THE ATLANTIC OCEAX. 215 



the singular fact recorded by Sir James Ross, of the perma- 

 nently low barometric pressure in high southern latitudes ; 

 or to the curious observations of Professor Airey and Mr. 

 Birt, on the periodical rise of the barometer in the course of 

 every month to some point above 30 ; suggesting the notion 

 of great atmospheric waves, ruffled by smaller waves in the 

 intervals between. We must look to the future for a solution 

 of these, and numerous other difficulties in meteorology, 

 which are beyond the reach of any tables or averages yet 

 obtained. Many of these phenomena may be best studied 

 under the equator, where there is little variation in the sun's 

 meridian altitude ; where the zone of observation is sym- 

 metrically related to each hemisphere ; and where the diurnal 

 fluctuation of pressure is so regular, that the time may 

 generally be determined within 15 or 16 minutes by the 

 barometer alone. 



The ' Depths of the Ocean,' and the methods employed to 

 determine them, form an interesting chapter in the volume 

 before us. Until a very recent time these methods were so 

 far imperfect that, though numerous soundings were made 

 into the more profound depths which sailors call Blue 

 water, it could seldom be affirmed ' that fathom line had 

 truly touched the ground ' in these abysses of the sea. In 

 the Southern Atlantic, more especially, results were given as 

 obtained by British and American officers, which indicated 

 depths varying from 26,000 to 50,000 feet, or from 5 to 9^ 

 miles; and in several of these instances without any as- 

 surance of the plummet having reached the bottom. Here, 

 in fact, lay the uncertainty of the whole process. Under- 

 currents might intervene, turning aside a slender thread and 

 insufficient weight from the right line of descent; or, if 

 allowing the weight to touch the ground, still acting upon 

 the bight of the line, so as to cause it to run out too far from 

 the reel in the vessel above. 



