552 



NATURE 



[.Oct. 5, 



philosophy of rainfall, a "rain-band," or rain-predicting 

 spectroscope. 



There are some persons who will persist in opening the 

 slits of their spectroscopes too wide, and obtaining 

 thereby, when they look at the light of the sky, only a 

 brilliant continuous spectrum of showy colours, or who 

 let the sun, or some strong light glance across the slit, 

 and can then see nothing satisfactorily. But all those 

 others who narrow down the slit almost to extinction, 

 and focus the eyepiece nicely to their own eye, looking 

 from a shaded corner out to a portion of the low, day 

 illumined sky in front of them — all who in fact just do the 

 simply right and proper thing to begin with, have no 

 trouble in seeing, as they extend across the spectrum strip 

 of the daylight, besides the thin solar Fraunhofer lines, 

 and certain hazy lines and bands parallel thereto, and 

 depending on the absorption of the dry gases of our 

 atmosphere — they all, I say, agree and acknowledge that 

 they can also see one, two, or three other bands, which 

 from their places amongst the colours and solar lines, are 

 known to be the spectroscopic imagings of watery vapour. 

 Hence among the recent discussions in the Times, the 

 Scotsman, and other daily or weekly papers, there was 

 practically no disputation that the spectroscope has the 

 faculty of showing the presence of the otherwise quite 

 invisible watery vapour in the atmosphere. But some of 

 the writers contended that it showed the fact either so 

 faintly, or so capriciously, that the method was of little 

 use even as a hygrometer ; could only give deceptive 

 disheartening results in predicting the probable occur- 

 rence of rain, and must be looked on merely as one of a 

 number, and by no means the best, of " weather prog- 

 nostics. '' Is it worth while, therefore, to pursue the 

 method further ? 



If with the hope of overcoming the already formed 

 idiosyncratic prejudice of some one human mind, it is 

 not worth while. For there is nothing so easy for an 

 unwilling observer, as to ignore the nicety, and overlook 

 the precision of any quantitative spectroscopic observa- 

 tion ; especially when this mode of employing the 

 instrument in our present inquiry has been loudly con- 

 demned in public under a depreciating name, which 

 would bring it into the same category as the herd-boy's 

 confident advice to Dean Swift : " Sir, when you see that 

 bull turn his tail to the hedge, then you may be sure it is 

 going to rain." 



But we need not after all be offended at the mere 

 name of " prognostic ; " for are there not prognostics 

 and prognostics in meteorology ! What are not the 

 risings and fallings of the wind-compelling barometer 

 itself, but a weather prognostic for those who can inter- 

 pret them. And even a chart of isobars collected instan- 

 taneously from the whole extent of Europe by telegraph, 

 and mapped down in a central office in London, is only 

 another weather prognostic — of a very grand and expen- 

 sive kind truly ; but neither perfect in its forecastings 

 for every part of the country, nor so generally available 

 as could be desired to each private individual therein. I 

 myself, though charged with the meteorological reduc- 

 tions for all Scotland, have never been favoured with a 

 single telegraphic communication of fore-casted weather 

 from the London Office since its establishment. And if 

 I wait, as I did recently, for the isob.ir imp in the Thins, 

 it arrives here twenty-four hours late of the meteorological 

 events it records ; an interval quite long enough to allow 

 of an unwarned-for cyclone having meanwhile entered 

 the country on one side, and left it on the other, after a 

 devastating course across it. 



Wherefore a very good apology may surely be set up 

 for many, very many persons in the provinces continuing 

 to observe and speculate on the weather for themselves at 

 their own places of abode, supplementary to any forecasts 

 that may be issued onee a day from London. And if 

 such worthy persons do propose to take up the study of 



the atmospheric water-vapour, or rain-band spectroscope, 

 I do beseech them not to trust to it alone ; but endeavour 

 to observe simultaneously with it barometer, thermometer, 

 and wet-bulb hygrometer, not forgetting both wind and 

 cloud. But in that case do you ask, "can the spectro- 

 scope give such an observer anything he has not yet 

 already?" It can; for it gives him an instrument far 

 more portable than any other, seeing he can carry it (in 

 its most usual form) literally in his waistcoat pocket ; can 

 use it at a moment's notice, when in motion as well as at 

 rest ; besides which it gives him such a feeling of certainty 

 and security to know, that even from ever so confined a 

 crib or cabin, with no more than a few cubic feet of 

 peculiar, and for science-purposes vitiated, air about it — 

 he is nobly looking through the whole atmosphere from 

 the surface of the earth right through to space outside, 

 and analysing its condition as to watery vapour (the raw 

 material of rain, as the Times happily phrased it) in one 

 instantaneous, integrating glance. 



On the other hand, no doubt there is the drawback 

 that no meteorological spectroscope can be used at night, 

 nor in a London fog. It is a daylight instrument, and 

 requires the best part of the daylight too. But such 

 natural light usually lasts long enough to enable anyone 

 to make fifty observations a day, and more too if he be so 

 inclined ; though one will be usually quite enough in all 

 ordinary weather, if the observer attend to such necessary 

 precautions as these, viz. : — 



Observe always low down near the horizon, for atmo- 

 spheric effects in the spectroscope are there nearly twenty 

 times as strong as in the zenith. Get an opening between 

 clouds if you can to observe through. Prefer that the 

 sun itself shall be angularly distant from your observing 

 direction ; and behind a cloud also, if possible, at the 

 instant, so as not to illumine the motes in the air of your 

 neighbourhood with its high altitude light. Especially 

 avoid the minutes of sun-rising or setting, for that act, or 

 rather position, brings certain of the dry gas bands into 

 a short-lived maximum of intensity, without any other 

 signification than that the sun is then on the horizon. 

 But good observations may often be taken through falling 

 rain, though not through falling snow, and also between 

 the earth and the under sides of the clouds, if they 

 entirely shut out all view of the air of the heavens 

 beyond them. 



Sometimes dense coal smoke, or thick low fogs and 

 mist may prevent the observer obtaining his usual spec- 

 troscopic shot at a very low angle of altitude ; and if he 

 then points the instrument higher, the telluric rain-ban i 

 is necessarily weaker. How then is he to eliminate that 

 mere accidental, though most forcible, effect ? Simply by 

 making his notation of the strength of the rain-band not 

 absolute or solitary, but differential in terms of another 

 band which is not connected with watery vapour. And 

 herein he will find himself much assisted by the arrange- 

 ments of nature, or thus : — 



The strongest of the water-vapour or rain bands in the 

 spectroscope is on the red side of the solar D iine, and 

 apparently attached to it ; while at a very little distance, 

 removed away on the yellow side occurs a dry gas band, 

 called at home, in a lady's journal, the " low-sun band." 

 But throughout the greater part of the day forming only a 

 faint, constant shade, in terms of which the rain-band 

 may be entered ; and as they are both affected in the 

 same degree by merely being looked at in a high or low- 

 sky, the proportion between the two, which is all that we 

 require for the intended quantification of watery vapour, 

 remains the same. 



Again, before deciding on what conclusion, as touching 

 rainfall, is to be drawn from any particular degree of 

 darkness of the said water-vapour band, let the observer 

 consider the temperature of the air at the time. Run 

 down the columns of Mr. Glaisher's invaluable tables for 

 reducing hygrometric observations, and obtain thereby a 



