86 | THE ATLANTIC. [cuap. 1. 
be made to test, by the “ current-drag,” whether any underflow can be 
shown to exist from either polar basin toward the equatorial region. 
A suitable locality for such experiments in the North Atlantic would 
probably be the neighborhood of the Acores, which are in the line of 
the glacial flow from the North Polar Channel. The guide to the depth 
at which the current-drag should be suspended will be furnished by the 
thermometer, especially where there is any abrupt transition between 
one stratum and another. It would be desirable that not only the rate 
and direction of surface-drift, but those of the subsurface stratum at 
(say) 200 fathoms’ depth, should be determined at the same time with 
those of the deep stratum. 
Tidal Observations. — No opportunity of making tidal observations 
should be lost. Careful observations, made by aid of a properly placed 
tide-pole in any part of the world, will be valuable. Accurate measure- 
ments of the sea-level once every hour (best every dwnar hour, i. e., at 
intervals of 1" 2™ of solar time) for a lunar fortnight (the time, of 
course, being kept) would be very valuable information. 
Beach-marks.—In reference to the interesting question of the eleva- 
tion or subsidence of land, it will be very desirable, when sufficient tidal 
observations can be obtained to settle the mean level of the sea, that 
permanent beach-marks should be established, recording the date and 
height above such mean level. Even recording the height to which the 
tide rose on a certain day and time would render a comparison possible 
in future years. 
A good determination of the mean sea-level by the simple operation 
of taking means may be made, in less than two days, with even a mod- 
erate number of observations properly distributed so as to subdivide both 
solar and lunar days into not less than three equal parts. Suppose, for 
example, we choose eight-hour intervals, both solar and lunar. Take a 
lunar day at 24" 48™ solar time, which is near enough, and is convenient 
for division; and choosing any convenient hour for commencement, let 
the height of the water be observed at the following times, reckoned 
from the commencement : 
Hrs. Min. 
8 0 
16 16 
24 32 
The observations may be regarded as forming three groups of three 
