1830.] 
On the Tides of the River Tlugli. 
15 
which we will suppose six hours, may not be sufficient to raise the surface of water 
in the cistern to an equal level with A, and that the discharge, during the same 
periods of six hours through B, 
2 , 
A. <ms 
-A. 
O. 
ad'l'ib. 
ra. 
may also be insufficient to depress 
the surface of water in the cis- 
tern to B. 
Commence with the water of 
the cistern standing at a sup- 
posed mean level (J) and open A 
for a half interval or three hours, 
then shut A and open B for six 
hours, after which the alterna- 
tion may proceed at regular in- 1 
tervals. 
It is plain, that if the two cocks 
had equal orifices, situated as in 
figure 2, the expenditure for six 
hours would not be equal through 
each. Such, however, is not 
a natural state of things, and it would be better to make the supply and discharge 
cock at B, as in figure 3, in which case, as the average head of water would be the 
same to the supply and discharge, 
an equilibrium must evidently r? 
take place in the undulation of u 
the surface of the cistern, con- 
sequent upon the alternate man- 
oeuvring of the cock. And in this 
case, all other things being equal, 
the average height of the water 
in the cistern would correspond 
with the mean between A and B 
or O. 
To bring the illustration, how- 
ever, nearer to nature, it becomes 
necessary to suppose the orifices 
of the two cocks to alter every 
moment during their action, the 
supply cock increasing in section in a very rapid geometrical ratio to the time of 
high water, and the discharge cock to decrease nearly in the same rapid ratio to the 
time of low water. It is also necessary, in applying the illustration to a tide which 
has to travel over any distance, to allow for the time of passing, or in other words, 
to suppose the supply cock to open before the discharge cock has ceased its action : 
and in the like manner, the discharge cock to open before the supply cock is closed. 
The effect of this elapsing time is, however, only partially represented by such a 
supposition, inasmuch as the check formed by the first flood at the mouth of a creek 
to the water still ebbing inside, will, on the principle of afflux, create an artificial 
level, while the commencement of the ebb, at the mouth of the creek, can have no 
parallel influence upon the level inside the creek. So that it is evident, that al- 
though, by the above illustration, while the orifices of discharge and supply are equal, 
or even changing in equal ratios, it may be difficult to prove that the average surface 
in the cistern will vary from the mean height between A and B at O ; when this latter 
circumstance of the check created by the first flood is taken into the consideration, 
the balance will ever be in favour of the flowing tide, and the average height m the 
cistern will be above O, the mean height between A and B. 
The application of the above illustration is of course simplest with maritime 
lakes and gulfs, such as the lakes of the delta of the Mississipi, or perhaps t ie 
Chilka lake in the Cuttack district, and Pulicat, or Madras lakes, m all ot whicn 
the body of the lakes is only separated from the sea by short inlets of greater oi e - s * 
transverse section. The bed of these inlets, although generally coqsmet a > \ < 
low water, still so much partakes of the nature of a bar, that the mean eve o 
lakes is almost universally raised, as shown in the diagram, above t lC ' nt '* , 
outside ; — and here the lake will resemble the cistern of the illustration \eiy < ) • 
The extended tide creeks of the Sunderbunds, on the other ban , ‘ > • 
represented, by supposing a series of such cisterns extending to m ni \ , 
wise, according as the point ® of final dissipation is attained oi n ’ 
