MAGNETIC OBSERVATIONS. 7 



momentarily visible through breaks in the clouds, that the few sights necessary in 

 order to complete the magnetic observations were obtained. 



With a single exception, all the magnetic and astronomical observations were 

 made at a spot from which the true bearing of the entrance on the north face of 

 Fort Caraguata (erroneously spelled Gravata on the English charts) is S. 70° W., 

 and its distance fifty-five feet. There were no guns in the fort at the time. The 

 surrounding country is very hilly, the bare, coarse, granite rocks cropping out every- 

 where from the hill-sides, but in the more level places they are thinly covered with 

 earth. Assuming the position of Fort Villegagnon to be lat. 22° 54' 42" S., long. 

 2 h 52 m 36 3 .0 W., then, according to the English Admiralty Chart, the position 

 occupied by the instruments is in 



Lat. 22° 54' 5" S. 



Long. 2" 52 m 30 5 .7 W. 

 The exception referred to above is some observations of the sun for time, made 

 on January 9th. They were -got on Rat Island, the spot where naval officers 

 usually go to rate their chronometers when lying in this harbor. Assuming the 

 position of Fort Villegagnon as above, then, according to the English Admiralty 

 Chart, the position of Rat Island is 



Lat. 22° 53' 45" S. 

 Long. 2 h 52 m 37 s .9 W. 



Monte Video, Uruguay. The ship was swung in this harbor, on January 24th, 

 1866, in the usual manner. We first attempted to get her around about 1 P. M., 

 but owing to the force of the wind and tide we only obtained ten points, viz., those 

 from E. by S. to S. S. W. Just at sunset we tried it again, and succeeded in getting 

 the remainder of the circle. It was nearly dark when we finished, but as the dis- 

 tant object used for an azimuth mark shone plainly against the sky, there was suf- 

 ficient light to see pretty distinctly when it was in range with the sights of the 

 compass. 



The readings of part of the circle on the After Ritchie compass were lost, owing 

 to the failure of daylight and delay in procuring a lantern. The officer who usually 

 read the After Azimuth compass was on shore at the time, and the duty of making 

 the observations at that instrument was assigned to another, but it turned out that 

 he did not understand how to read an azimuth compass, and his observations were 

 worthless. 



While we were lying at Monte Video the tide was very irregular. Most of the 

 time the ship only swung to it about 90°, but two or three times she swung 

 180°. At the time we swung her to obtain the deviation of the compasses her 

 position was lat. 34° 55' S., long. 56° 13' W., and joint XII on the after turret was 

 4.5 inches to port. 



The greater part of the magnetic observations on shore were made on January 

 18th, at a station on the ground occupied by Tomkinson's slaughtering establishment. 

 The instruments were set up at a spot where there are four large umbu trees stand- 

 ing in a line. The exact position may be recovered by means of the following true 

 bearings. The corner made by the south and west sides of the dwelling-house 



