THOUGH Mars is, with the exception of Mercury, the smallest of the chief planets in the solar system, exceeding but little in magnitude Titan, the sixth of Saturn's satellites, yet it has always excited great interest, from the fact of its being the only planetary body which in its physical conformation bears a close resemblance to that of our earth.

With a good telescope we see its surface mapped out into what are believed to be continents and seas, though these would seem to be distributed in the contrary order to that which they occupy on our globe-the great seas being situated near the poles of the planet, while a broad belt of land surrounds it at the equator. Near the poles, but not corresponding exactly with them, are seen ice and snow, which melt as the poles are respectively presented more directly towards the sun. Like our earth, Mars has a cloudy atmosphere, betokening the presence of both air and water.

The accompanying drawings of Mars, so beautifully rendered in colours, were made in Mr. Barnes's Observatory, at Upper Holloway, with his telescope, an equatoreal reflector of my own make, having a silvered-glass speculum, 8 inches in diameter, which was parabolised for me by Mr. With. Achromatic eye-pieces of positive construction were alone employed. The power used was generally 300, but, occasionally, on nights when the air was unusually steady, I have used powers as high as 600, with great advantage.

Many persons who have seen the original drawings, have addressed such questions to me as these:-But, did you get such definition as you have represented? Could you really make out all those details? To answer these questions I must describe the method I adopted in making the drawings.

A number of circles were previously prepared, each being a

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white disk on a black ground. On looking at the planet through the telescope, it frequently happened that only the general character of the markings upon it were discernible. The most prominent of these markings were at once noted on one of the prepared circles, as quickly as possible, consistent with accurate estimation and determination of their position. It is important that this portion of the work should be promptly executed, because the position of the markings rapidly changes with the rotation of the planet.

Having obtained the outline of the principal markings in the manner above described, intervals of good definition were watched for, and upon their occurring, details of minuter markings, colours, and shadows were successively filled in, until in about an hour the drawing was completed.

It will be understood that a drawing made in the manner stated will show many features of interest that would escape notice in a cursory observation. It will, in fact, represent the most that can be made out by patient watching, with a telescope of the aperture employed on the particular night.

In the present drawings the planet is shown as it was seen in the telescope. As inverting eye-pieces were alone used, the lower white markings always represent the ice or snow on the north pole of the planet, while the smaller light patches on the upper part of some of the disks indicate the ice near the south pole. I have made nearly thirty sketches at the telescope, but I have carefully copied only thirteen. These were all made when the air was tolerably steady, and the definition so good that I could work well with powers above 200. Of this number eight of the most interesting have been selected for repre

sentation.

When several sketches were made on the same evening, they were taken at intervals of two hours, if the weather permitted. In only one instance have I failed to make out that the form of the markings was permanent, due allowance being made for the effect of perspective in foreshortening them as they approached the edge of the disk.

It is highly probable that in the view of the planet taken on February 16th, at 6:45, the two pointed markings on the extreme left, one above, and the other below the equator, would have been seen united if they could have been observed when they were on the centre of the disk. This drawing would then have agreed pretty closely with one of Mr. Dawes's views engraved in the "Astronomical Register" for September, 1865.

The colour of the body of Mars I have found vary from rose-madder to burnt ochre, the colour appearing ruddiest when there was most mist in our atmosphere. The comparative absence of the ruddy colour towards the edges of the

disk of the planet, Mr. Norman Lockyer has ascribed to the presence of clouds in the planet's own atmosphere.

Mr. Huggins in his valuable paper on the "Spectrum and the Colour of Mars" ("Monthly Notices of the Astronomical Society," March 8, 1867), has referred the absence of colour at the edges of the planet to some peculiar effect of the surface of the planet itself. That the ochreish colour is due to some peculiarity in the surface is, I think, almost proved by Mr. De la Rue's exquisite drawings of Mars, as in these markings of that tint are seen with definite outlines. As I have said, when our atmosphere is free from mist the colour of the equatoreal part is pale, and has not nearly so strongly marked a ruddy tint. This is accounted for by supposing that mist stops the most refrangible rays of light, that is, those towards the blue end of the spectrum, whose waves have the greatest velocity; the red light thus being allowed to preponderate.

When observed under these conditions, the edges of the disk appear Naples yellow, the centre orange, tinged with burnt ochre, while the parts immediately under the dark markings, near the south pole, are whitish, with a tinge of salmon colour. The colour of the dark markings on Mars has been described as greenish or bluish grey; they always appear bluish grey to me, and with this colour I have depicted them. Occasionally the north polar ice has been seen strongly tinted with a bluish colour. I have examined the spectrum of the planet with a direct-vision spectroscope, fitted in the eye-piece of a telescope; the spectrum of the dark markings presented no distinctive peculiarity. This would scarcely prove the entire absence of a blue or greenish shade of colour in the markings, as I have found that if white light be reflected five or six times from surfaces of metallic silver, and then received on white paper, it will be strongly tinged with a chocolate colour. Yet if the light reflected from white paper thus illuminated be examined by means of a spectroscope, no appreciable difference can be seen between the spectrum of this light and that of white light.

In consequence of the effect of irradiation, I have not been able to make out satisfactorily the outline of the north polar ice. I have frequently seen faint white spots appear on the disk, and as these spots approached the edge of the disk, they increased in brilliancy, until, when nearly at its extreme edge, they almost rivalled the polar snows in whiteness; these white cloudy patches never had any definite outlines. They were generally nearly circular in form, and they always appeared in the region of the equator. One of these white cloudy spots I have shown in the sketch taken on February 8th, at 10.30. The spot is represented passing off the left hand edge of the disk.

Mr. De la Rue has shown the dark markings near the south pole as darkest towards the edge nearest to the centre of the planet, and just below the edge of the dark markings the ruddy colour of the body as much fainter than it is nearer to the centre of the body about the equator. I see these appearances distinctly.

In the drawing taken on January 28th, at 9h, a number of breaks will be seen in the edge of the dark marking near the south pole, and forming a series of light streaks directed towards the pole. Mr. Barnes also sketched this appearance, without seeing my sketch, and as, with the exception of a slight difference in the angle given to the light streaks, the two sketches agreed, I cannot have been mistaken in their appearance having been as I have drawn and described them.

On the 31st of March, at 7h, I obtained an exact repetition of the markings in precisely the same position shown in the drawing taken on February 23rd at 9h.; the two drawings coincide so perfectly that I can only distinguish between them by the dates affixed to them. In these two drawings the mark usually termed the hour-glass mark is represented as having just passed the centre of the disk of the planet. The movement, in the time I have stated, includes a period of 35 revolutions, and the time of a single revolution on its axis deduced from the observed recurrence, would be 24h. 38m. 8s. Beer and Mädler, who have observed the largest number of revolutions, give the period of rotation as 24h. 37m. 23s. My own determination is, however, less than Sir Wm. Herschel's.

Could a repetition of the markings in exactly the same position have been observed after a much greater number of revolutions had been completed, a period of revolution more closely accordant with B. and M.'s determination would probably have been obtained. The very unfavourable weather we have had to contend with for some months, has, I regret, rendered a repetition of the observations impossible.

I have said elsewhere that between my own drawings, Mr. De la Rue's, and Mr. Dawes's, there exists a great similarity, and to one of Secchi's drawings one of mine has a considerable resemblance, but I cannot trace a likeness between any of my views and those of Beer and Mädler. This observation leaves on the mind a suspicion that in the course of time some change may have taken place. With regard to the point that no flattening of the planet at the poles has been detected, even by that admirable observer, Mr. Dawes, I would remark that although if the sphere of the planet were oblate to the extent of onesixteenth, or one-tenth of its diameter, as in the case of Jupiter and the globe of Saturn, the flattening might be easily discerned, yet, if the flattening should not exceed, in proportion,

that of the earth, as would probably be the case from its having nearly the same axial velocity, we could not hope to perceive it, for, under such circumstances, the flattening of the disk of Mars would not exceed twelve miles, and this when the planet is nearest to us, would subtend an angle of only one-tenth of a second.

Professor Phillips's drawings of Mars, of which three are engraved in the "Proceedings of the Royal Society," No. 55, were taken when the north pole of Mars was invisible. Allowing for the difference produced by this cause, the drawings agree with mine in the general form of the markings, but the white margin which I only see in the bays, Professor Phillips shows completely fringing the whole of the markings.

Now half a second is the smallest amount of difference in diameter we could hope to detect, even by the aid of the most delicate micrometric apparatus. The double-image micrometer, devised by the Astronomer Royal, would be the best to employ for this measurement.*

No satellite attending on Mars has yet been discovered. Pursuing the analogy between the planet and our earth, if such a satellite existed of a size proportionate with our moon, as it would be one-quarter the diameter of the primary, we might expect it to be easily visible, but should a satellite exist, not exceeding in size proportionately the second satellite of Jupiter, it would only be visible in very powerful instruments, still it would not, I think, have escaped the notice of the persistent observers who have searched for it hitherto vainly.

The discovery of a satellite to Mars is looked for with interest, as the effect of the sun and the primary in producing perturbations of a satellite would enable the density and mass of the planet to be accurately calculated. At present these are only imperfectly known, but the density is considered to be almost exactly the same as that of the earth.

Unfortunately, observations of value can scarcely be made upon this difficult planet with telescopes of less than six inches aperture, and, unless the observer has extremely good sight, eight or ten inches will be found necessary. During the next opposition I hope to be able to continue my observations with an instrument having a silvered glass speculum twelve inches in diameter.

* In measuring, micrometically, the diameter of Mars, it is very difficult to avoid obtaining too large a result for the polar diameter, the effect of the irradiation causing the white spots near the poles to appear to project slightly from the disk of the planet. The best method, probably, of overcoming this difficulty is by using a single reflecting solar eye-piece.