continent. It is rather singular that no part whatever was assigned to America until 1882, when 'the utmost reliance might be placed on the zeal of our American brethren ' for observing the transit of that year at stations in the United States.

But the next few years saw all these ideas changed. It was found that an error had been made in the original investigation of the conditions of the two transits, and that it is for the later not the earlier transit that Halley's method fails. Accordingly, ample preparations were made for observing the duration of the transit of 1874 from suitably selected stations. Among these were several which had been already chosen for the other method; but others were new. In particular the Russian Government provided for a whole range of stations in Eastern Siberia where the transit had a lengthened duration, while America, France, and Germany arranged to occupy Crozet Island, St. Paul's Island, Campbell, Auckland, and other islands in the subAntarctic Seas, which, with Rodriguez, Kerguelen, and other places to be occupied by England, formed ample provision for the observation of the shortened duration.

A little disappointment was occasioned to those who may have hoped, from the plans published in 1868, that Antarctic exploration would have been undertaken for observing the transit of 1882. So soon as it was pointed out that no good could result from the occupation of Antarctic stations in that year, all those plans were very properly abandoned. It had now become known, however, that Antarctic stations would be more useful during the transit of the present year than it had been supposed they would be in 1882. Some imagined that the authorities who had been so enthusiastic in favour of Antarctic exploration for one transit would not be altogether opposed to such exploration for the other. This hope was doomed to be disappointed. In fact the Astronomer Royal and the Admirals grew quite facetious in ridiculing the idea of Antarctic exploration; though they suddenly became serious,

even to severity, when reminded of the views they had themselves expressed in 1868. Thenceforth they deprecated jesting with a touching solemnity.

But after all, Antarctic exploration was not a point of great importance for the transit of 1874. So admirably was the method of durations suited for this transit, that without incurring the dangers of Antarctic voyaging-whether these dangers be excessive, as now stated by the Admiralty, or slight, as they stated in 1868-a large number of stations could be occupied both in the northern and southern hemisphere, whence the whole transit could be seen. And fortunately for science the opportunity was recognised early enough to be turned to good account. Russia, as we have stated, occupied no less than eleven northern stations for observing the whole transit, America, Germany, and France occupying between them seven or eight others in Siberia, North China, and Japan, while England occupied one in North India.

In the southern hemisphere nearly all the stations were such that the duration of the transit could be observed, except Cape Town, which had special value as a station for observing the middle of the transit. England occupied four stations in the southern hemisphere, besides Cape Town, Melbourne, Sydney, and other places already provided with astronomical instruments; and America, France, and Germany occupied many other southern stations for applying Halley's method. But it is not by any means to be supposed that Delisle's method was neglected. England, for instance, occupied the Sandwich Islands where only the ingress could be observed, and the Isthmus of Suez whence only the egress could be observed, and Russia had a yet larger number of astronomers devoted to the observation of egress only. Moreover, all the stations whence the duration could be seen were excellent stations for observing ingress and egress alone, so that where bad. weather unfortunately prevented the observers from noting the duration, they still had a chance of doing useful work.

This, in fact, was one great reason why it would have been little less than a disaster for science had the value of the transit for Halley's method not been noted in good time; because it was hardly to be expected that other nations would occupy second-rate Delislean stations when England and Russia had all the best stations of that kind, whereas under the actual circumstances a large number of secondrate but excellent Delislean stations were occupied because they were first-rate Halleyan stations.

STAR-CLOUDS AND STAR-MIST.

THERE are some scientific questions which have excited an interest seeming at a first view disproportioned to their intrinsic importance. Sometimes the distinguished position of those who have taken part in scientific discussion, at others the skill and acumen with which rival theories have been maintained, have directed exceptional attention to particular questions. Too often personal animosities have become associated with a subject of discussion; for unfortunately it sometimes happens that

The man of science himself is eager for glory and vain,
An eye well practised in nature, a spirit bounded and poor.

But there have also been occasions when the singular progress of a dispute, the swaying hither and thither of contending evidence, and even the repeated apparent settlement of the question, by what seems like overwhelming evidence on one side or the other, have given the discussion a singular (perhaps a factitious), and in a sense an almost romantic interest.

Among questions of this sort, few have been more interesting than the subject of which I propose to give now a short sketch, partly historical, partly explanatory—the history running up to this present time, the explanation to be completed only hereafter, and perhaps at a very remote date, if at all.

When the depths of the heavens are explored with a powerful telescope a number of strange cloud-like objects

are brought into view. It is startling to consider that if the eye of man suddenly acquired the light-gathering power of a large telescope, and if at the same time all the single stars. disappeared, we should see on the celestial vault a display of the mysterious objects called nebulæ or star-clouds, exceeding in number all the stars which can now be seen on the darkest night in winter. The whole sky would seem mottled with these singular objects. With reference to nebulæ, or rather to certain classes of them, opposite views were for a long time maintained by astronomers. 'Whether,'

said Humboldt, half a century since, 'the cloudy masses referred to be indeed composed of luminous matter, or whether they are merely remote, closely crowded, and rounded clusters of stars, is a question which has for more than two hundred years been agitated among astronomers.' Galileo and Kepler and Halley discussed the question when as yet the nebulæ might be counted on the fingers of one hand. I might, indeed, even go further back, and quote the views of Tycho Brahe, who wrote before the telescope was invented. Later, the Cassinis and Mitchel, on one side. Durham, Lacaille, and Kant, on the other, supported the rival theories according to the evidence available in their day. Then came the wonderful labours of Sir William Herschel, who in an age when nebulæ had been counted by tens, continued to send in to the Royal Society lists of thousands of these objects. His discoveries attracted fresh attention to the subject of controversy. Holding first the opinion that all nebulæ are star-clouds—that is, closely aggregated congeries of stars, reduced by distance to the appearance of clouds-Herschel came round in the latter part of his career to the contrary view, and gradually, after many disputes among his followers and opponents, the question was held to be settled in favour of the opinion he maintained. How, since then, a contrary opinion gradually struggled into favour, and was eventually established, as was thought, upon the firmest possible basis; how this opinion, at the moment when it seemed that all astronomers were