314 Proceedings of the Royal Society 
square miles of country, enters the Black Sea by three separate 
mouths — the northern called the Kilia, the central the Sulina, and 
the southern the St Greorge’s mouth. The first duty of the Com- 
mission, with the advice of Sir Charles Hartley, who was appointed 
their engineer, was to select one of the three mouths for improve- 
ment, which was by no means an easy task, as each of them pre- 
sented advantages peculiar to itself, and after much consideration 
the Sulina or central channel was selected, and although consider- 
able difference of opinion existed as to the propriety of the choice 
the result has shown that the course adopted was judicious. 
What gives professional interest to works on the Danube is their 
object — the improvement of a bar harbour, in all cases a trouble- 
some engineering problem, due probably, in some measure at least, 
to the varying circumstances which the engineer has to consider in 
dealing with the formation and improvement of bars. In this 
country, for example, we find bar harbours at the Tyne, the 
Wear, and other similar localities, caused by the storms on our 
coasts, which, by the action of the waves on the bottom, throw up 
sand and shingle, and but for the scour produced by other agents 
a continuous line of beach would soon be formed across the mouths 
of our tidal rivers and inlets. But this action of the sea is counter- 
acted by the scouring flow of the tidal and fresh-water currents, 
which, in spite of the waves, tend to keep open a deep channel 
through the beach. In this way the antagonistic action of the 
waves of the sea on the one hand, and the currents of the estuary 
or river on the other, produce the well-known feature of a sub- 
merged beach or sand-bank extending across our inlets, and having 
a channel through them of more or less depth, according to cir- 
cumstances, which channel is termed “ the bar.” The conditions 
under which such accumulations are formed are as I have else- 
where stated : — * 
1st, A bottom composed of materials easily moved; 
2 d, Water of depth so limited as to admit of the waves acting 
on the bottom ; and 
3d, Such exposure as shall allow of waves being generated of 
sufficient size to operate on the submerged materials. 
But in such places as the Firth of Forth, for example, before the 
* Encyclopaedia Britannica, Art, Inland Navigation. 
