PROGRESS OF OUR KNOWLEDGE OF THE TIDES. 125 
regularly as one proceeds either northward or southward 
from that point. It has been observed that when it is high 
water to the north of Tor Bank it is low water to the south of 
that place, and vice versa. Further, it is found that when high 
water occurs a few miles north of Tor Bank it is also high 
water throughout all the northern end of the gulf at nearly 
the* same time, although it is about 114 miles to Suez at its 
head, where the spring tide is about seven feet. As the water 
at Tor is tolerably deep and the channel canal-like, no inter¬ 
ference of tides such as often takes place in island regions can 
occur, and it has been a puzzle to understand what causes 
operate to destroy the tides while still permitting tidal cur¬ 
rents to exist there. The theory of fractional areas having 
dependent stationary waves seems to furnish a satisfactory 
explanation. 
For the purpose of illustration of what this theory paeans, 
let us assume a trough of water resting upon some support 
like an ordinary table which is not perfectly rigid. By 
adapting the depth of water in proportion to the length of 
the trough, it is easy to impart to the water a regular period 
of oscillation of half a wave length or any desired odd mul¬ 
tiple of half a wave length by gently pushing against one 
end of the trough at properly timed intervals. If small 
pieces of cork or other floats are scattered over the surface of 
the water, one can observe what is going on. For simplicity, 
suppose an oscillation of half a wave length is started in the 
trough, it will be high water throughout one-half of the 
length of the trough while it is low water throughout the 
other half length at the same time, and in the middle there 
will be no vertical displacement. The pieces of cork at either 
end will rise and fall with the water, but have very little 
horizontal motion, while the corks in the middle will oscil¬ 
late back and forth for some distance on either side of the 
middle, without any sensible up and down motion. 
Now, returning to the Gulf of Suez once more, it has been 
found, by a study of the soundings, that the distance between 
Tor Bank and Suez is just about one-quarter of a wave length. 
18—Bull. Phil. Soc., Wash., Vol. 14. 
