TO RIO JANEIRO. 37 



reaching to the zenith, and passing over the moon's disk, encircling 

 her with a faint halo of twenty degrees in diameter. It continued an 

 nour, and although it was bright moonlight, the phenomenon was very 

 distinct and beautiful. 



On the 5th, the winds drew to the south-southeast, and we crossed 

 the line, as we had intended, in longitude 17° W., which enabled us to 

 pass over and examine the supposed locality of the Triton Bank, in 

 longitude 17° 46' W., latitude 00° 32' 00" S. The current was 

 found this day to be setting to the northeast, fifteen miles in the 

 last twenty-four hours. This night the sea was extremely brilliant, 

 showing in large luminous patches. The light proved to be occasioned 

 by a large species of Pyrosoma, some of which were ten inches in 

 length, and two inches in diameter. Many phosphorescent animal- 

 cula were taken, and some rips that were seen, exhibited long lines 

 of brilliant light. Temperature of water 76-5°. Our dipping-needle 

 on the equator gave 23° 30'. Hourly observations were made for 

 forty-eight hours, to ascertain the oscillations of the barometer 

 under the equator (for which see Appendix XVI.) The periods of 

 oscillation were found to be as follows : the maxima at nine a. m. 

 and nine p. m., and the minima at three a. m. and three p. m. The 

 variation was -1 of an inch, and was found to be very regular, from 

 latitude 3° 30' N., to 4° S. 



We had now heavy deposits of dew, on several fine and cloudless 

 evenings. Indeed the sun had scarcely set before the ship was quite 

 w r et w r ith it. One of the essential requisites supposed necessary by Dr. 

 Wells for a deposit of dew, was certainly wanting in this case, viz., 

 that " the temperature of the body on which it was deposited, should 

 be considerably lower than the surrounding air ;" — the temperature of 

 the air and ship having remained the same for several days at about 

 78°: all objects, hammock-cloths, spars, sails, and rigging, so far as 

 could be ascertained, showed the same. And at the time when the 

 dew was observed to be most copious, we had a fine breeze. It has 

 generally been supposed that dew never falls off soundings. This at 

 least is an old saying among seamen : but our observations are at 

 variance with this notion ; for, as far as every indication went, both by 

 sounding and blue water, we certainly had no bottom. 



The supposed position of the Triton Shoal was now passed over, 

 and examined carefully in the same manner as heretofore described, 

 sounding at the same time with two and three hundred fathoms of 

 line. Nothing of the kind was perceived, nor was there any indication 

 of soundings in the discoloration of the water, or any change in its 

 temperature. 



