would be melting itself instead of freezing the water it came in contact with. There is no more reason for the lakes it caused being frozen than that the Manjalen Sea-which is formed in a similar way-should be so.

The advance of the circumpolar ice-sheets in the form of low ridges, rising very gradually from their outer edges and culminating at a height of perhaps not more than 8000 or 10,000 feet above the present level of the sea, and then decreasing in height towards the Poles again, in consequence of the moisture being precipitated on the outer slopes, is, I think, more in accordance with our experience than the usual form of the theory of ice-caps, which would make them highest at the Poles. The latter supposition, it has been shown, is opposed to the facts that the ice flowed northward from the northern end of Scandinavia; that the high peaks of the Lofoden Isles are not glaciated; and especially that at the present time the northern end of Greenland is much more free from ice than the southern extremity. Whilst every inlet of South Greenland is occupied by ice flowing from the interior, and breaking off into great bergs when it arrives at sufficiently deep water, in the extreme north the glaciers do not reach the level of the sea. And in the Glacial period, instead of there being more ice than now within the Arctic and Antarctic circles, there was probably much less. Within the ridge of ice that then, I think, irregularly encircled the southern hemisphere, and, in the northern, bridged across the northern ends of the Pacific and Atlantic Oceans, both plants and animals that could hybernate through the severe and long winters might have existed in greater abundance than now. Even now, in the Antarctic, behind the ridge of ice that surrounds the South Polar continent, it is possible that there may be large areas of land free from ice in summer, and supporting a flora and fauna, which, if they could be studied, might throw much light on the distribution of animal and vegetable life in the South Temperate zone. And were that barrier of ice once passed a journey to the South Pole might be found to be less impracticable than one to the northern extremity of the globe.

Whilst the physical phenomena of the Glacial period in New Zealand and South America are so similar, there are differences in its effect on the pre-glacial fauna that must be noticed. Although there was great destruction of life. amongst the individuals of the great apterous birds of New Zealand, there was not the same extirpation of species as

in America, nor even so much as in Europe amongst the great mammals. Numerous bones of the moas are found in the stratified deposits of the plains, but the same species reappear in the post-glacial surface-beds. Their complete destruction did not take place at the same time as the megatherium and its associates in America, or the mammoth and the woolly rhinoceros in Europe, but must rather be correlated with that of the Irish elk and the American mastodon. The preservation of the large birds through the great vicissitudes of the Glacial period may have been owing to the following causes :-Long before the ice from the Antarctic reached the coasts of New Zealand, a great stretch of land to the north would be laid dry by the lowering of the sea-level, so that there was possibly more space suitable for their occupation during the greatest extension of the ice than now. The same lowering of the sea-leve! took place all over the world, and in South America a similar extension of land surface must have ensued; but there the new land, to the north of the districts glaciated or submerged beneath the waters of the great glacial lake, would be occupied by northern animals who would resist the immigration of those fleeing the catastrophes of the south. In New Zealand, also, there was not the same competition with smaller animals, which appears to have led to the extirpation of many of the bulky species of the continents during the changing conditions of the Glacial period. Had, for instance, New Zealand at that time become connected with Australia, the great extension of area thus obtained, instead of tending to the preservation of the moas, would have probably led to their extermination by bringing them into competition. with the marsupials of the continent. But a channel, 2600 fathoms deep, separates the two countries; and that there was then no land connection between them is evidenced by the absence of the gum-trees and acacias of Australia from the flora of New Zealand, and of the marsupials, the cockatoos, the grass parroquets, and the pigeons of the former from its fauna.

Although the large apterous birds of New Zealand were not exterminated during the great glaciation of the country, there are other signs of the impoverishment of its fauna. About 200 species of beetles had been described in 1872, and these belonged to no less than 110 genera, giving an average of less than two species to each genus. Many of the species live also in Australia, or have nearly allied forms there. The great paucity of insects, the isolation of the species, and their affinity with those of Tasmania and

Australia, is probably due to a great destruction of the native species during the Glacial period, and the arrival since of several from the countries to the north-west. I do not know how else some of the facts can be explained, such as that of there being only eight butterflies, and amongst these some of wide distribution; and that of the Heteroptera there are thirteen known species belonging to thirteen different genera and nine distinct families.* Such gaps as these in the fauna of a country are as significant as the grooved and polished surfaces of its rocks, and the naturalist may as surely point to the evidences of the Glacial period as the geologist.

Mr. Wallace has also drawn attention to the large destruction of species of insects in the Chilian sub-region, evidenced by the great number of peculiar genera of beetles of extremely isolated forms, and I might multiply instances from the faunas and floras of southern lands, all tending to the conclusion that the southern hemisphere has been glaciated as much as, or more than, the northern; but I could not do justice to this phase of the question within the limits of this article, and I have only glanced at some of its most salient points with the object of indicating that the physical evidence of glaciation does not stand alone, but is strengthened by that of the present distribution of animal and vegetable life.

Capt. HUTTON, Trans. New Zeal. Inst., vol. v., p. 227.

VOL. VII. (N.S.)

2 B

IV. RECENT ADVANCES IN TELEGRAPHY.

By J. MUNRO, C.E.

F recent years the most important development of practical telegraphy has been the duplex system of sending messages. The idea of transmitting more than one message along a single wire at once appears first to have been conceived by M. Zantedeschi, as early as 1829; but we may take it for certain that it was not until 1853, or about twenty-five years ago, that any serious attempts were made to carry it out in practice. From that time the attention of electricians, both in Europe and America, has been directed to this interesting problem, but, until the last ten years, with so little success that in the 1867 edition of Sabine's "History of the Electric Telegraph " we find the following sentence:-" Both these systems of telegraphing in opposite directions, and of telegraphing in the same direction more than one message at a time, must be looked upon as little more than 'feats of intellectual gymnastics' -very beautiful in their way, but quite useless in a practical point of view."

Experience, however, shows that it is unwise to repudiate an electric novelty; and since these words were written the duplex system of telegraphing in opposite directions has become the ordinary means of communication over thousands of miles of land-lines in England and America, and of submarine cables in Europe and the East; while the multiplex system of sending several messages in the same direction is rapidly being brought into practical service by means of the Meyer instrument in France and the telephones of Elisha Gray in America and La Cour in Denmark. Indeed, by a future combination of the duplex and multiplex systems, we may yet have a single wire transmitting as many as twenty or more distinct messages, ten either way.

In order to explain the general principle of duplex or counter transmission, it is necessary to be perfectly familiar with the ordinary method of simplex or simple transmission. An ordinary telegraphic circuit invariably consists of the battery or source of the electric current; the key or sending instrument, by which the circuit is opened or closed

and the current admitted into the line; next, the line or wire stretching from the Station A, where the message is sent off to the Station B, where it is received; and next, the receiving instrument at Station B, actuated by the current from the line, so as to give sensible signals. These parts of the circuit are all connected together, and at

Station A one "pole" of the sending battery is put in con

nection with the ground by means of an "earth-plate," which is generally an unoxidisable metal, such as copper; while at Station B one "terminal" of the receiving instrument is connected, similarly, to an "earth-plate" there. A complete external circuit for the sending battery is thus formed through the line, the receiving instrument, and the