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E. WALTEK MAUNDER. F.E.A.S.. ON THE 



fifth of an inch in diamet'er. the telescope would have to l)e 

 167 feet in length. At Blount Wilson a telescope has actually 

 been constructed with an equivalent focal length of 150 feet; 

 if this were mounted like an ordinary telescope, it would he 

 impossible to give it the necessary rigidity, and any wind would 

 set up tremors in it which would be fatal to the chance of 

 securing good photographs. But by firmly fixing the telescope 

 and reflecting the light from the planet into it, from a moving 

 mirror, this difficulty has been overcome. At the Yerkes 

 Observatory and at IMr. Lowell's smaller telescopes have been 

 used and the image of Mars has been enlarged afterwards. But 

 though a wonderful success has attended these efforts of 

 Mr. Lowell and of Professors Barnard and Hale, the photographs 

 have not settled the controversy. Mr, Lowell finds canals " 

 on Ijis photographs, though it must be added tijat in appearance 

 they are more like M. Antoniadi's representations than Mr. 

 Lowell's own drawings. Professor Barnard's photographs, 

 which appear to be the best that have yet been Jsecured, show, 

 on the other hand, nothing that is canaUfonij, but they 

 reproduce most closely the l)eautiful paintings made by the 

 late ^Ir. Green, thirty-five years ago. 



The actuality of the geometrical network " is, therefore, 

 still in dispute : is there anything about the planet that is not 

 in dispute ? 



Two facts about the planet had \jeen ascertained long before 

 the invention of the telescope : its distance from the sun as 

 compared with that of the earth was known to be more than 

 half as much again. This implies that it receives from the sun 

 only three-sevenths the amount of liglit and heat, surface for 

 surface, that the earth does. The length of its year was also 

 known ; it is much longer than that of the earth, being only 

 six weeks short of two full terrestrial years ; expressed in day& 

 it is 687 as compared with om- 365^ days. 



Since the invention of the telescope the distance of Mars 

 fiom the Sim has been measured, not only relatively, but in 

 miles, and the size and weight of the planet have been deter- 

 mined. The latter was inferred from the movements of the 

 two tiny sateUites discovered in 1877. We know that Mars is 

 but little more than half the earth in diameter ; in volume it is 

 only about one-seventh ; and in mass only one-ninth that of the 

 earth. Its density, therefore, is about five-sevenths of the 

 earth, and the attraction of gra\itation at its surface is not 

 much more than one- third as much as it is here. On the earth a 

 falling weight will pass through sixteen feet in the first second ; 



