THE SATELLITES OF MARS.—The approaching opposition of this planet does not hold out much probability of satisfactory observations of the satellites except with the larger instruments, though in European latitudes the meridian altitude, which is an element in the case, will be considerable. Taking Prof. Asaph Hall's unit for brightness in 1877, viz. that on October 1, when the outer satellite was seen with the 9.6-inch equatorial of the Naval Observatory, Washington, we find the maximum brightness at the next opposition will be represented by 0.4, which is a less value than corresponds to the last date of observation with the 26-inch refractor at the same observatory. It may be remembered that Mr. Common observed Deimos on the morning of September 2, 1879, without much difficulty with his reflector of 3-feet aperture, when the degree of brightness in terms of Prof. Hall's unit was 0.50; at the last Washington observation in 1879 it was 0.52. The earth being only about 10° from the line of nodes of the satellites' orbits at the opposition in December next, their apparent paths are reduced almost to straight lines. The longitude of the ascending node of Deimos is 88°.