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menon, even during an interval of 141 years, as indicating real change.

The only way, as it appears to me, to test an alleged change such as we have in the phenomena of Linné, is to collect and well weigh all the information bearing on the subject, both ancient and modern. Hitherto, the progress of selenography as depending on the comparison of drawings for fixing characters, or for determining change whether they be early or recent, has signally failed. I speak more of my own experience, at the same time with regard to others I am unable to lay my hand on any delineations of the moon's surface that would satisfy me, from a comparison of them alone, that real change has taken place. All the circumstances above alluded to are so many drawbacks to a true scientific determination of change. Under these adverse and almost disheartening circumstances we have, it may be, a clue to the unravelling of some of the mysterious phenomena occurring on the moon's surface; that which has been attended with such signal success in the investigation of binary stellar systems :-viz. measurement-may be employed with advantage on the moon's surface. Already it has enabled us to determine with equal accuracy as on the earth, the positions, heights, and depths of objects; and a judicious application of it to size may still further help us. I am not aware how far B. and M.'s and Lohrmann's measures for size can be depended on, the extents of objects are very freely scattered over both works, and B. and M. give, on pp. 88, 89 of "Der Mond," the formula, with an example of computation, also a table of determinations. Be this as it may, we are certainly able to obtain measures, accompanied by a description of the objects measured, which treated in a certain way, are likely to help us in our investigations. For very close determinations, each measure must be submitted to a somewhat troublesome computation, and this is by no means desirable, as small differences will inevitably be mixed up with the circumstances before mentioned. To decide on real change, we must have salient and large differences. Now we know that at the centre of the moon one second of arc covers a certain linear space, according to my computation, 1.1585 English miles, or 6116.7 English feet. At any given angular distance from the centre, 10 will cover a greater space in the proportion of the secant of the angle. If, therefore, we measure the diameter of an object on the radius from the moon's centre, it is not difficult to calculate the extent of the measured line in English feet, and we can thus express the size in English feet and seconds of arc. Both Lohrmann and B. and M.

give the diameters of craters in geographical miles of 3807·1 toises each, which are equivalent to 24,344 English feet, and we have Schmidt's value for Linné 15, and B. and M.'s 14 geographical miles; these values subtend respectively 517 and 4"83 at the position of Linné on the radius at mean distance. The value of 10 7056.6 English feet. On Oct. 18, 1866, Schmidt considered the whitish cloud which he found in the place of Linné, to be two geographical miles in extent; and on Dec. 27, 1866, only 2000 toises. In Dec. 1866, on the 15, 18, 19, 21; and on Jan. 14, 1867, I measured the white cloud on Linné, in a direction at right angles to the parallel. The measures varied from 675 to 11"61 or 4"86, a greater quantity than can reasonably be ascribed to errors of observation. In July, 1867, measures made very carefully, varied from 533 to 70 in three days. Mr. Buckingham, on March 14, 1867, found the cloud 60 in diameter. Combining these (of course involving the small errors arising from libration +), we have a series of diameters referred to the radius varying from 12,790 to 81,920 English feet-a very large difference, and certainly unconnected with libration. The difference in seconds 9"80, which is very much greater than the difference of 25 sets of measures of Dionysius-viz. 4"67. This difference in the measures of craters appears by no means to be a rare occurrence. Now, what does this depend upon? The measures of the white cloud "Linné " were made so near each other in Dec., 1866, and in July, 1867, that the varying distance of the moon could not produce so great a difference. It is difficult to conceive that the increasing or decreasing altitude of the sun could alter the extent of a reflective surface, although it would the intensity. Again, unsteadiness in the state of our atmosphere would tend to enlarge the measures, but the difference of 4"5 in three days can hardly be referred to this cause. The series appears to point, 1°, to errors of observation or reading; 2°, to the state of our own atmosphere; or 3°, to an increased and decreasing extent of matter of a greater reflective power than the surface of the mare.

Were all the observations made by myself, I might well distrust them, but when Schmidt and Buckingham agree to within 10, and I agree with Huggins to 0008, some of mine at least come into the category, and, with the exception of Schmidt's value of Dec. 27, 1867, and a measure by Joynson of 4'42 on January 3, 1868, they give a larger extent for the white cloud than Lohrmann and B. and M. gave for the crater. The small crater of Secchi, which he estimated at of a second, with the small hill and black

point estimated by Schmidt at 1900 and 1700 English feet, would subtend an angle of about of a second. But Respighi makes the small crater 4"0. Gathering up the results, we have four separate objects on record. The crater (B. and M.) (Grube, Lohrmann), 1087 English feet deep, having a mean diameter of 35,265 English feet; the whitish cloud (weisse wolke, of Schmidt), the largest measured diameter being 81,932 English feet, and the smallest 31,190 English feet. Schmidt's estimation on Dec. 27 was much smaller, viz., 12,790 English feet. The little hill of Schmidt, seen by Buckingham and others, of about 1900 English feet in diameter; and the little crater of Secchi, of about 2000 English feet in diameter, which has been seen by several observers.

How are these different objects to be treated? The questions that suggest themselves to my mind are numerous. Did the state of our atmosphere in Greece so effectually conceal the white hill and black point, that Schmidt did not see them during October and November? Was the state of our atmosphere so greatly improved in April that the little crater was seen more distinctly then than at any other time? Has a deep crater, which B. and M. probably measured, and to which three selenographers have assigned a diameter over one geographical mile (German), been so affected by some remarkable influence during the last few months, as to present the appearance of a hill or dot, and afterwards of a small crater of less than its seventeenth part. Has the older crater during a period of short duration shown itself-still contracted in size-to Respighi, while other observers speak of it as small. If it appears smaller to us in 1867 than it did to Lohrmann and B. and M. in 1823 and 1831, it is exceedingly important to ascertain the nature of the cause that is capable of so materially contracting it in appearance, while it remains precisely of the same size. If, on the other hand, the crater was never larger than it at present appears to us, it is quite as important to prove that B. and M. and Lohrmann were mistaken! Respighi says on this point: "Que si la carte lunaire de MM. Beer et Mädler donne au cratère des dimensions plus grande, ou doit l'attribuer a l'inexactitude du dessin inexactitude dont ou pourrait produire d'autres exemples bien plus manifestes; ou est d'autant plus fondé à le faire que dans le cas actuel il s'agit d'un objet représenté par un signe de convention plutôt que par un dessin veritable."

Another question suggests itself: What influence has produced the appearance of the whitish cloud seen on one occasion more than

twice as large as the crater recorded by Lohrmann and B. and M. Schmidt, in his paper on Linné, gives no observations between 1843, August 17, and 1866, Oct. 16. In 1842, July 14, Linné was drawn "als sehr kleiner crater." It is therefore very uncertain when the appearance recorded by Schmidt first occurred. On the other hand, it is quite certain that between 1866, Oct. 16, and the present date (June 28, 1866), we have enough on record to prove the present state of Linné, viz., the larger shallow crater, with the smaller crater within it a state of things not at all hinted at in the earlier records. The whitish cloud is still existing, and is a very proper object for measurement in the present state of affairs. The black dot may or may not be seen again.* It appears to have been described as a crater, and a diameter of 40 assigned to it. Its permanence, or fugitive character, it is very important to determine; but nothing I apprehend can settle the question but close and unremitting observation, which Linné has been the subject of since Schmidt's announcement.

On April 11, May 11, and June 10, I carefully examined Linné, and could find nothing like a crater with apertures of 4 and 81 inches.

* The small crater was seen and measured by Mr. Joynson on January 3, 1868. Its diameter was 1223, which is 0487 less than Mr. Huggins's measure of it on July 9, 1867.

THE THUNDER-STORMS OF NATAL.

(With a Map and a Plate.)

THERE is a small colonial land, belonging to England, and furnishing a field for the growing enterprise of a now increasing band of English settlers, which lies in the further hemisphere of the earth, reaching at one point to within but little more than 200 miles of the Southern Tropic, where the scorching sun shines vertically down upon the ground at noon at the period of midsummer; but which, nevertheless, has a yearly mean temperature that, even on the low sea-coast, does not exceed 68° of the heat-scale of Fahrenheit, and an absolute temperature that, excepting upon rare occasions, when a hot land-wind, or Sirocco, prevails for a brief period, does not at the warmest period of the day exceed the heat experienced in an English summer. In this favoured land the sugar-cane, evergreen coffee, tobacco and cotton, Indian corn, the pine-apple, the banana, and the orange are at home; and so also, strange to say, are cattle, sheep, and horses, the grain and root-crops, and to a considerable extent, the trees, vegetables, and fruits of temperate Europe.

This remarkable result is brought about by the instrumentality of a power that is ordinarily deemed one of the most terrible of nature's agencies, but which here stands so manifestly stamped with a beneficent character and function, that it is literally stripped of its terrors and welcomed, day after day, nay almost courted, for the good it brings. The writer of this memoir, indeed, as having occupied somewhat the unfortunately responsible position of a clerk of the weather and, as it were, Rain-doctor, out in the land of which he is about to speak, has actually found himself on not rare occasions, exposed to the unmistakeable and very imperfectly repressed reproach of a partner and companion-who certainly had no particular affection for thunder and lightning in old days in England, before she had had the advantage of making its acquaintance in South Africa-because the afternoon thunder-storm lingered a little behind its time. It will be apparent, then, from this incidental avowal, that the clerk of the weather and Rain-doctor in question is about to describe this agent in the operations of nature, the thunderstorm, as it is capable of being studied in its favourite haunt and home, among the green hills of South-Eastern Africa, where it ordinarily assumes its grandest and most majestic aspect, while performing its beneficent office.

It is matter of familiar knowledge that the conversion of invisible

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