755 
of Edinburgh, Session 1871-72. 
supposing the carbonic acid and water equally efficient as absorb¬ 
ing agents of the vibratory energy (although each has a specific 
absorption for certain qualities of rays), then the decomposition of 
the two compound molecules may take place continuously side by 
side, owing to the equality of the thermal equivalents of carbonic 
oxide and hydrogen. We already know, from the laborious re¬ 
searches of Tyndall, how thoroughly aqueous vapour retains 
thermal radiations; and Janssen has further shown that the same 
substance has a strong absorptive action on the rays of light of low 
refrangibility (just those rays that are in part selected by chloro¬ 
phyll), producing the well-known atmospheric lines of the solar 
spectrum. The presence, therefore, of varying quantities of 
aqueous vapour in the atmosphere in all probability produces a 
considerable difference of rate in the decomposition effected by the 
leaf, and may, in fact, end in carbonic acid and water being 
attacked in another ratio than that given as the fundamental 
equation of decomposition. Thus the same plant in different 
atmospheric conditions may elaborate different substances. 
2. On the Rainfall ot the Continents of the Globe. By 
Alexander Buchan, Secretary of the Scottish Meteoro¬ 
logical Society. 
This paper was illustrated by two large charts of the world 
showing, by isohyetal lines, the rainfall over the different conti¬ 
nents in January and July; two large charts showing the months 
of least and greatest rainfall in Europe, north Africa, and west 
Asia; and by six sets of smaller charts of thirteen each, showing, 
by isohyetal lines, the monthly and annual rainfall of Europe, 
Asia, Australasia, North America, Africa, and parts of South 
America. The data laid down on these eighty-two charts were 
taken from a Table comprising about 2000 good averages of rain¬ 
fall, calculated or collected by the author. 
On comparing the results of the rainfall with the author’s charts 
of Atmospheric Pressure and Prevailing Winds, published in the 
Society’s Transactions,* the broad principles regulating aqueous 
precipitation are chiefly these :— 
* Yol. xxy . p. 575, et seq . 
