538 C. PIAZZI SMYTH ON THE 



usual term of "Humidity" among Meteorologists, but which is in reality 

 " Relative Humidity " only ; and means nothing for spectroscopic purposes, nor 

 for absolute chemical composition either, unless accompanied by a statement of 

 the temperature at the time. This may be well illustrated by the excellent 

 annual account of New York Meteorology published by Dr Daniel Draper, at 

 the Central Park New York Observatory for 1884. For, while his annual list 

 of monthly means of "Elastic force of Vapour" numbers (which are but 

 another form of " Grains of water-gas in a cubic foot of air ") shows a grand 

 curve through the year, having its minimum in January, and Maximum in 

 August or September, — agreeably with Professor CooKe's observed growth of 

 " Aqueous lines between D 1 and D 2 " — yet the New York " Relative 

 Humidity," runs up to its terrible maximum of 96, not in August or 

 September, but in January ! On referring however to the Doctor's Tempera- 

 ture return for the same month, and same hour, it is found to be only 

 21 "42° P. Wherefore the amount of water-gas in each cubic foot of air at that 

 time could only have been 13 grains ; under which scarcity, all thin water-gas 

 lines would have been practically invisible. 



Even at Edinburgh, in the present month of May, looking with the identical 

 Winchester grating spectroscope — it was almost startling to find hardly 

 anything except the one Nickel line, by contrast most pronounced in the middle 

 of an apparently waste, empty space between the two D's ; the water-gas amount- 

 ing in this case to 2*6 grains per cubic foot of air. While at Winchester, when 

 assisted by double that weight, I seldom looked at the D lines without seeing 

 eleven or more finer, and evidently water-gas lines between them, besides 

 the solar Nickel line ; and some of the former were almost as strong as that. 



Although therefore definite and constant lines of water-gas have only 

 hitherto been much noticed towards the red end of the Spectrum, where 

 they are undoubtedly strongest, and most easy to see, yet in warm, moist 

 weather we may expect to find them elsewhere also. And there is a narrow, 

 but positive band of them, noticed by Angstrom as " strong in summer," so far 

 away as in the further Green, or beyond little b. This band too, from the 

 interesting manner in which it has lately been independently rediscovered, and 

 even utilised in Jamaica as a Rain indicator, has been proposed to be named 

 " Maxwell Hall's Jamaica Rain-band." 



Besides which, Spectroscopists should be warned, that as the whole violet 

 end of the spectrum is sensibly dulled by water-vapour when in abundance, 

 there is most probably a formation over the whole of that region of infinitely 

 fine linelets. And though these have not yet been distinctly resolved to view 

 by any one, — still with every increase in the power of new spectroscopes 

 we may expect to fall across some cases of them made visible ; especially if 

 we look under the most powerful of the appropriate Meteorological conditions. 



