7o8 THE POPULAR SCIENCE MONTHLY, 



very near its perihelion. The body was, therefore, in the best possi- 

 ble position for close examination. At the approach of this favorable 

 epoch the new twenty-six inch refractor of the Naval Observatory at 

 Washington, under the skillful direction of Prof. Asaph Hall, was 

 turned upon the planet. On the night of August 11th a small star 

 was observed near the disk of Mars, but its true character was not 

 then suspected, or at least not determined. On Thursday night, the 

 16th, at ll*"' 42", Prof. Hall again noticed a star of the thirteenth or 

 fourteenth magnitude, very close to Mars, and measured its apparent 

 distance from the planet. On the same night, about two o'clock, he 

 again examined the planet, and to his great surprise found that the 

 small star had moved in company with Mars. He had therefore dis- 

 covered a Martial satellite. On Friday morning the observations were 

 submitted to Prof. Simon Newcomb, who, from the data furnished by 

 a watch of five hours, calculated the time of revolution, which he 

 fixed as a first approximation at 31 or 32 hours. This showed that 

 the satellite must pass behind Mars some time during the following 

 night. It was accordingly invisible when first looked for in the 

 evening, but, as predicted by Newcomb, it reappeared about one 

 o'clock. On Saturday morning the discovery was made known to 

 Admiral Rodgers, the superintendent of the observatory. It was de- 

 termined, however, to wait for another observation before formally 

 announcing so important a discovery. On Saturday evening the 

 satellite was again found very nearly in its predicted place, and its 

 exact position was measured by several astronomers. 



About four o'clock on the morning of August 18th Prof. Hall dis- 

 covered a second satellite, interior to the orbit of the first, and of 

 about the same apparent magnitude. The astronomers of Europe 

 were officially notified of the facts by the following dispatch : 



"Washington, August 18, 187T. 



Two satellites of Mars have been discovered by Hall at Washington. First, 

 elongation west, August 18th, eleven hours, Washington time. Distance, eighty 

 seconds ; period, thirty hours. Distance of second, iifty seconds. 



" Joseph Heney." 



The statement of fifty seconds as the distance of the inner satel- 

 lite was subsequently found to be quite erroneous. 



On Monday, August 21st, Rear-Admiral Rodgers, superintendent 

 of the observatory, communicated the discovery, together with Prof. 

 Newcomb's approximate circular elements of the orbits, to the Hon. 

 R. W. Thompson, Secretary of the Navy. 



Distances from the Ckntre of Mars. The distance of the in- 

 ner satellite from the centre of the primary is about 5,700 miles ; that 

 of the outer, 14,200. The distance of the former from the surface of 

 Mars is but 3,300 miles no greater, in fact, than that of London from 

 New York. The apparent magnitude of Mars as seen from this satel- 



