OF THE VNITEU STATES. 3911 



do all this before any derangement of the instrument lias 

 taken place, that any discrepancy in the results may be tested 

 and corrected immediately. 



This will most likely not allow an early rest to the obser- 

 ver. Tliercfore it will hardly ever be |)ossible to do what 

 would now be advantageous, viz. to observe ciicunimeridian 

 zenith distances of stars in the night, particularly to the north, 

 to compensate the influence of the instrument upon the ob- 

 servations taken to the south with the sun, and coricct the 

 result by their means. The observer will l)e oljligcd to re- 

 serve Ills strength for the work uf the next day, as his ob- 

 servations would lose much of their accuracy, if he should 

 be overfatigued. Such night observations are therefore only 

 possible, wiicn he has assistants able to take a part of the 

 task of the day from him. 



Night observations require besides a temporary observa- 

 tory, built for the purpose, and appropriated to the instrument 

 used, like one which I had constructed for the work of the 

 boundary line with Canada. 



It is proper therefore that a peculiar and suitable station 

 be selected, to make a regular and well coml)ined series of 

 observations on the latitudes, with all the necessary means 

 and arrangements for accuracy. As it will then be possible 

 to obtain many more observations of stars than of the sun, 

 their number and kind may be so combined, as to serve as a 

 full compensation for all tiie ol)servations of the sun made 

 at the other stations, and which are to be reduced to this by 

 calculation. 



It will natinally occur, that at such a station a complete 

 series of azimuths may also be observed by a variety of me- 

 thods, and with every means of accuracy ; and that after 

 such a station has been mad(% it will not be necessary to ob- 

 serve them on the neigiibouring stations. 



The indispensable oliservations of longitude are of course 

 to be referred to the permanent ol)servatory of the country, 

 if such a one exists : if not, they also become necessary at 



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