cylinder of compressed oxygen gas which is carried by the diver. The advantage of such an arrangement is that the diver is free from pumping apparatus and can go where an ordinary diver could not. Mr. Fleuss was able, by diving with this apparatus, to prevent an immense loss of property by arresting the flooding of the Severn tunnel which was imminent during its construction. A difficulty in regard to the Fleuss apparatus has been that oxygen gas is a poison, causing inflammation of the lungs and convulsions when under a pressure of from two to three atmospheres that is to say, at from 30 ft. to 60 ft. depth in water. The pressure exercised by the air of the atmosphere at sea-level is equal to that exercised by a column of water 30 ft. high, and hence at 30 ft. depth in the sea the oxygen gas would be under the pressure both of the atmosphere itself and of water to the same amountwhich is expressed by saying that it is under two atmospheres' pressure, or twice the atmosphere's pressure. The pressure of the atmosphere is, in plain figures, 15 lb. on every square inch of surface. Of course, the oxygen is compressed far beyond this point in the cylinders in which it is carried. In using it, it is allowed to escape by opening a valve leading into an elastic sac, and is then and there subject to the pressure depending on the depth of water to which the diver has descended. It is found to be dangerous for a diver with this apparatus to descend to a depth of more than 30 ft. having pure oxygen in his apparatus, because the oxygen is then compressed under a pressure of two atmospheres. Accordingly, Dr. Haldane, of Oxford, has proposed that the oxygen should be diluted with atmospheric air, so as to give a mixture of equal volumes of oxygen and nitrogen. With this mixture the diver can safely descend to a depth of 60 ft. The apparatus is provided with a partition containing caustic soda, which absorbs the carbonic gas thrown out

of the lungs in expiration. With such an apparatus a diver can safely remain under water at a depth of 60 ft., and walk about and explore for as long as two hours. A most important application of this self-contained diving apparatus is found in its use in the exploration of mines, where smoke or gaseous products resulting from an explosion render it impossible for rescue parties to penetrate without its use. It has been the means of saving many lives in such circumstances. A form of this apparatus is made in which the oxygen is supplied, not by a cylinder of compressed gas, but by granules of a chemical compound called pneumatogen, a peroxide of sodium and potassium, which when breathed into absorbs carbonic acid from the air expired by the lungs, and gives off pure oxygen. Submarine ships are now being provided with a dress or outfit of this description for each member of the crew, so that in the case of the entrance of water into the submarine, every man can put on his “ oxygen helmet," and one by one, when the ship is full of water, they can pass out by the conning tower and float to the surface. The perfected diving dress, with self-contained diluted oxygen supply and other improvements, has been constructed by Siebe, Gorman, and Co., and was exhibited by Dr. Leonard Hill at a soiree of the Royal Society.

XXVIII

SPARROWS, TROUT, AND SELECTIVE

THE

BREEDING

HE talk about the urgent need for the destruction of sparrows reminds me that the word "sparrow" is applied commonly in this country to at least two very different but common birds. No doubt farmers and gardeners know well enough the house-sparrow (Passer domesticus or Fringilla domestica of Linnæus), which is the one they consider injurious. But some boys and some newly-fledged proprietors of country places may inadvertently confuse the house - sparrow with a very different bird, though only a little smaller and of a general brown colouring, also called "sparrow," namely, the hedge-sparrow (Accentor modularis).

The hedge-sparrow is a true denizen of the country. It does not live on grain, but on insects and grubs, and is useful on that account to agriculturists. Its eggs are pure blue. A spotted egg of a cuckoo laid amongst them readily catches the eye, so that cuckoos' eggs are often found in hedge-sparrows' nests. It seems that it is all a mistake on the part of the cuckoo hen when this occurs. The strain of cuckoos properly attached to hedge-sparrows lay a beautiful blue egg differing only in its somewhat larger size from those of the hedge-sparrow itself, and hence difficult to detect. These blue cuckoo-eggs-proper

to cuckoos which make use of the hedge-sparrow as fostermother-escape detection both by boys and the fosterparents, and successfully hatch out and propagate the race of blue-egged cuckoos with a memory and a sense of smell which bring them back if they are hen-birds to the little hedge-sparrow's nest when they are grown up and have an egg to dispose of. The spotted grey or brownish eggs are, if not discovered by boys, ejected (there is reason to believe) by the hedge-sparrows themselves. They were deposited by mistake by some pippet-loving or warblerseeking strain of cuckoo in a hurry, or are throw-backs to a common ancestral colouring of the egg due, perhaps, to the male parent not being of the true blue strain. A very fine series of "clutches" and nests of hedge-sparrow, robin, shrike, reed-warbler, pippet, yellow-hammer, and other birds with the accompanying cuckoo's egg may be seen in the Natural History Museum, and they show how closely the parasitic egg often resembles that of the fosterparent, though striking failures also occur.

The hedge-sparrow is placed in that group of small birds which includes the robin, the thrushes, and the warblers; it is not a finch. On the other hand, the house-sparrow is a finch, allied to the chaffinch, the goldfinch, and the brambling. It has, like all the finches, a very powerful broad-based beak, and is more than a match for bigger birds than itself. It is really a parasite or " commensal" (messmate) of man, living and flourishing entirely by helping itself to the grain and the young buds of shrubs grown by man, and in towns to the waste fragments of his food and the grain left in horse-dung. Whether it does any good in the early part of the year by eating grubs seems to be doubtful, but the conclusion is justified that it does more harm than good, especially as it drives away other small birds which are exclusively insectivorous. It has gone with European man to all

temperate climates. There are Spanish, African, Italian and Indian species, closely related to the common housesparrow, which I should like to see put out side by side with it and some of its varieties for the public edification in the Natural History Museum. These are the true sparrows," and should be compared side by side with the hedge-sparrow, and the differences pointed out.

There is another true sparrow in England, called the "tree-sparrow," which is not nearly so common as the house-sparrow. They are, however, so closely allied to one another that hybrids have been produced between the two. On the other hand, the hedge-sparrow is a great deal too remote from the finches to interbreed with the house-sparrow or any other of the finch group.

There ought to be a careful report on the probable effects, in every direction, of a great destruction of housesparrows before any very drastic measures are taken in that direction. The employers of gamekeepers should remember that by destroying owls, hawks, and weasels they may not only enable small injurious birds to flourish in excess, but that they may encourage disease and weakness in the game-birds which they so eagerly desire to multiply, since the natural extermination of weakly birds by birds and animals of prey is put an end to when the latter are abolished. In all such matters more knowledge is needed, and reasonable people will not take irretrievable action until they have taken the trouble to obtain thorough knowledge.

It is a curious fact that though the house-sparrow does not naturally sing, yet hand-reared house-sparrows have been made, by association with bull-finches, to acquire the song of that bird-a truly astonishing instance of hidden or latent capacity.

A lover of trout-fishing has been writing lately upon the question as to whether the trout in much-fished rivers