March 1, 1895.] 



KNOWLEDGE. 



57 



Rome on October 18th, 1862, at 7b. 21m., Greenwich time, 

 whilst just 36 minutes later Lockyer completed another 

 drawing in London. The two astronomers were, there- 

 fore, probably actually observing the planet for some little 



Fig. 5. — The "Eye" of Mars, as drawn by Lockyer, October 18th, 



1862, at 8h. Om., G-reenwich Mean Time. From Flammarion's 



" Mars," p. 549. 



time together, for the Italian may very well have continued 

 to watch it after the completion of his sketch, whilst the 

 Englishman must certainly have commenced his watch 

 some appreciable time before that recorded for the com- 

 pletion of his design. On the whole, the latter must be 

 accepted as by far the more accurate. Kaiser's drawing 

 of six days later (Fig. 1 of the plate) is in too close accord 

 with it for there to be any doubt as to its substantial 

 accuracy. It follows, then, that we must accept it as 

 established that an astronomer, even of such eminence as 

 Secchi, could make most serious errors in his representa- 

 tion of one of the leading features of the planet. The 

 spiral "canal" that sweeps round the Lake of the Sun, 

 and finally into it, is clearly due to some confusion with 

 the outer coast of Thaumasia — the similarity of shape of 

 neighbouring outlines on Mars is one of the most fi-uitful 

 causes of errors in drawings of it ; the heavy W to the north 

 of the lake is a manifest exaggeration of the Lacus Tithonius, 

 seen so faintly on Lockyer's drawing. Lockyer, on the 

 other hand, probably under-estimated this latter marking, 

 and failed to detect any trace of the estuaries of the 

 Chrysorrhoas and Fortuna, where they open into the 

 Lacus Tithonius, to which no doubt the lower points of 

 Secchi's W correspond. 



But even discrepancies like these are not sufficient to 

 overthrow the evidence for real change that exists. Com- 

 pare the two little sketches given below ; Fig. 6, from a 

 drawing by Dawes, under date January 21st, I860, and 

 Fig. 7, from one by Green, of September 29th, 1877. 



In the earlier drawing, the two lakes, Solia and 

 Tithonius, are shown as " bottle-necked " seas, to use 

 Proctor's term. Both are connected with the Mare 

 Erythrseum by " canals," and of the two Tithonius is the 

 larger and darker. In the later, Solis is completely cut off 

 from the Erythi-iean Sea, but is joined by a short canal, 



the Eosphorus, with the little Lacus Phoenicia. Other 

 drawings of Mars made by Dawes on November 3rd, 10th 

 and 12th, 1864 (see B.A.S. Monthly Notices, Vol. XXV.), 

 show Tithonius as most distinctly larger than Solis, and 

 like it in shape, and all agree in showing the Nectar canal 

 to the east of the Lake of the Sun, whilst of the Eosphorus 

 and the Lacus Phoenicis on the west there is no trace. 

 On the other hand, Schiaparelli (Fig. 2 of the plate) fuUy 

 confirms Green with respect to the Eosphorus and Lacus 

 Phcenicis in 1877 ; so do my own drawings made at the 

 same opposition. Schiaparelli, indeed, shows Tithonius as 

 a narrow, winding canal on September 26th, whilst Green 

 gives no trace of it tliree days later. But this was prob- 

 ably no mere oversight, but due to a real disappearance, 

 since on September 2nd he had shown it as a faint 

 streak. So it appeared to myself at about the same 

 time, and so to Flammarion, Dreyer and others, whilst not 

 a few good observers, Terby amongst them, failed to detect 

 it at all. It is quite clear that there was a complete 

 change, therefore, in the intensity and definiteness of 

 Tithonius between the oppositions of 1861 and 1877. 

 Although as large and as dark as the Lake of the Sun in 

 the former year, it was scarcely perceptible in the latter, 

 and would indeed appear to have faded entirely away by 

 the date of Mr. Green's last observation. 



One difference which the nine drawings in the plate 

 bring out is manifestly due to the artist ; the difference in 

 the sciilt; of certain of the markings. This is a point which 

 every comparison of a number of drawings by difl'erent 

 hands is sure to bring out. The cause of it is obvious. 

 The observer has first to study the planet at the telescope, 

 to patiently trace out the different details, and then depict 

 them, more or less from memory, in his sketch. The 

 drawing cannot be made at the very instant that the 

 original is being viewed. The degree, therefore, to which 

 some point attracts special attention will have much to do 

 with the relative proportion given to it. Individual 

 discordances are often considerable, and " changes on 

 Mars " have bsen advertised in the most reckless fashion, 

 on the faith of a single drawing, quite neglectful of this 

 source of error even with the best artists. 



Yet the evidence for an actual change, both in the size 

 and in the intensity of the Lacus Solis, as instanced in 

 Figs. 4 and 5 of the plate, is very considerable. Mr. 

 Stanley Williams, for instance, like M. Guiot, found the 

 lake both small and faint during September, 1892. 



The case is clearer when we come to the tnnii>i of the 



Fig. 6. — liars, by Dawes, 

 Jauuary 21st, 1865. 



Fl&. 7. — Mars, by Green, 

 September 29th, 1877. 



markings. Thus drawings 2, 3, and 6 in the plate show 

 the western edge of Thaumasia under a very different 

 aspect to drawings 7, 8, and 9. The former three show 

 the Sinus Aonius most accordantly ; the latter three show 

 no trace of it. This evidence is further supplemented by 

 that of Mr. Lowell, at the Flagstaff Observatory, Arizona^ 



