SURVEYING INSTRUMENTS. 75 



once perfected, kept secure : so much so, that it would re- 

 quire considerable violence to derange them. The horizon- 

 glass, D, alone has a contrivance for adjustment at a and 

 d, both to set it perpendicular to the plane of the instru- 

 ment, and to correct or reduce the index error, which, in 

 this instrument, had better be kept correct, as it is not so 

 likely to get out of order as in the large sextant, which, as 

 we have before observed, seldom admits of its index error 

 being rectified. The key, c, is formed to fit both squares at 

 a and d, to make the adjustments, and it is generally tapt 

 into some square place in the instrument, as at c, that it 

 may be always safe and at hand. 



It is supplied with a telescope, E, which screws into a 

 shoulder-piece, F, and can be attached to the box by the 

 screw G : this can be applied or not, at the pleasure of the 

 observer, as there is a contrivance at H to enable him to 

 observe without the telescope, if he prefers plain sights. 

 Two dark glasses are placed within the box, and there is 

 also one adapted to the eye-end of the telescope. 



The angle is read off by the help of the glass, I, which 

 being mounted with a joint, can be moved over the vernier 

 on any part of the limb. The instrument is divided to 30 

 minutes of a degree, and by the vernier is subdivided to 

 single minutes, one half of which, or 30 seconds, can be ob- 

 tained by estimation. 



The divided limb is numbered both to the right and left, 

 commencing at to 120. 



The lid of the box is contrived to screw on the bottom (as 

 is shown in the plate), where it makes a convenient handle 

 for holding the instrument. Price $35.00 to $40.00. 



Reflecting Circle. (Fig. 98, next page.) This instrument, 

 in principle and use, is the same as the sextant. It has 

 three vernier readings, ABC, moving round the same cen- 

 tre as the index-glass, E, which is upon the opposite face 

 of the instrument. One of the verniers, B, carries the 

 clamp and tangent-screw. D represents the microscope for 

 reading the verniers ; it is similar to the one used in reading 

 the sextant, and is adapted to each index-bar, by slipping it 

 on a pin placed for that purpose, as shown in the figure. 

 The horizon-glass is shown at F. The barrel, G, contains 

 the screws for giving the up-and-down motion to the tele- 

 scope ; it is put in action by turning the milled head under 



