levers as it vibrated. 

 When it was desired to delineate a section of a rail-road survey, it was drawn 

 on the ground, on the proper line, and of course the undulations gave a pro- 

 portionate amount of vibration to the pendulum, which again actuated the 

 series of peculiarly combined levers mentioned. One of them operated a pen- 

 cil, and traced the undulating line of the road on a piece of prepared paper, 

 which was wound off on a cylinder. The other two moved counters constructed 

 on the principle of a calculating machine, which showed the actual amount 

 of solid contents to be excavated, from elevations, and the fillings-up to be 

 made in depressions of the road to a specific level. 



