OF THE UNITED STATES, :Jl)7 



should therefore be made during the azimuth ohscrvatioii it- 

 self, if assistants for that [)urpose are at hand. 



After tht'se observations, the instrninont liuvino; l)cen vcri- 

 tied again, the observations of terrestiiul an'j;les will l)c con- 

 tinual, upon the signals from the fore mentioned situation to 

 about due north utitil near ten o'clock, if the illuniination of 

 the (lay is iavourablc, Al)out ten o'clock, coirespondins; al- 

 titudes may be taken with the sun, for which tlie easiest, and 

 probably most accurate method will be, to take with a re- 

 flecting instrument the contact of both linil)s of the sun, no- 

 ting the time, and laying the instrument undislurl)ed and cure- 

 i'ully aside, to observe tlie same again after noon. Tiiis will 

 avoid every influence of any error of tiie instrument. 



Durinii; the middle of the day, the state of the atmosphere 

 and the illumination are so unfavoural)le to the observation of 

 terrestrial angles, that none must be observed, tlic objects 

 being all seen in the shade, and the reflection from the sig- 

 nals and any object in general passing high over the oI)ser- 

 ver. In a hot summer day, the illumination will even cease 

 to be favourable after 9 o'clock. But when the sun has a 

 low southern declination, it may sometimes be possible to 

 observe somewhat longer u[)on signals nearly nortli. 



This is therefore the time to transcribe the oI)servations 

 into the day book, to take out the results of the terrestrial 

 angles, to examine them, make such preliminary calculations 

 as may l)e required to determine the time an(l rate of the 

 chronometer, and arrange the work of the afternoon, ^'C. 



AI)Out thirty minutes before noon, it will be necessary to 

 adjust the vertical circle, and prepare for the circummeridian 

 observations of the sun. For though it is not absolutely ne- 

 cessary in a triangulation, to determine the latitude of every 

 station by actual ol)servation, still it is not proper to let the 

 sun pass the meridian without observing it, as every observa- 

 tion is an addition to the mass of this kind of observations, 

 which it is necessary to have as large as po-*sib|c. and they 

 are all reduced to one point by the gcodcsical calculations, 



VOL. II. — 3 F 



