1902.] 



LOWELL — AREOGRAPHY. 



227 



the period, they make transition to them. Dawes thus made twi- 

 light to the third period in 1864; W. H. Pickering and the Lick 

 observers to the fourth in 1892. 



7. Distinct phenomena characterize the three periods. Patches 

 of light and shade make the markings shown on the maps of the 

 first stage of cartography. Of a piece though these patches are, 

 their shapes appear well defined. At first one might suppose such 

 to be due to the handicraft of the draughtsman and to possess no 

 scientific value. But inspection of the several charts, one after the 

 other, shows that the shapes are not artistic embodiments of ill-seen 

 shadings, but are intrinsic traits of the shadings themselves, for 

 chart after chart reproduces the same turns and twistings. 



8. To see this we have but to take up in sequence the maps from 

 1840 to 1876. No. I of the series shows a cordon of patches 



Fig. I. 



r«9 «D ^ gao^oaiaao SSOM> ^SOMOi 



MAR5 (640 



Map of Beer and Mailer, 1840. 



Stretching round the map at about 30° south latitude. Their height 

 is greatest at 90° of longitude, and from this slopes down through 

 360° to 20° longitude, whence it gradually rises to the maximum. 

 At the point of maximum is an oval marked out by broad shading 

 on the south, by narrow penciling on the north, and holding a 

 roundish dark spot in its centre. This is the Soils Lacus, the eye 

 of Mars. To the right of it follows a leech-like patch, the Mare 

 Sirenum and the Mare Cimmerium seen as one. After this comes 



