THE SPECTROSCOPE AND THE WEATHER. 241 



terse philosophy than those which are to be found in the ancient hook 

 of Job, wherein, of the wondrously "balanced clouds" high up in 

 mid-air, it is said, "They pour down rain according to the vapor 

 thereof." 



More or less of this water-vapor is always in the air, even on the 

 very clearest days, and a happy thing for men that it is so ; for, as Dr. 

 Tyndall and others have well shown, it moderates the excesses of hot 

 solar radiation by day and cold radiation of the sky at night, and is 

 more abundant in the hotter than in the colder parts of the earth. 

 Wherefore, according largely to its temperature for the time being, 

 the air otherwise consisting almost entirely of nitrogen and oxygen 

 can sustain, and does assimilate, as it were, a specified amount of 

 this watery vapor, invisibly to the naked eye, the microscope, or the 

 telescope ; but not so to the instrument of recent times, the spectro- 

 scope. And if the air vertically above any one place becomes pres- 

 ently charged with more than its usual dose of such transparent 

 watery vapor (as it easily may, by various modes and processes of 

 nature), the spectroscope shows that fact immediately, even while the 

 sky is still blue ; clouds soon after form, or thicken if already formed, 

 and rain presently begins to descend. 



But how does the spectroscope show to the eye what is declared to 

 be invisible in all ordinary optical instruments ? It is partly by its 

 power of discriminating the differently colored rays of which white 

 light is made up, and partly by the quality impressed on the molecules 

 of water at their primeval creation, but only recently diseovei'ed, of 

 stopping out certain of those rays so discriminated and placed in a 

 rainbow-colored order by the prism and slit of the spectroscope, but 

 transmitting others freely. Hence it is that, on looking at the light of 

 the sky through any properly adjusted spectroscope, we see, besides 

 the Newtonian series of colors from red to violet, and besides all the 

 thin, dark Fraunhofer, or solar originated lines, of which it is not my 

 object now to speak, we see, I say, in one very definite part viz., be- 

 tween the orange and yellow of that row of colors, or " spectrum," as 

 it is called a dark, hazy band stretching across it. That is the chief 

 band of watery vapor ; and to see it very dark, even black, do not 

 look at a dark part of the sky or at black clouds therein, but look, 

 rather, where the sky is brightest, fullest of light to the naked eye, 

 and where you can see through the greatest length of such well-illu- 

 mined air, at a low, rather than high, angle of altitude, and either in 

 warm weather, or, above all, just before a heavy rain-fall, when there 

 is and must be an extra supply of watery vapor in the atmosphere. 

 Any extreme darkness, therefore, seen in that water-vapor band be- 

 yond what is usual for the season of the year and the latitude of the 

 place is an indication of rain-material accumulating abnormally ; 

 while, on the other hand, any notable deficiency in the darkness of it, 

 other circumstances being the same, gives probability of dry weather, 



VOL. XXII. 16 



