OF THE UNITED STATES. 349 



under the direction of a German, and on account of the ad- 

 vantages for which I gave it the preference in this work. 



The principles of the alhidade are simple. It may bcac- 

 curately constructed, and easily verified, ih properties 

 should be : — 



1. To level a plane. 



2. To describe an exact vertical upon this plane. 



3. To draw, or rather indicate, upon this horizontal plane 

 a line parallel to the vertical plane of an object. 



All further complications are not only useless, but always 

 prejudicial to the accuracy of some of its main properties, 

 particularly in applications similar to those intended in this 

 work. 



In Plate IX. fig. S, 3, ^ 4, a, b is a rule of about sixteen 

 inches long, three inches broad, and one-tenth of an inch 

 thick. Four pillars c, c, near its middle, rise about three 

 inches high. They support a frame d, d perpendicularly 

 across tiic rule, about six inches long. Upon the two ends 

 of this arise perpendicularly two uprights e, e of four and a 

 half inches in height, forming the supports of tlie axis of the 

 telescope. 



The telescope/.^ is a regular small transit, describing an 

 exact vertical upon the horizontal axis A, h without clamping, 

 stop circle, or any similar contrivance. It is about fifteen 

 inches long, and of about an inch and a quarter aperture. It 

 slides forwards and backwards in a tube/, ^ of foin- inches in 

 length, fixed to the square centre piece, and may, by that 

 means, be placed in equilibrium, so as to remain in all po- 

 sitions by the mere friction of the axis in its supports, and 

 the level hooks-. 



In one of the supports is the adjustment for levelling the 

 axis of the telescope, by a capstan head screw i,i. There is 

 expressly no horizontal adjustment for this axis, to eflect the 

 parallelism of the vertical plane of the telescope with the 

 sides of the rule, because this is intended to be fixed and 

 adjusted by the proper filing of the Y's in tiie sup[)oits, or 

 the placing of the frame d, d on the columns c, c. 

 vol.. II. — z 2 



