292 ANNUAL OF SCIENTIFIC DISCOVERY. 



orbit nearest to Jupiter, than it does when the earth is in that part 

 of its orbit most distant from Jupiter. These four moons are of 

 progressional sizes. The magnitude of the nearest is about a 

 quarter greater than that of our own ; that of the second is equal 

 to ours ; the diameter of the third, however, is nearly double 

 that of our moon, and it is nearly equal to the planet Mercury; 

 the diameter of the fourth satellite is about one-half greater than 

 that of our moon. When these satellites get into the shadow of 

 Jupiter they become invisible to us, and hence we know they are 

 opaque bodies, which shine, like the moon, by the reflected light 

 of the sun. All the circumstances connected with their eclipse are 

 visible to us. We see them enter the shadow and leave it, and we 

 can estimate the duration of each eclipse, and observe exactly its 

 beginning and ending. These eclipses have been instrumental, 

 not only to useful purposes in art, but also to great discoveries in 

 science. It is by them, among other means, that the longitude of 

 places on the surface of the earth is determined ; but by far the 

 most important discovery connected with these bodies is that of 

 the motion and velocity of light, it having been shown, by their 

 means, that the velocity of reflected light is the same as that of 

 direct light. 



The planet was unaccompanied by any visible satellites fora 

 period of 1| hours. The disappearance and reappearance, as seen 

 in England and America, were as follows: at 8h. 14m. G. M. T. 

 (soon after sunset), the third satellite entered on Jupiter's face ; at 

 9h. 9m. the second was eclipsed in Jupiter's shadow; at9h. 28m. 

 the fourth, and at lOh. 4m. the first entered on Jupiter's face. All 

 four satellites were then invisible, three on account of their 

 passing simultaneously over the planet's disc, and the fourth from 

 being immersed in the planet's shadow. At llh. 49m. the third 

 satellite passed from Jupiter's face ; at 12h. 13m. the second 

 reappeared from behind the body of the planet ; at 12h. 23m. the 

 h'rst, and at 13h. 54m. the fourth satellite passed off the disc. 



The shadow of the fourth satellite appeared larger than that of 

 the third, though the latter is the larger body. This observation, 

 if confirmed, \vould show that the apparent dimensions of the 

 shadow depend rather on the extent of the penumbra than of the 

 true shadow. 



THE PLANET MARS. 



According to Mr. Huggins' observations during the late opposi- 

 tion of Mars, Jan. 10, 1867, in this planet's spectrum groups of 

 lines were seen in the blue and indigo, but they could not be 

 measured so as to determine whether they are solar or due to the 

 planet's atmosphere. Many marked lines were also seen in the 

 red. On Feb. 14 faint lines were seen near D, and were thought 

 by him to be due to absorption by the planet's atmosphere, as, al- 

 though similar to lines seen in the solar spectrum when the sun is 

 low, Mars was not low enough for the production of the lines, 

 which were not seen in the moon's spectrum, though she was 

 lower than Mars. The spectrum of the darker portions of the 



