Art. IX. — Notes on Hydrology. 

 By G. K. B. Steane, Esq., C.E. 



[Read 14th June, 1883.] 



The subject which I Lave the honour to submit to you this 

 evening, Hydrology, is a very extensive one ; but I purpose 

 referring only to the subject of rainfall, more particularly 

 the rainfall in Sandhurst, and some of the results of my own 

 observations during a long residence there. 



Kainfall is, I believe, the most capricious of the elements, as 

 it is governed or influenced by so many varying forces, and 

 the laws which govern it are most complex. The general 

 laws which govern it are being studied, and it is anticipated 

 that some of them may be generalised, so that though it 

 may be impossible to govern the rain, yet we may in the 

 future be better enabled to prepare for the inevitable. 



The object with which I have observed rainfall has been, 

 primarily, its effect in causing floods, and to arrive at 

 reliable data for providing for it. With that object in view 

 I constructed a simple recording rain-gauge, consisting of 

 a cylindrical gauge, into which rain from a known area was 

 conveyed. A light metal float actuated a couple of pencils 

 which recorded the rainfall, first on a drum which revolved in 

 an hour, and secondly on a drum which revolved in twenty- 

 four hours. After a heavy fall of rain it was an easy 

 matter to scale off the quantity that fell in any period of 

 time. 



Rain, as is well known, is water which has been 

 evaporated carried by currents of air and condensed 

 by cooling. Air has the power of absorbing varying 

 quantities of moisture increasing with the temperature. 

 Hence a cube foot of air at 32° can absorb 3 J grs. 



at 86° „ „ 14 grs. 

 at 140° „ „ 56 grs. 

 and so on, so that if warm air, saturated, is cooled it will 

 discharge water, and it is well known that the greater the 

 heat the more rapid the evaporation. 



The movements of the winds which carry these vapours 

 have been most ably explained very recently by our esteemed 

 President. 



We will briefly consider under what circumstances rain 

 falls generally. First, the air must be laden with moisture 



