74 H. Mitchell on recent soundings in the Gulf Stream. 
face of the fresh water, even in the dry season, is above this 
level—not so much above, however, as to equalize ‘the difference of 
specific weight between it and the sea water, so that the latter, during 
the summer months, flows in along the bed of the stream, while 
the former overflows into the ocean. 
In the recent nese , observations upon surface densities were 
carried over several hundred miles. These show decided con- 
trasts between the ocean and the stream, but no greater than the 
differences of temperature might lead us to expect, 
The Gulf Stream is essentially confined to the southern half 
of the Strait in the portion crossed by this survey, but no west- 
wardly drift —— he rth shore was observed except at one 
time a feeble flood tidal easiness setting close along the reef. It 
is not reed = the widths of the Gulf Stream vary, as its 
velocities are known to do, and both of these may in many 
aa ae ge upon er continued gales of wind. During the 
period of the recent survey, however, the weather was exceed- 
ingly calm in the Gulf, and as far as learned, sam — at 
sea, yet the velocities of the stream altered in a marked m nner, 
and so much so that the oo became a pianen of csieaiill 
among pilots and ship masters arriving at Havana. It would 
be exceedingly interesting and practically useful to ascertain 
from systematic inquiry the order of these variations. We 
would suggest as a reasonable hypothesis that these variations 
follow those changes of mean-sea-level which depend upon 
declinations of the sun and moon—more especially the latter. 
There are no two seas upon the earth whose tidal phenomena 
differ more essentially than those of the Gulf of Mexico and 
the Atlantic Ocean; and it - a matter of certainty that the ele- 
vations of these two bodies of water are not affected in the same 
manner and degree by the half. monthly changes of the moon’s 
declination, Professor Bache’s aper on the “Tides of Key 
West,” published in the Coast Barvey Report of 1853 shows 
that the mean level of this station is one foot higher when the 
moon is in the equator than when she is at her t declina- 
tion. Inthe North Atlantic the order is the reverse of this; 
the mean level is there about three inches higher at the maxi- 
mum than at the zero declination.* Small as these relative 
changes of elevation may seem they must bear a large proportion 
to the total head of the Gulf Stream which suffers exceedingly 
little resistance in its course. 
* From computations of the Coast Survey, and from Phil. Trans. R. S., 1839. 
