MERCURY 



3743 



MERCURY 



compounds of mercury are very poisonous, and 

 none should be used as a medicine except un- 

 der the direction of a physician. 



Mercury is obtained by roasting the ore, cin- 

 nabar, in a current of air. This heating process 

 drives off the sulphur, which combines with oxy- 

 gen of. the air to form a gas, thus leaving the 

 mercury free. Spain, Mexico and the United 

 States produce most of the world's supply. 

 The chief mines in the United States are near 

 the city of San Francisco. J.F.S. 



For the best-known uses of mercury, see 

 BAROMETER ; THERMOMETER. Consult Lindgren's 

 Mineral Deposits. 



. MERCURY, the smallest of the planets and 

 the nearest to the sun. It .was named for Mer- 

 cury, the swift-footed mythological messenger 

 of the gods. Less is known about Mercury than 

 about any other of the planets, for its nearness 



MERCURY'S POSITION 

 Orbit of Mercury and of the other minor planets. 



to the sun makes it difficult to observe. When 

 an evening star, Mercury is seen to advantage 

 in the eastern sky soon after sunset. The 

 Greeks called Mercury Apollo when it was visi- 

 ble in the morning, and Mercury when it was 

 an evening star. 



Magnitude of Mercury. The planet has a 

 diameter of about 3,000 miles, somewhat more 

 than one-third of that of the earth; its surface 

 is about one-seventh and its volume about one- 

 eighteenth that of the earth. Its density is not 

 so great as that of the earth, and the gravity on 

 its surface is about one-third of the force of 

 gravity on the earth's surface ; a man weighing 

 150 pounds on earth would weigh about fifty 

 pounds on the planet Mercury. As Mercury has 

 no satellite with which to compare it, measure- 



ments given can only be regarded as approxi- 

 mate, #nd not exact, as in the case of other 

 planets. 



Rotation of the Planet. It was for a time 

 understood that Mercury rotated on its axis 

 once in 24 hours, 5 minutes, 28 seconds, but this 

 has been proved incorrect. It revolves in its 

 orbit round the sun in eighty-eight days, and it 

 rotates on its own axis only once during that 

 period, keeping the same face always towards 

 the sun, as the moon does towards the earth. 

 Some astronomers, however, refuse to accept 

 this statement of rotation, adhering to the prin- 

 ciple of daily rotation. The average distance 

 of Mercury from the sun is 36,000,000 miles, 

 but its orbit is so eccentric that the distance 

 varies from 28,500,000 miles, nearest distance, 

 to 43,500,000 miles as the greatest distance. The 

 synodic period of the planet, that is, the time 

 taken to travel from conjunction to conjunc- 

 tion, is 116 days. 



Transits of Mercury. At intervals of from 

 three to thirteen years the planet Mercury 

 passes across the sun's disk between sun and 

 earth, and becomes visible as a small black spot 

 against the sun. Such transits usually occur in 

 the months of May and November, the latter 

 month having almost twice as many as the 

 former. The last transit visible in the United 

 States was 'at 8 A.M., November 7, 1914; the 

 next transit will occur in May, 1924. Transits 

 are always carefully observed by astronomers, 

 as they yield valuable information. 



Light and Atmosphere. The reflecting power 

 (albedo) of Mercury is much below that of 

 the other planets and is inferior to that of the 

 moon. Such atmosphere as Mercury possesses 

 cannot be as dense as that of Venus or the 

 earth. Transits of Venus show by a circle of 

 light surrounding the dark mass against the sun 

 that it has an atmosphere, but in transits of 

 Mercury this light circle is not seen. It is 

 known, however, that water exists in the plan- 

 et's atmosphere. Revolving in its orbit, Mer- 

 cury presents to the people of the earth exactly 

 the same phases or positions as the moon, 

 the crescent, the full moon and the half moon. 

 The surface of the planet receives nearly seven 

 times as much heat and light as is received in 

 the same area on earth, and it has at least two 

 seasons annually. Seen through a telescope the 

 surface of Mercury is not as interesting as the 

 other planets. The disk is brighter at the edge 

 than in the center, but little can be seen of 

 value as a guide to study of the nature and 

 formation of the planet. F.ST.A. 



