About the end of the first decade of the century Poisson attacked the phenomena of electricity analytically, and succeeded in demonstrating the right of electrical investigation to rank among the exact sciences. Of his most important mathematical propositions is one in which, assuming as a working hypothesis the existence of two mutually attracting fluids, he deduced formulæ covering the distribution of these fluids on the surfaces of two conducting spheres, in or out of contact.
A great deal of work was done during the end of the last century and the beginning of the present one on what is now known as pyro-electrification. The Abbé Haüy discovered that fragments of tourmaline crystal exhibited opposite electrifications on opposite extremities of their lines of cleavage. It is this crystal also which has unusually remarkable powers of polarizing light, and which, under electro-magnetic stress, suffers modifications of the latter property. Haüy investigated the field with much diligence, and succeeded in cataloguing a large number of natural crystals by the side of tourmaline. The subject was amplified later by Sir David Brewster, who added a series of artificial crystalline salts to the list of pyro-electrical materials, among them, notably, hydro-potassic (and sodic) tartrate. The property was found not always to reside on these substances, but to be developed by heating them. Brewster found that even powdered tourmaline exhibited opposite electrifications on the opposite extremities of each tiny particle, causing the latter to act, so far as attractions and repulsions went, as infinitesimal magnets.
Our rapid and imperfect survey has now brought us to the threshold of the great activity in electrical work elicited by the tremendous discovery, made by Professor Oersted, of Copenhagen, of the existence of the electro-magnetic field. It happens that two of the most amiable and estimable individuals that have ever devoted their lives to scientific research stand out in this connection head and shoulders above all other investigators—Ampère and Faraday, the latter sixteen years younger than the former and destined to long survive him.
WINGLESS BIRDS.
By PHILIPPE GLANGEAUD.
It is often said that there are no rules without exceptions. We purpose to test the truth of this maxim once more. Fishes are made to live in water, but some of them pass the greater part of their existence in mud. Some even perch upon trees, thus competing with birds, whose kingdom is the air, and which are able, with the aid of their wings, to plunge into space and travel rapidly over considerable distances. Yet there are birds, deprived by Nature, which do not possess the wing characteristic of the feathered tribe, and are consequently, like the majority of animals, pinned to the soil.
Birds do not all have equal power of flight, which is closely related to the extent of the development of their wings. There exist all grades in the spread of wings between that of the condor, which is four times the length of the body, whereby the bird is able to rise to the height of nearly twenty-five thousand feet, and the little winglets of the auk, which are of no use to it. The penguins have still smaller wings, which are nothing more than short, flattened stumps, without proper feathers and covered with a fine, hairlike down which might be taken for scales.
Another group of birds exists, called appropriately Brevipennes, the wings of which are so poorly developed as to be wholly unsuitable for flight. As an offset and just compensation for this, their long and robust legs permit them to run with extraordinary speed. For that reason they have been called running birds, in distinction from other kinds that constitute the group of flying birds. Among them are some gigantic birds, and also some that have no visible wings on the outside of their bodies, and may therefore be properly called wingless.
The ostrich is a member of this group. With its bare, callous head and short bill, its long, featherless neck, and its massive body, supported by long, half-bare legs, ending in two large toes; its very short wings, formed of soft and flexible feathers; and its plume-shaped tail, it presents a very special appearance among the birds.