SCIENCE-GOSSIP. 



283 



CHAPTERS FOR YOUNG ASTRONOMERS. 



By Frank C. Dennett. 



THE PLANET MARS. 



(Continued from page 249.) 



Later observers have mapped out on the land 

 surface of Mars a complete network of fine lines, 

 which for want of a better name have been called 

 canals. A chart lying before me shows more than 

 ninety such objects, but to see them at all tele- 

 scopes of over six-inch aperture are required, and 

 many observers altogether fail to find them. A 

 singular phenomenon has been noticed by some 

 observers to take place, as if many of the canals 

 doubled ; but this is thought by others to be due to 

 a want of perfect 

 focussing on the part 

 of the telescopist. 



The existence of an 

 atmosphere was very 

 early ascribed to this 

 planet. Cassini, in 

 October, 1672, found 

 the5th-magnitudestar 

 $ Aquarii, whilst it 

 was quite 6' distant 

 from Mars, became 

 so faint that it could 

 not be seen with a 

 three - feet telescope. 

 Some other cause, 

 however, must have 

 produced this dis- 

 appearance, because 

 Sir James South and 

 others have seen stars 

 actually occulted by 

 the planet, and Sir W. 

 Herschel saw a star 

 of the 13th or 14th 

 magnitude very near 

 to its disc. The spec- 

 troscopes of M. 

 Janssen and Sir W. 

 Huggins have quite 

 settled the question 

 of the presence of an 

 atmosphere, differing 

 little from our own, 

 including watery 

 vapour held in sus- 

 pension. This gives 

 encouragement to the 

 belief that the white 

 patches close to the 



poles are really, as usually called, snow caps. 

 This view is strengthened, because the caps 

 are largest when first they become exposed to 

 the sun's rays. As the sun rises higher, and 

 the exposure is lengthened, the caps become 

 smaller ; whilst the region edging them has been 

 observed to grow darker during the decrease, as if 

 from the presence of the water from the melting 

 snow. A singular phenomenon is that neither of 

 the poles are central to the snow-caps. The centre 

 of the south polar cap is five or six degrees of 

 latitude away from the South Pole, according to 

 the observations of Professor Schiaparelli, which 

 are confirmed by those made at Washington. The 

 north polar cap is not concentric, so that the two 

 caps are not opposite to each other. Both caps 

 are sometimes visible at once, and at other times 

 both are hidden from sight. Comparatively small 



Planet Mars 



instruments will confirm these observations. From 

 their great brilliance the snow caps often seem to 

 be raised above the surface of the planet, so that 

 some astronomers have made the suggestion that 

 they are really clouds suspended in the atmosphere 

 above the polar regions ; but the effect is doubt- 

 less brought about by irradiation. 



Other white spots have been occasionally ob- 

 served, especially a large one in the De-la-Rue 

 Ocean, which has 'been called Dawes Ice Island, 

 in compliment to the Rev. W. R. Dawes, who 

 observed it. That spot subsequently disappeared. 

 Sir William Herschel speaks of seeing both bright 

 and dark belts on the planet, but later observers 

 have not confirmed his observations. Dawes, 

 Browning, Mitchell, Crossley, Gledhill, and others 

 have seen small 

 bright patches which 

 are not always visible, 

 and so appear to be 

 due to clouds, or 

 something analogous 

 in the Martial atmos- 

 phere. 



In the cases of 

 Jupiter and Saturn 

 the flattening of the 

 poles may be readily 

 observed, but this is 

 not the case with 

 Mars. Some ob- 

 servers have even fan- 

 cied that the polar 

 diameter was the 

 longest, but doubtless 

 the irradiation from 

 the brilliant snow 

 caps has been answer- 

 able for the mistake. 

 At the Lick Observa- 

 tory in 1894, Prof. E. 

 E. Barnard measured 

 the polar and equato- 

 rial diameters as 4,312 

 miles and 4,352 miles 

 respectively, making 

 the flattening amount 

 to forty miles or xrfgth 

 part of the diameter. 



Mars is a planet 

 which bears high mag- 

 nifying powers well, if 

 the telescope itself is of 

 good quality. Indeed, 

 the drawings made by 

 Grover, above men- 

 tioned, were obtained when a power of 200 was 

 being employed upon a two-inch achromatic tele- 

 scope. The late Rev. T. W. Webb, in 1862, using 

 a five and a-half inch refractor usually employed 

 a power of 170, but when our atmosphere would 

 admit of it, found that even 460 could be used 

 with advantage. Powers from 153 to 372 were 

 used on the five and a-quarter inch Calver 

 reflector, in making the diagrams illustrating this 

 paper. A slight frosty fog on a calm night will 

 often help to intensify the markings of planets. 



Conjunctions of the planets sometimes prove 

 very interesting. For instance, on January gth, 

 1591, Mars is said to have passed in front of 

 Jupiter. On November 21st, 1875, Mars and Saturn 

 were together in the field of view of the telescope 

 with a power of eighty. [For " Satellites of Mars," 

 see page 270. — Ed. S.-G.] 



