172 
THE PHYSICAL GEOGRAPHY OF THE SEA. 
that the time of day 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, he observes his 
thermometer to rise higher and higher as he approaches it ; at last, 
entering the region of equatorial calms and rains, he feels the 
weather to become singularly close and oppressive ; he discovers 
here that the elasticity of feeling which he breathed from the trade- 
wind air has forsaken him ; he has entered the doldrums, and is 
under the " cloud-ring." 
Escaping from this gloomy region, and entering the southeast 
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, while 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. 
348. 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 
under 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 
thunder-storm, during which time torrents of rain fall, when the 
air becomes a little refreshed ; but a hot, glowing sun soon heats 
it again, and but for your awnings, and the little air put in circu- 
* See paper on Meteorological Observations in India, by Colonel Sykes, Philosoph- 
ical Transactions for 1850, part 2d, page 297. 
