The Astronomical Register.

No. 177.

SEPTEMBER.

1877.

THE SATELLITES OF MARS.

The following telegram has been received by M. Leverrier from Prof. Joseph Henry, Secretary of the Smithsonian Institute: "Two satellites of Mars, by Hall, at Washington, first elongation west, August eighteenth, eleven hours Washington, distance eighty seconds, period thirty hours, distance of second fifty seconds," which being translated from telegraphic English into ordinary language probably means that Prof. Asaph Hall, of Washington, making use of the great 26-inch refractor, believes that he has discovered two satellites to the planet Mars. The first was at its greatest west elongation on August 18th, at eleven hours Washington time. Its distance was then eighty seconds of arc from the centre of the disc, or in other words, when at greatest elongation it was only three diameters of the planet from the western limb. (The diameter of Mars at Greenwich noon on the 18th August was 23"·75.)

It would seem that Prof. Asaph Hall has observed this satellite sufficiently often to be able to predict that its period is only 30 hours. The distance of the second satellite is given as 50 seconds, but it does not appear whether it is really nearer or further from the planet than the other satellite.

VOL. XV.

OPTICAL SPECTROSCOPY

OF THE RED END OF THE SOLAR SPECTRUM, AT A HIGH ALTITUDE IN THE SKY.

PART I.-METEOROLOGICAL APPLICATIONS IN THE SMALL

AND ROUGH.

"All the most important features of the solar spectrum cluster about the blue and violet end thereof," wrote, not long since, a great spectroscopist, to whom we all owe much; and he may be right, for it is that end which chiefly represents the higher volatilizations of the intensest heat, all the most sensitive known reactions to actinic influences, with their train of permanent impressions of even the minutest visible and some invisible features, and the chief triumphs of human labours in both theory and observation.

But precisely because all that may be most absolutely true, and because the blue and violet spectrum end has, therefore, troops of able observers, devoting themselves magnificently to further researches therein; so a duty has fallen into my province, in my particular mode of trying to work, to the best public advantage, the small and poorly fitted-out Royal Observatory of Edinburgh, to take up the too generally neglected, even by some despised, red end of the solar spectrum, and endeavour to do a little more justice to its innate, natural claims to attention, in a manner also suitable, if possible, to the wants, and ready to serve the demands, of the present scientific day.

Hence, after arranging a spectroscopic apparatus peculiarly for work towards the extreme red rather than any other part of the spectrum, and having had this grand practical truth proved again and again, in spite of me unwilling, that for any really fine optical examinations of solar light it was pure waste of 9,999 out of 10,000 of precious moments of time spent upon it to attempt to make these observations in the latitude and locality of the Royal Observatory of Edinburgh, because that locality is condemned to a low sun, almost constant clouds, frequent rain, and completely eternal haze of dun-coloured, greasy, coal smoke, thrown out from legions of chimneys, both factory and domestic, close around the observatory's site, on Sundays, too, as well as week-days, and by night as well as by day-I therefore determined to try what could be done by taking the instrument for a time to some southern land.

Of course, to have got the best results, I ought, besides going south, to have followed Newton's sage, prospective advice of making the experiments in the pure and calm air on an elevated

mountain top, raised far above the principal clouds and grosser impurities of the lower atmosphere; and much indeed had I desired in this cause to repeat the experiences of the Teneriffe Astronomical Mission of 1856. But as no public funds or other assistance of any kind appeared, and my private purse was already nearly run dry by the cost of the spectroscope itself (requiring, as it eventually did for travel, no less than four large boxes and three tripod stands), I could only make a compromise with opposing requirements; or rather found that I had only just enough money left, this year at least, to take the instrument by an economical sea-voyage to some southern city, where the climate should be proverbially dry rather than wet, where little stone coal is burned, and where the summer sun should rise nearly 20° higher in the sky than it ever does in Edinburgh.

Such a city proved to be Lisbon, and the means of getting there were the ships of the Liverpool Pacific Steam Navigation Company, forming quite a link of connection between the coasts of cloudy Great Britain and sunny Portugal-which many others besides an occasional astronomer and his wife might well profit by; though the two latter have also to acknowledge the handsome liberality of the Company, who conveyed all the spectroscopic packages both out in their s.s. "Aconcagua" and home in their s.s. "Cotopaxi as luggage (in a case where no merchasdise is permitted), and without any charge. Such action on the part of the directors tended to make matters look cheerful, if not hopeful also, from the very moment of embarking. But even before that, we had been greeted with something connected with the red end of the daylight spectrum, which promised to be useful to others as well as interesting to ourselves, though it was but a hasty observation, and made in a very petty instrumental manner.

Small it was obliged to be, because neither in a railway carriage on shore nor in a vessel at sea can a large spectroscope of many feet long and broad be employed, any more than an astronomical telescope of the same size; and just as every navigator still depends in his nautical astronomy, on board the grandest ship that floats, upon a diminutive sextant which he holds daintily in his fingers, so the spectroscopist when travelling must content himself with the smallest conceivable spectroscope, in fact a "waistcoatpocket spectroscope," after a fashion now made by several opticians.*

But what may any traveller hope to accomplish with such a

*By no one better than Mr. Adam Hilger, 192, Tottenham Court Road, London. The particular example, whose use is presently to be described, was made by him; and was contained in a tube about 4 inches long and -inch in diameter only.

little mite of a tool? Nothing that can touch the particular researches into details of the solar spectrum as they are carried on with large instruments in fixed observatories, you may be sure. But here is an example of what bountiful Nature produces from time to time out of her abundant store of wheel within wheel of many phenomena, if carefully and conscientiously observed.

It was summer, and on the morning we left home, the 11th of June, a bright morning with a fairly high barometer (30·00) and a pleasant temperature of 64°, my wife had written down in her little meteorological journal, in the column entitled "Spectrum of daylight sky," and in its respective sub-columns of "Rain band" and "Low sun band," the figures 2 and 2, implying, where 10 was an expected and possible maximum, a very moderate degree of each of those effects in the red region of the spectrum. So we looked for a fine day, and a very fine day, too, it turned out, nor had either of us the least suspicion of anything else being literally in the wind, until, by the time the train was passing through the lake-district of Cumberland, we positively started on perceiving in the pocket spectroscope a rain band so strong as to be instantly marked down with a 5 to its account.

This was an observation made, be it remembered, when travelling at the rate of 20 miles an hour, and through a thick glass window; and being an uncomfortable feature in prospect, was attributed at first to that lake district where it is always raining, and where Mr. Isaac Fletcher, of Workington, has found, he believes, the rainiest mountain nook in all Europe. The lake district, however, was soon passed, but not the rain band, for that remained in the air and continued in the spectroscope, accompanying us into Liverpool; and though no rain fell there, and the clouds, barely hiding the sun's disc, were thin, yet the rain band at the red end of the spectrum was something that was a caution to behold, looking "like a black ribbon in the sky," said the lady, until darkness closed in.

And what came of it? Simply this, that next morning broke in steady unmitigated rain, pouring almost all the day, and making us thankful that the ship was not to sail until the day after.

That day, June 13, began dark and threatening, with heavy clouds. Half a regiment of soldiers passing before St. George's Hall carefully encased in their long grey great-coats showed that their commanding officer expected another rainy day. But we appealed to the spectroscope, and found that in spite of vulgar appearances there was no rainband among the clouds (the entry made in the journal was 1 only) and no rain fell. In fact the day turned out positively fine; the afternoon, as we went on