SURVEYS. 



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the chronometers during the entire cruise are given in the table, 

 which will show their performance and what confidence is to be placed 

 in them. 



MAGNETIC OBSERVATIONS. 



At an early period of the cruise, Barlow's plates were adapted to the 

 azimuth compasses, and the position kept throughout. These azimuth 

 compasses were after Kater's construction, with prismatic eye-pieces. 

 The greatest amount of local attraction, in high latitudes, observed on 

 board the Vincennes, was but 5° ; in lower latitudes it was not per- 

 ceptible. The situation of the compass was about half way between 

 the mizzen-mast and tafFrail amidship, and the stand was of sufficient 

 height to enable an observation to be taken at all times without diffi- 

 culty. The variation was observed at sea, when the chronometric 

 sights were taken, both morning and evening. The result of the mean 

 of ten observations is given. At the observatories, during the time 

 they were established, once every day, with the variation transit. 

 The results are embodied in tables under the proper heading. The 

 dip and intensity were observed frequently throughout the cruise, at 

 sea, but at times it was found impossible to obviate the motion so as 

 to get any results that I deemed worthy of record, even by means of 

 swinging-tables and other contrivances which were resorted to with 

 the intensity needles. The Expedition was not furnished with the 

 apparatus of Fox. The dipping-needles were used with more success 

 at sea, and by noting the vibrations of the needle, and taking the 

 mean, the result could be depended upon. I resorted to the plan of 

 always putting the ship's head north or south during the time of 

 making the observation : this brought the line of local attraction in 

 the plane of the magnetic meridian, and obviated the necessity of any 

 reductions for the effect on the needle caused by the local attractions 

 on the various rhumbs of the compass, — an element constantly under- 

 going changes with the latitude, and also affected by any alterations 

 of the iron in the vessel. 



Although I experimented by changing the poles of the needles, I 

 cannot but believe that it increases the liability to errors more than 

 it obviates them. The manner of changing the poles should always 

 be uniform, and, if possible, by the same person. Great care is neces- 

 sary to avoid scratching the needles ; and in order to prevent this, 



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