OF IHE UNITED STATES. S87 



Description of the Two-feet Theodolite. 



Plate V. presents a general perspective view of the in- 

 strument ; and Plate III. fig. 7^8, the details of the centre- 

 work in a horizontal and veriical section. 



The horizontal circle aa is of two feet diameter, divided 

 on a silver arch to every five minutes. It is fastened on 

 six conical hollow radii bb, proceedinp; from a strong hollow 

 hexagonal centre piece c of six inches in diameter, to which 

 tliey are screwed fast each by four strong screws passing from 

 the inside of the piece r into the strong base of the cone. 

 'I'lic diameter of the cones is three inches at the base, and ah 

 inch and a half at the outer circumference of the circle. 



Of these six radii, three reach only to the outer circum- 

 ference of the circle, and the three otheis, intermediate be- 

 tween these, project about two inches farther, to receive the 

 spheric nobs dd, through which vertical double screws pass, 

 wliich level and support the instrument upon the truncated 

 cones ee. These elevate the instrument sufficiently above 

 the stand, to admit the verification telescope under the cen- 

 tre of tlie instrument. 



The construction of these double screws for the adjust 

 ment of the instrument in levelling is best seen in the sec- 

 tion of the Repeating Tlieodolitc, Plate IX. fig. 2 ; as this 

 construction has been ap|)lied to all tlie larger instruments, 

 in order to render their adjustment more exact, c' is the 

 outer screw, which is of I)rass, going into the mother screw 

 perpendicularly through the nob. and having its milled head 

 c' below it. This screw being hollow receives the steel screw 

 a' through its whole length : tlie milled head d has below 

 it a sei>ment of a sphere, left rough so as to impress itself 

 into the lead which is in the top of tlie cone r. The screws 

 being placed so as to give play to the screw c' both in the 

 nob and inner screw a', the motion of the screw c' serves to 



