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THE GUIDE TO NATURE. 



perature of Mars is low, but differ on the 

 degree of temperature there. Poynting, 

 from theoretical considerations, finds that 

 the average temperature on Mars is 22° 

 Fahrenheit below zero. At such a degree 

 of cold, there would be no question of 

 canals ; the water would all be frozen 

 solid. Though 54° below freezing may be 

 below the real temperature, all infer- 

 ences we can draw from the earth points 

 to the fact that if the atmosphere at all 

 resembles our own, it should then be al- 

 ways below freezing. To have water in 

 the canals it is necessary to assume that 

 the atmosphere differs radically from our 

 own. We can postulate that Mars is 

 rich in radium, or with Prof. Lowell as- 

 sume there is a copious supply of water 

 vapor present in the atmosphere of Mars. 

 Water in the atmosphere will act like 

 glass in a greenhouse, will let the sun's 

 rays in, but will entrap them as they are 

 radiated back, and Mars may thus really 

 have a temperature capable of supporting 

 life. How are we going to discover the 

 presence of this water vapor? It cannot 

 be done by direct observation, and re- 

 course must be had to the all-powerful 

 spectroscope, which has already settled 

 so many strange problems in astronomy. 



The principles which enter into the ap- 

 plication of the spectroscope to Mars are 

 readily understood. The planet, we know, 

 shines by reflected sunlight. Its spec- 

 trum, therefore, must be identical with 

 that of the sun, except as it is modified 

 by the (supposed) atmosphere of Mars, 

 The spectrum of the sun would be con- 

 tinuous, i. e., without any lines or breaks, 

 were it not for the comparatively cool 

 layer of gases around the sun, a sort of 

 solar atmosphere which introduces thous- 

 ands of dark lines in the solar spectrum. 

 Our own atmosphere modifies the solar 

 light which passes through it, and adds 

 to the solar spectrum at least 1,200 lines 

 called telluric lines, which constitute 

 what we may term the spectrum of our 

 atmosphere. 



The rays or light coming into our in- 

 struments from Mars originate in the 

 sun ; they enter the atmosphere of Mars, 

 are reflected from the surface of Mars, 

 back again through its atmosphere, and 

 reach us after passing through our own 

 atmosphere. The double passage of the 

 solar rays through the Martain atmos- 



phere will cause a modified spectrum. 

 How are we going to pick out from the 

 enormous number of lines in the Mar- 

 tian spectrum those which are brought 

 there previously by the double passage 

 through the planet's atmosphere? Were 

 it not for the moon, we would be entirely 

 unable to attack the problem. The moon 

 has no atmosphere, and if our compari- 

 son of the lunar and Martian spectrum 

 show additional lines in the latter we 

 should be sure they come from the Mars 

 — but of this we are not certain until the 

 utmost pains have been taken with the 

 observations and the infinite variety of 

 detail has been looked into. Without 

 going into a summary of these details 

 it may be said that two years ago Prof. 

 Lowell thought he had proved that there 

 was a great supply of water vapor pres- 

 ent in the atmosphere of Mars. This past 

 summer Prof. Campbell, director of the 

 great Lick Observatory, carried to the 

 top of Mt. Whitney, a very carefully 

 prepared equipment, and by his observa- 

 tions there showed that the spectra of 

 Mars and moon were identical, and that 

 there was no proof whatever of the as- 

 sumption that water vapor is present in 

 large quantities in the atmosphere of 

 Mars. Prof. Campbell is probably the 

 greatest authority on spectroscopic mat- 

 ters to-day, and his opinion should carry 

 great weight. Indeed, a planet, barren 

 and desolate, on which life would be 

 impossible, were it not for irrigation, 

 but one which is wafted by an atmos- 

 phere laden with water vapor, seems 

 to an anomaly difficult to understand ! 



THE PLANETS. 



Mercury will be at the greatest elonga- 

 tion east from the sun 19° 3' on January 

 9, and so will be visible as an evening 

 star a week before and a week after that 

 date. Though its great southern declina- 

 tion prevents its being well seen, a close 

 observer may readily find it in the south- 

 west, about half an hour after sunset. 

 Mercury sets less than an hour after the 

 sun, about 30° south of west. It is said 

 that the founder of the Copernican the- 

 ory never saw Mercury. It is an easy 

 object to pick up, for its brighntess will 

 be relatively great. 



Venus, the bright evening star, will be 

 higher up in the southwestern sky about 

 30° towards the east from Mercury. ' 



