210 THE PHYSICAL GEOGRAPHY OF THE SEA. 



gives a high and low barometer every day with such regularity 

 that the hour within a few minutes may be told by it. The rise 

 and fall of this tide, measured by the barometer, amounts to about 

 one tenth (0.1) of an inch, and it occurs daily and every where 

 between the tropics ; the maximum about lOh. 30m. A.M., the 

 minimum between 4h. and 5h. P.M., with a second maximum and 

 minimum about 10 P.M. and 5 A.M.* The diurnal variation of 

 the needle changes also with the turning of these invisible tides. 

 Continuing his course toward the equinoctial line, the navigator 

 observes his thermometer to rise higher and higher as he ap- 

 proaches it ; at last, entering the region of equatorial calms and 

 rains, he feels the weather to become singularly close and op- 

 pressive ; he discovers here that the elasticity of feeling which he 

 breathed from the trade-wind air has forsaken him ; he has enter- 

 ed the doldrums, and is under the " cloud-ring." 



585. Escaping from this gloomy region, and entering the south- 

 east trades beyond, his spirits revive, and he turns to his log-book 

 to see what changes are recorded there. He is surprised to find 

 that, notwithstanding the oppressive weather of the rainy latitudes, 

 both his thermometer and barometer stood, Avhile in them, lower 

 than in the clear weather on either side of them ; that just before 

 entering and just before leaving the rainy parallels, the mercury 

 of the thermometer and barometer invariably stands higher than 

 it does when within them, even though they include the equator. 

 In crossing the equatorial doldrums he has passed a ring of clouds 

 that encircles the earth. 



586. I find in the journal of the late Commodore Arthur Sin- 

 clair, kept on board the United States frigate Congress during a 

 cruise to South America in 1817-18, a picture of the weather un- 

 der this cloud-ring that is singularly graphic and striking. He 

 encountered it in the month of January, 1818, between the paral- 

 lel of 4° north and the equator, and between the meridians of 19° 

 and 23° west. He says of it : 



" This is certainly one of the most unpleasant regions in our 

 globe. A dense, close atmosphere, except for a few hours after a 



* See paper on Meteorological Observations in India, by Colonel Sykes, Philosoph- 

 ical Transactions for 1850, part ii., page 297. 



