or retardation, the transition between them being usually very abrupt. The anomalies occasionally met with seemed due to the prevalence of opposite winds at the two ends of
the straits.Putting aside for the time the influence of wind, the double current of the Black Sea straits may be accounted for as follows : The excess of fresh -water discharged into the basin of the Black Sea is always tending to raise its level ; and this produces an outward surface-current, which as regularly tends to keep it down. On the other hand, the reduced salinity of the Euxine column gives to the JEgean column an excess of lateral pressure, which causes its lower stratum to flow back into the Black Sea basin; and as the equality in the amount of salt thus carried back by the under-current to that which escapes by the surface-current is indicated by the maintenance of the standard salinity of Black Sea water, it follows that, as the water which escapes contains about half as much salt in equal measures as the water that enters, the volume of the latter must be about half that of the former.
Now, when the rate of the surface-current is augmented by a N.E. wind, there will be not only a more rapid lower ing of the Black Sea level, but a tendency to elevation at the ^Egean end of the strait ; and as this will augment the difference between the downward, and therefore the lateral, pressures of the two columns, the force and volume of the inward under-current will be augmented. When, on the other hand, the S.W. direction of the wind reverses the surface-current, it tends, by piling up the water at the N.E. end of the strait, to augment the weight of the Black Sea column, the excess of which (notwithstanding its lower salinity) over that of the yEgean column, will then produce a reversal of the under-current also. When the S.W. wind is moderate enough to check the surface outflow without reversing it, the -inward under-current will likewise be brought to a stand ; for a slight rise in the level of the Black Sea column will cause its greater height to compen sate for the greater salinity of the yEgean column, so that their lateral pressiires will be equalized. We have here a " pregnant instance" of the potency of slight differences in level and in salinity to produce even rapid movements of considerable bodies of water ; and a strong confirmation is thus afforded by direct observation to the doctrine that differences in density produced by temperature are adequate to give rise to still larger, though slower, movements of the same kind in the great ocean basins.
Zoology.—The basin of the Black Sea is frequented by seals, dolphins, and porpoises ; and it is said that in the neighbourhood of the mouths of the Danube the porpoise is perfectly white, so that the Greek mariners, when they catch sight of it, know that they are in the current of that river, although in 30 fathoms water, and many leagues from land. The fish of the Black Sea appear to be for the most part the same as those of the Caspian and the Sea of Aral Its northern rivers bring into it the sturgeon, sterlet, and other fresh-water fish, which can live in and near their estuaries. On the other hand, its waters are elsewhere salt enough for the mackerel, whiting, mullet, turbot, and sole. The pelamys spoken of by Strabo as issuing from the Mceotis (Sea of A.zoff ) in shoals, and as following the coast of Asia, is still abundant ; though commonly spoken of as the herring, it seems to be a large sprat. The principal fish that enters this basin from the Mediterranean is the tunny, which comes into the Black Sea in large numbers at the spawning season, The other inhabitants of the Black Sea have not been especially studied ; but it may be noted that a species of Teredo is very common and destructive both to ships and to wooden harbour-works, and that it is not con fined to the salter waters of the basin, but frequents the estuaries where the water is almost fresh.
BLACKBIRD (Turd-its menda), belongs to the Turdidce or Thrushes, a family of Dentirostral Birds. The plum age of the male is of a uniform black colour, that of the female various shades of brown, while the bill of the male, especially during the breeding season, is of a bright gamboge yellow. The blackbird is of a shy and restless disposition, courting concealment, aiid rarely seen in flocks, or otherwise than singly or in pairs, and taking flight when startled with a sharp shrill cry. It builds its nest in March, or early in April, in thick bushes or in ivy- clad trees, and usually rears two broods each season. The nest is a neat structure of coarse grass and moss, mixed with earth, and plastered internally with mud, and here the female lays from four to six eggs of a blue colour speckled with black. The blackbird feeds chiefly on fruits, worms, the larvae of insects, and snails, extracting the latter from their shells by dexterously chipping them on stones ; and though it is generally regarded as an enemy of the garden, it is probable that the amount of damage done by it to the fruit is amply compensated for by its undoubted services as a vermin-killer. The notes of the blackbird are rich and full, but monotonous as compared with those of the song-thrush. Like many other singing birds it is, in the wild state, a mocking-bird, having been heard to imitate the song of the nightingale, the crowing of a cock, and even the cackling of a hen. In confine ment it can be taught to whistle a variety of tunes, and even to imitate the human voice. It is found throughout Europe, Palestine, and the northern parts of Africa; and Darwin states that he observed it as far west as the Azores. Individuals reared in Britain, it is said, do not migrate ; but annually great flocks arrive on the eastern shores of England from more northern countries, remaining for a few days only, and then proceeding southward.
cipal borough of England, situated on a stream called, in Domesday Book, the Blackeburn, but now only known as " The Brook," in the north-eastern division of the county of Lancaster, 209 miles from London by railway, 15 E. of Preston, and 30 N.N.W. of Manchester. Besides its numerous churches and chapels, the public buildings of Blackburn comprise a large town-hall, finished in 1856, a market-house, an exchange, built in 1865, a county court (1863), public baths (1864), and, outside the town, an infirmary (1862). A public park of about 50 acres was opened in 1857. Since about 1865 a variety of extensive and important improvements have been effected in the general condition of the town, which is now well paved and lighted, has an elaborate system of drainage, and receives an abundant supply of water. Previous to that date the so-called streets were, over a large area, almost useless for purposes of traffic. The staple trade of Black burn has long been the manufacture of cotton, for the development of which a great deal was done by some natives of the town, such as Peel and Hargreaves, in the last century. The subordinate branches include woollen factories, engineering works, iron foundries, and breweries. In 1871 there were employed in the cotton factories 14,220 men and 17,075 women, of twenty years of age and upwards; the engineering works gave employment to 356 men, and the iron manufacture to 794. Coal, and lime, and building stone are abundant in the neighbouring district, which is, however, very far from fertile. The Leeds and Liverpool Canal passes the town, which has also extensive railway communication. Blackburn is a place of some antiquity, and its parish church of St Mary s (for the most part taken down in 1813), dated from before the Norman Conquest. It was for a time the chief town of a district known as Blackburnshire, and as early as the reign
of Elizabeth ranked as a flourishing market town. About
