July 8, 1892.] 



SCIENCE. 



17 



the slough. On a few there are constrictions and ridges 

 around the cap, that recall those on the spines of the lizard. 

 As it happens, those marked in this manner are the nearest 

 living allies of the rattlesnakes. In the paper on the Evo- 

 lution of the Rattle, above cited, the copperhead, Ancis- 

 trodon (Fig. 5), was brought forward as most nearly rep- 

 resenting the ancestor of the smaller rattlesnakes, Sis- 

 trurus; and the bushmaster, Laehesis (Fig. 6), of northern 

 South America, was suggested as the most likely for the 

 large rattlers, Crotalus. These forms were pointed out as so 

 nearly approximating a condition from which the possession 

 of a rattle was a necessary consequence that we might at 

 any time expect to find individuals on which the caps were 

 mechanically retained. My conclusions in regard to the 

 inception of the rattle seem to be indirectly confirmed by 

 what obtains on the lizards. This will be the more apparent 

 if it is borne in mind that the present development of the 

 rattle (Figs. 7-9) embraces much that is a consequence of its 



Figs. 1-2, nuchal spines, and 3-4, a dorsal spine of Couolophus subcristatus ; 

 Fig. 5, tall of Anelstrodou contortrlx ; Fig. 6, tall of Laehesis mutus ; Fig. 7, 

 Sistrurus catenatus, at birth ; Figs. 8-9, Crotalus confluentus. 



possession, much that has been induced bj' its presence and 

 use. The greater part of the shortening-forward in the ex- 

 tremity of the tail, of the compacting and consolidation of 

 the posterior vertebrae, with the enlargement of the cap to 

 include them, and much of the development of the caudal 

 muscles must be eliminated before one can realize the pri- 

 mary condition of the rattle, a condition which was, no 

 doubt, but a little advanced upon that now existing in An- 

 cistrodon and Laehesis, as sketched in Figs. 5 and 6. 

 Mus. Comp. Zool., Cambridge, Mass. 



OPPOSITION OF MARS. 



ET EDGAR L. LARKIN. 



The coming opposition of Mai« will be of interest to as- 

 tronomers throughout the world; and extensive preparations 

 are being made to observe it. The face of the god of war is 

 sure to be watched, drawn, and photographed with more 

 care than ever before. And the most perfect spectroscopes 

 made will be turned on his ruddy disk. The sun, earth, 



and Mars will be on the same straight line nearly, on Aug. 

 3 at 13 h. 13 m., or at 1 h. 13 m. a.m., Aug. 4, 1892. The 

 time of the opposition will be favorable for observation, since 

 the earth passes its aphelion on July 1, while Mars does not 

 pass his perihelion until Sept, 7. That is, the earth will be 

 34 days only past the time when at its greatest distance from 

 the sun; and Mars but 35 days from its nearest approach. 

 If these dates could coincide — opposition take place when 

 the earth is at a maximum and Mars at a minimum distance 

 from the sun — then would the earth and Mars be at a 

 minimum distance from each other, or 33,864,000 miles; in 

 which computation a solar parallax of 8.8" and a mean dis- 

 tance of Mars of 141,500,000 miles were employed. However, 

 since the opposition will occur midway between, it is proba- 

 ble that, at the moment of the nearest approach of the two 

 planets, they will be distant about 35,500,000 miles. 



The last opposition favorable for close observation was on 

 Sept. 5, 1877; at which approach, Prosessor Asaph Hall dis- 

 covered two minute moons in revolution around our neigh- 

 boring world. This important discovery is best given in 

 Professor Hall's own language: "The sweep around the 

 planet was repeated several times on the night of Aug. 11, 

 and at half-past two o'clock I found a faint object on the 

 following side and a little north of the planet, which after- 

 wards proved to be the outer satellite. On Aug. 16 the 

 object was found again on the following side of the planet. 

 On Aug. 17, while watching for the outer satellite, I discov- 

 ered the inner one." Perhaps this optical discovery reveals 

 the power of modern telescopes in a manner more impressive 

 than any other, thus: "The outer one was seen with the 

 telescope at a distance from the earth of 7,000,000 times its 

 diameter. The proportion would be that of a ball two inches 

 in diameter viewed at a distance equal to that between the 

 cities of Boston and New York " (Newcomb and Holden, 

 "Astronomy," p. 338). 



These moons were seen with the 26-inch glass at Washing-- 

 ton ; but now a 36-inch telescope is in waiting for Mars, and 

 none can predict what will be discovered. The satellites are 

 estimated to be 6 and 7 miles in diameter; and they have a 

 most rapid motion. It is well to note some ot the facts 

 about these bodies that served a great purpose, in sweeping 

 away that mythology of astronomy, the nebular hypothesis. 

 Distances from centre of Mars : Deimos, 14,600 miles ; Phobos, 

 5,800 miles. Times of revolution: Deimos, 30 h. 18 m. ; 

 Phobos, 7 h. 39 m. But it requires 24 h. 37 m. for Mars to 

 turn on its axis, which divided by 7 h. 39 m. equals 3.22; 

 that is, the inhabitants of Mars have 3.22 months of Phobos 

 every day. This moon rises in the west and passes through 

 a phase in 1 h. 55 m. Deimos is 130 h. 37 m. from rising to 

 rising, or 65 h. 18 m. from rising to setting. Its gain over 

 the rotation of Mars is 3° 24' per hour, hence it requires 106 

 hours to gain a whole revolution, which, added to the diurnal 

 rotation of the planet, gives the 130 h. 37 m. But 65 h. 18 

 m. equals 2.155 months of Deimos; therefore the other satel- 

 lite passes more than two full sets of phases while above the 

 martial horizon, with plenty of eclipses beside. 



The main interest in the next opposition rests in the hope 

 that an accurate map of Mars can be made, or that good photo- 

 graphs can be secured, or that the spectroscope may make 

 further revelations concerning the absorption of solar rays 

 by its atmosphere, or that the lines due to the vapor of 

 water may be seen to better advantage, if possible, than at 

 the last. Professor C. A. Young, "Astronomy," p. 337, says: 

 "The probability is that its density is considerably less than 

 that of our own atmosphere. Dr. Huggins has found with 



