OP THE UNITED STATES. 307 



evident tlmt the result of the calcul ition of the azimuth of 

 the sun will give at tlie same time the azimuth of tlie ohject, 

 the time heing determined hy proper moans as near to this 

 observation as possil)le. In this manner the azimuths of all 

 ohjects in the vertical of whicii the sun passes in the day 

 (on hotji sides of the zenith) may he determined, and the 

 terrestrial ani;les between them be toleral)ly well ascer- 

 tained. 



By the same method also an astronomical circle well ad- 

 justed in the vertical may serve for the determinaiion of 

 azimuths; and when it is required to lay ofi' certain direc- 

 tions on the earth, sigtials may be |)laced purposely to ob- 

 serve azimutlis upon them i)y this method ; and if the sufi 

 slundd not at tl)e time pass such a vertical, a star properly 

 situated should be chosen for the purpose. 



It is pioper in this place to introduce some practical re- 

 marks relative to the illumination of the division, whicii has 

 a eonsi(ieral)Ie influence on the accuracy and facility of tlic 

 reading. The light upon the divisions must be reflected 

 from a white unpolished plane ; it must fall upon the divi- 

 sions in the direction of tlieir lenglli, and not fiom the side ; 

 and if the reflection from tiie limb enter the microscope, the 

 greati'st liglit will lie ol)tained. All glaring light will i)e im- 

 proper, I found white paper the most proper suiHice; and 

 fortius pmpose I folded a quarto sheet into an octavo foi'm, 

 and to give it more solidity folded rims to the open sides ; 

 then giving to this again a fohl in the middle, so as to make 

 the two parts stand at rii!;ht angles with each oth'-r. in the 

 middle of one half I cut a circular hole r-xaetly litting the 

 microscope tulie below its holder, and adapted to it by its 

 close fitting. The screw going over the whole length of 

 the tul)e, it kept its position, and the other half hung down 

 to within a short distance of the li i)b of the circle outside ; 

 the reflecting surface being thus a tangent to it, and neaily 

 perpendicular to the liml). When the observer stands in 

 the direction of the radius of the microscope, the liL'ht comes 

 as from the centre of the instrument, rcflLCted in tlie same 



