MARS. Observed at Hopefield Observatory. No 6. Nov. 26. 1864 11h.46.m. G.M.T W.R.Dawes,delt Aperture 80 in Remarks. Ꮇ Ꭺ Ꭱ Ꮪ . No 8. Observed at Hopefield Observatory. Nov. 20. 1864 11 h.36 m. G.MT. Focal length Power used 258. Aperture 80 in Focal length_ Greenish white. N.E.Green, lith. Power used 258. Remarks a. Remarkably white, and whitish all along the edge of the Coast from the Promontory a little west of centre marked x b. Greenish white. c. Light orange. d. Greenish white. e. Small roundish white spot near the South Pole. MARS. No 12. Observed at Hopefield Observatory. Nov. 12. 1864 12 h. 30. m. G.MT. Aperture 80 in Focal length Power used 258. Aperture 80 in Focal length Power used 258. The Astronomical Register. No. 33. SEPTEMBER. 1865. PHYSICAL OBSERVATIONS OF MARS NEAR THE OPPOSITION IN 1864. BY THE REV. W. R. DAwes. (ABSTRACT of a Paper read before the Royal Astronomical Society, June 9, 1865.) THE sketches exhibited, 16 in number, were made by the help of an excellent 8-inch object-glass, by Cooke and Sons, equatoreally mounted. No pains were spared in procuring the most correct views of the various features of the planet, and in determining their relative positions on the disk. To check the estimations by the eye, the micrometer was frequently employed; and the G.M.T. was noted to which the aspect of the planet referred.* Several curious and interesting features were brought out during the last opposition which the writer had never before seen so distinctly. One of the most remarkable was the long narrow strait, running NE. and SW., in the northern hemisphere, which was repeatedly well seen and depicted. This was observed, but much less distinctly, in 1852 with Merz's 63-inch objectglass. Another curious object was the forked shading, depicted more or less distinctly on every night when that part of the planet was visible. The same was seen frequently in 1852, but always as an oval bay with a regular edge; and no division in it was noticed until the opposition of 1862, when the forked appearance was once or twice noted, and on one occasion (1862, Sept. 22) was distinctly depicted, by help of Alvan Clark's * The original drawings were made by Mr. Dawes upon the forms supplied to the Subscribers to the Register; the numbers attached being those which they bear in Mr. Dawes' Series of 16. The 'focal length' was not filled in. 81-inch object-glass. The bifurcated appearance gives the impression of two very wide mouths of a large river, which, however, could never be traced. This feature seems to be new, and it will be interesting to note in future oppositions whether it is permanent or variable. No striking difference was observed in the relative distinctness of the various features of the planet at different times. The principal instances consisted in the white spot which was observed on the nights of Jan. 20th, 21st, and 22nd,-giving the impression of a mass of snow in the southern hemisphere, where there certainly was nothing of the kind on Nov. 10th and 12th, when the same part was well seen. It might be the reflection from the upper surface of a dense mass of cloud. That the ruddy tint of the planet does not arise from any peculiarity of its atmosphere, seems to be fully proved by the fact that the redness is always deepest near the centre, where the atmosphere is thinnest; and near the edges the colour of the light reflected from the dense atmosphere is white, or greenish white. It is to be regretted that it was not at the time thought of, to ascertain, by concealing the ruddy centre, whether the greenish hue was or was not the effect of contrast. At about the time of the planet's opposition, the polar and equatoreal diameters were measured with an excellent four-glass double-image micrometer by Simms. The difference amounted to o".02 only, by which the polar diameter exceeded the equatoreal; and the movable image being placed in exact contact with the fixed image, was made to revolve round it without in any part being observed either to overlap it or to separate from it. In the Radcliffe Observations, vol. xi. p. 292, are given the results of some measures of the diameters of Mars, by the late Mr. Johnston, by means of the Oxford Heliometer, in which, when reduced to the planet's mean distance, the polar diameter is greater than the equatoreal by o"6. In the Monthly Notices, vol. xiii. p. 56, these measures are unfortunately reversed-the larger being assigned to the equatoreal; and the Radcliffe volume is referred to as the 21st, instead of the 11th. Among the few instances of change among the observed features during the three months of observation, one of the most |