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WATER X'APOl'R ON MARS. t<\ 



By J. DE Fextox. F.R.A.S. 



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At no time in the history of knowledge was specidlisiii^ carried 

 on to a greater extent than it is to-day : and yet I will venture 

 to say that if there is one fact which more than another strikes 

 the observant student, it is that the day of the " one-line " man 

 in science is gone for ever. For such is the inter-relationship 

 of the different branches of knowledge, due in great part to the 

 wonderful discoveries of the latter half of the last century, that 

 to adequately pursue even one line of study, a wide range of — 

 to the outsider — extraneous knowledge must be covered. - Thus 

 to the up-to-date medical man cheniistry, physics and electricity 

 are a suie qua iwn. 



Least of all can the astronomer of to-day afford to remain simply 

 astronomer and mathematician. For the very existence of his 

 claim to the title, he needs must be conversant with chemistry 

 and jTiysics. electricity and spectroscopy, photography and 

 meteorology, geology and paktontology : and he will find it 

 to his advantage to know something of mechanics and engineering 

 a'so. From every field of human endeavour the astronomer 

 chooses the weapons with which to attack the enigma of the skies. 

 In fact, astronomy is the one branch of science which calls all 

 others its handmaidens. 



Were this better understood and put in practice, were more 

 of those interested in astronomical pursuits equipped with an 

 all-round knowledge, we might perhaps not see such disbelief 

 or at least unwilling adherence, expressed in facts which are, 

 after all. but simple and inevitable consequences of natural phy- 

 sical and chemical laws. 



I refer more especially to the question of the presence of water 

 vapour in the atmosphere of Mars. 



Owing to the practical absence of an atmosphere on the Moon, 

 we are enabled to make use of our satellite as though it were a 

 mirror, to reflect the spectrum of the Earth ; so that if a planet 

 and the Moon are compared spectroscopically under analogous 

 conditions, any outstanding difference in the two spectra can 

 be definitely ascribed to the planet. That owing to various causes, 

 chief among which is the small quantity of light emitted by the 

 planet, comparison is a matter of extreme delicacy, is at once 

 apparent. 



The 1,100 odd spectroscopic lines denoting the presence of 

 aqueous vapour have been divided by Thollon into seven groups, 

 extending from wave-length 745 in the extreme red to wave- 



