OP THE UNITED STATES. g89 



ill riate V. fig. 13, let 



S=Station |)oiiU observed. 



C=CeiUie of the station or si'j;nal. 



A ^' B= I he two signals observed from S 



Then is evidentlv, 



C+A+B=l80°=S+(A±SAC)+CB±SnC) 

 where the signs are easily known Irom angles liSC and ASC 

 compared with ASB. 



I tbund tliis method tl»e simplest, sliortest, and least Iial)lc 

 to mistakes. 



When the ground at the station point is not perfectly solid, 

 it is necessary to place in the ground strong plua;s or short 

 posts, sawed ofl" horizontally, and nearly level with each 

 other. 



'I'he sand hills on the low sea shores, and tlie looming 

 which takes place upon them, would have rendered necessary 

 for these places the construction of a stand of al»out twenty- 

 four leet elevation, solid and fnmly attached to the ground, 

 upon which the instrument with its stand mi2;hf l)e placed. 

 I intended to liave had one constructed in the form of a tri- 

 angular pyramid ; but it is evident that the use of such means 

 is to be limited as much as i)ossible, on account of the many 

 inconveniences attending them. 



The instruments must be well sheltered from all weather, 

 and the sun must not shine even upon the stand or the ground 

 near its Ici:;?. It is therefore necessary to have a separate 

 and suitable tent constructed for each instrument, with cur- 

 tains all around, but separate, so as open the side necessary 

 for observing, without (lepriving the instrument of its shelter. 



The instruments arc to be taken out of their boxes, plac- 

 ed, levelled, and adjusted in all i especfs. as soon a-? possible 

 after arriving at the station : in order that iliey may recover 

 from any unequal pressure to which they niay have been 

 subject in the boxes and in travellinj;, so that the parts may 

 come to rest in their proper position some time before the 

 instrument is used. 



VOL. 11. — 3 E 



