106 Messrs. K. Honda, T. Terada, and D. Isitani on the 
place to mention some experiments in oscillations made with 
tanks. We tried many forms of vessels, but will venture to 
describe only one of them. 
A circular vessel of constant depth (ttX 15 2 x 8 cm, 3 ) made 
of sheet zinc was partially filled with water. On a table two 
india-rubber tubes, each about 20 cm. long, were laid some 
30 cm. apart, parallel to each other. A wooden plate, on 
which the circular vessel rested, was set upon these tubes in 
a horizontal position. By moving the plate to and fro with 
different periods, we could produce any desired mode of 
oscillation. The aluminium powder was scattered over 
the surface of the water, and then the oscillations were 
photographed as usual. PI. Y. fig. 1, shows the stream-lines 
in the fundamental oscillation ; PL V. fig. 2 and PI. VI. 
fig. 1, those of the second and the third harmonics respectively. 
Thej- clearly indicate how each water particle moves. The 
theoretical treatment of this tank motion is given in Lamb's 
6 Hydrodynamics/ 
Next, instead of periodically moving the wooden plate> 
two diametrically opposite points on the wall of the vessel 
were held by the fingers and the zinc walls simultaneously 
pressed, both inward and outward, with a proper period. 
PL VI. fig. 2 indicates the stream-lines of the oscillating 
water thus started ; a, a the points touched by the fingers. 
These stream-lines formed a system of hyperbolas. Here it 
is to be remarked that in such a complex motion of water, 
it was impossible to judge by the naked eye what form these 
stream-lines actually have. 
Thus the above investigation affords a good method of 
experimentally solving some difficult problems in tank 
motion, for which mathematics so far has failed to be of 
avail. 
§ 5. Formula; for Calculating the Periods of the Oscillation 
in Bays. 
The oscillation in a bay is nearly the same as the seiches 
in a symmetrical lake, each half of which has exactly the 
same form as the bay under consideration. The hydro- 
dynamical condition at the mouth of the bay is, however, 
slightly different from that at the middle part of the lake. 
Hence the period of oscillation in the bay is not exactly the 
same as that in the symmetrical lake ; this difference is here 
called the mouth correction. If this correction be known, 
the problem of finding the period of oscillation of the bay 
water reduces itself to that of finding the period of oscillation 
in the lake. 
