of miles across, and flame-tongues one hundred thousand miles high, whirl and leap in the sun in order that soft winds may breathe, gentle rains fall, verdant plants grow, and endless generations of animals succeed each other and run through the appointed round of sentient being, on the islet worlds that have been scattered through space, each at the appropriate span of remoteness that fits it to the end secured.

It is scarcely possible to turn from the subject which has been briefly sketched in this article, in answer to a question that is just at this time met with in every direction among cultivated people, without hazarding a yet further development of the explanation in order that the subject may be really complete, and that the point of a controversy, that has recently been attracting some public notice, may be fairly understood.

This

The old orthodox form of observing the transit of Venus, which was adopted at the time of Captain Cook's expedition, will upon the approaching opportunities be checked, and, indeed, on the first occasion, be in some measure superseded, by another method of procedure that has an especial interest and a particular recommendation of its own. method is based upon the fact that the planet will have moved through a considerable and appreciable part of its orbit between the instant when it is first seen upon the sun's face from any part of the earth, and the instant when it is first seen there from all parts of the earth that are, at the time, directed towards the sun. The interval which will be occupied by the planet in performing this stage of its journey will be an ascertainable part, or proportion, of the time occupied in the whole orbital revolution, for it can be observed in seconds, while the whole journey is known to occupy 224-7 days, or 19,414,122 seconds. But the planet also passes during that observed interval through an actual length of space which is less than the distance separating the two spots, or stations alluded to, on the earth, in the proportion in which 723 is less than 1,000, the relative distances of the planet and the earth from the sun. From these simple data the astronomers will calculate: 1. The exact size of the entire orbit of Venus, a quantity as much more than the distance separating the two stations of observation on the earth as 224-7 days, or 19,414,122 seconds, is greater than the number of seconds consumed in the observation, an allowance in the proportion of 723 to 1,000 having been made in reduction of the estimate. 2. The length of the radius of the orbit of the planet, which always has a known fixed relation to the circumference, and which it will be observed is the planet's distance from the sun. 3. The earth's distance from the planet,

which is simply to the sun's distance from the planet as 277 to 723. And 4. The earth's distance from the sun, which is merely the planet's distance from the sun added to the planet's distance from the earth. This method of deducing the sun's distance from observations of the transit of Venus, which is known as Delisle's, from the name of its originator, has the great recommendation that a good result can be obtained if one observer has a favourable view of the commencement of the transit from the spot where it begins earliest, while another observer sees it from the spot where it begins latest; but it has also the disadvantage that the exact longitude, and true local time, of both stations must be known. In the more generally adopted method of observation, which was first employed by Halley, and which is therefore distinguished as his method, on the other hand, the entire progress of the transit from beginning to end must be watched from two remote stations, and the exact interval of time between the beginning and end must be ascertained at both. But the special recommendation of this method is that neither the longitudes nor the local times of the stations are required. It is obviously a very desirable thing that two plans of observation so radically distinct in principle and in detail should be employed in order that the one may be used as a check upon the other. But there is a still stronger reason for the employment of both methods, depending upon the fact that it most fortunately happens that sometimes the one and sometimes the other method of observation is best adapted to the circumstances of the time when the transit takes place. It occasionally occurs that these good results can be secured by Delisle's process when stations suitable for the employment of Halley's process are not to be had, and sometimes the converse is the case. A very long interval commonly lies between successive transits, because during that time Venus passes either above or below the face of the sun as it sweeps round that luminary, at nearer distance, and with greater speed, than the earth. The planet is only seen on the face of the sun when the orbital paths of both itself and the earth are so arranged as to permit the planet to be diametrically between the earth and the sun as it traverses this part of its journey, passing the earth on account of its greater speed. It, however, generally happens when this planet has once passed over the sun's face after the long interval, that it may be seen there again after the comparatively short lapse of about eight years. This occurs because the first time the planet goes round again to overtake the earth in the position in which it lies between it and the sun, the relative courses of

the two tributary bodies have not departed from each other enough to carry the planet clear of the sun's face either above or below. It is therefore again seen upon that face as it sweeps by. This is the reason why there was a transit of Venus in 1761, eight years before the historical transit associated with the name of Captain Cook, and this is why there will be a transit again in 1882, eight years after the transit of 1874, for which preparations are now in rapid progress. After the year 1882 no further opportunity for the revision of the estimates of the sun's distance by this royal method of observation will occur again until the years 2004 and 2012.

Now it has been ascertained that, as a general rule, the first of the closely coupled transits of Venus is so circumstanced as to be more favourable for Delisle's method of observation, and the second more suitable for the old method of Halley, and in the early preparations for the observation of 1874 it was accordingly assumed that the arrangements should be directed towards carrying out the operations upon Delisle's plan. The Astronomer Royal, after a careful consideration of the whole subject, came to the decision that the five best stations that could be fixed upon for Government observers to be employed at would be Alexandria,-Woaho, one of the Sandwich Islands in the North Pacific,-Rodriguez Island to the east of Mauritius, Christchurch in New Zealand, and Kerguelens Island far south of the Indian Ocean; and that on these stations Delisle's method of observing first and last contacts at different stations would have to be mainly, if not exclusively, adopted.

A grant of public money to the extent of the requirements of the equipments on this base was arranged, and the preparation for the work was systematically entered upon. A new prophet, however, shortly afterwards arose. In the midsummer of the year 1869 Mr. R. A. Proctor presented a memoir to the Royal Astronomical Society, in which he questioned the soundness of the view which had been authoritatively adopted, and supported his argument in the matter by a method in which he is peculiarly skilled, namely, the diagrammatic, or pictorial, representation of the conditions of the question in the form of six orthographic maps representing the course of the transit for different parts of the earth. These maps were constructed by a large expenditure of industry and ingenuity, involving as many as three thousand distinct geometrical measurements. With these maps for the device upon his shield, Mr. Proctor affirmed that the old Halley method of observing the entire duration of the transit from remote stations of the earth would be found to promise better results in 1874 than

Delisle's method, even with the Astronomer Royal's own stations of observation, and that it would yield most materially better results, if other possible positions were also occupied in the Antarctic Ocean.

The method of attack which was adopted on this occasion is in most cases an efficient and powerful one when skilfully, pertinaciously, and unflinchingly carried out. In the evolutions of modern literature it is very much what the Prussian scattered formation of advance is in the evolutions of modern war; and it has the further recommendation, in determined hands, that the expedient of buckram men' and 'suits of 'Kendal green' can also be drawn upon. One resolute man can play the part of a swarm of skirmishers by himself. Upon the present occasion, however, it happened that before the defences of Greenwich were entirely invested, the Admiralty drew the attention of the Astronomer Royal to certain of the newspaper articles which had appeared, and asked for some information concerning them. The Astronomer Royal, having examined the articles and considered their statements, drew up an official report to the Admiralty, and sent it in with the suggestion that, although from the nature of the case he could not enter into newspaper controversy concerning the matter, the report might be, perhaps, communicated to the Royal Astronomical Society for the information of the members, who were naturally much interested in the proceedings of the Government expeditions. The Admiralty adopted the suggestion and officially sent in the report to the Society, and it was thereupon read at a meeting of the Society and printed in its Transactions, known as the Monthly Notices. The report is entitled A Letter from the Astronomer Royal to the Secre'tary of the Admiralty, expressing his Views on certain articles 'which had appeared in the Public Newspapers in regard to 'the approaching Transit of Venus.' From this letter it was plainly manifest that the views of the Astronomer Royal were that those articles were substantially the development and extension of Mr. Proctor's memoir of 1869, and that Mr. Proctor had taken a too geometrical survey of the question at issue, and had strained the idea of separating the Northern 'and Southern stations,' which were to be occupied by observers, somewhat more than was practically wise. In order to show that he was not alone in his own opinion the Astronomer Royal quoted a communication from the distinguished astronomer, Dr. Oppolzer of Vienna, in which the remark occurs that the main reason for the preference of Halley's method of observation was its freedom from the need of very VOL. CXXXVIII. NO. CCLXXXI.

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exact determinations of the longitude of the station of the observer- -an immunity of great practical importance in past days, when the problem of exact observations of longitude was in a rude and imperfect state, but of no moment whatever now, when that difficulty has been entirely removed by a more perfect knowledge of the moon's irregularities. Dr. Oppolzer clearly considers that Halley's method of transit observation 'possesses no special advantage at the present day.' The Astronomer Royal points out that it is an essential part of his programme that new determinations of longitude will be formed at each of his five stations, by observing at least thirty meridional, and one hundred and twenty extra-meridional, transits of the moon, and that from these he will have the true longitudes of the observers' place within a single second of time, and therefore exquisitely exact and infallible means of making his final estimate from comparison of absolute Greenwich times of the occurrence of the transit at these several stations; and after an exhaustive re-consideration of Mr. Proctor's argument, estimating it chiefly by a deduction which implies that in no case can Halley's method be more valuable than Delisle's, unless the parallactic difference of displacement of the planet's course from the opposite stations under consideration amounts to at least 32.8 minutes of duration of the phenomenon, he concludes that only under the improbable circumstance of a good mid-winter observation being secured at Nertschinzk in Siberia, in the high northern latitude of 51°, and in a continental situation 1,000 miles from the nearest sea, could this difference of duration be secured by stations suggested by Mr. Proctor as desirable. The ultimate issue of the Astronomer Royal's reconsideration of the question is, that he finds himself unable to recommend a Government expedition to high Antarctic latitudes, with a view to the more extended adoption of the Halley method of observing.

To this communication Mr. Proctor, although apparently his own numbers had been used in the deduction, replies that the Astronomer Royal has assumed the least favourable stations of comparison for his case, and that if other obvious and specified stations had been taken for comparison, a difference of parallactic displacement in excess of the Astronomer Royal's criterion is secured; and that yet other stations, accessible in the early part of the month of December, are available which would give better results by Halley's method than even the Astronomer Royal himself looks to obtain from Delisle's. Mr. Proctor's last word in the controversy assumes very much the ring of a prophetic denunciation. Again urging an ex