OP THE UNITED STATES. 351 



strument is rendered very light, and at the same time very 

 solid, by the nature of its framing. 



It may be useful to give here the description of an addi- 

 tion which this instrument admits, and by whicii it answers 

 the purpose of a very good goniometer. 



In the middle, between the pillars c, c, is a socket which 

 receives an axis of at least one inch in length and about one- 

 third of an inch in diameter. At both ends of the rule, a 

 ^' b, there are two very thin pieces, either added to the end 

 edges, or worked out in it, in which two points are made, at 

 equal distances from the centre, and diametrically opposite 

 to each other. 



Another rule similar to the above, and of equal breadth 

 and length, is kept from bending below by an edge bar taper- 

 ing from the centre tcnvaids the two ends. This has in its 

 middle the steel axis to the socket of the other rule, whicli is 

 put upon it, and held by a screw from above, when the in- 

 strument is to be used as a goniometer. 



fn the middle, below the rule, is a centre piece, either 

 square or round, with a screw sufficiently solid to hold well, 

 and adaptable to any stand, which will of course be conve- 

 niently contrived so as to suit the motion work of the plane 

 table. This table will be descril)cd hereafter. 



A decimal scale will then be constructed on the radius of 

 the instrument considered as a unit, which therefore tnay be 

 chosen so as to serve at the same time as a scale for the 

 l^lanc tal)le operations. The cords must be measured by a 

 beam compass of proper length, ^'C. The numbers on the 

 scale may be marked at one half their value, so that when 

 the cord measured by the beam compass is applied to it, they 

 will immediately indicate the size of half tiie angle, which 

 Ijeing taken from a table of natural sines, will give the an- 

 gles with great case, and, if proper care is taken, with consi- 

 derable accuracy. 



'I'he levelling, and all adjustments of this instrument, are 

 of course exactly the same as when used on the plane table. 



