Description of the Patent reflecting Semicircle. 187 



at once protracted in actual inagnitude on the sketch, with- 

 out the trouble of reading off. This is particularly useful 

 in military survey, where the true situations of objects can 

 at once be determined, and the sketch corrected at the same 

 time that it is taken. 



The description of the instrument is as follows: 



To the radius or limb of a semicircular (or circular) pro- 

 tractor ABC (Plate VI.) the index glass DE is fixed. The 

 horizon glass FG is fixed to a bar HI, which has a motion 

 ou the centre K. This bar slides upon a pin O attached to 

 the limb or radius carrying the index glass, which pin is 

 adjusted so that there shall be no apparent index error, and 

 exactly in the same circle with the point K. The sliding 

 bar will then move over half the real angle measured, and 

 the principal limb protract the true angle. 



Thus, the new reflecting circle or semicircle is divided 

 into 180' or 360° instead of the double number as in the 

 repeating circle, and the arc on which the divisions are en- 

 graven is equal to that of a sextant, whose radius is equal 

 to the length of the sliding bar, that is, diameter of the 

 circle. 



A vernier is applied to read off with accuracy. 



A 4-inch plotting or diagonal scale of a mile divided into 

 yards, is engraven on the fixed limb of the instrument, bv 

 which addition all the cases of trigonometry can be solved 

 by construction. 



To those who have used the common sextant, the use of 

 the reflecting semicircle will be obvious. It should be held 

 in the right hand by the end of the fixed limb, and directed 

 so that the left or the direct object is seen through the un- 

 silvercd part of the horizon glass. Apply the thumb of 

 the left hand to the end of the moveable limb, and move it 

 till the other object is seen reflected in the lower part of 

 the horizon glass, and the angle is measured, which can be 

 protracted at once. The errors or mistakes arising from 

 reading off in a hurry are thus avoided. There is no neces- 

 sity for keeping a field-book on these rapid occasions. The 

 operations of protracting the points and sketching the fea- 

 tures of ground are combined ; and the transfer of the 

 sketch to the lair sheet is much sooner accomplished. 



The above instrument is made by Mr. Gary, optician. 

 No. 182, Strand. 



XXXVH. Dc- 



