J 0.6 PHYSICAL GEOGEAPHY OF THE SEA, AND ITS METEOPtOLOGY. 



tliis part of tlie Atlantic is not quite 5 millions of square miles, and 

 it does not include more than one thirtieth of the entu'e sea sm^face 

 of our planet, while the water-sheds under consideration contain 

 one sixth part of its entire land surface. The natural proportion 

 of land and water smface is nearly as 1 to 3. According to this 

 ratio, the extent of sea sm-face required to give rain for these SJ- 

 millions of square miles would be a little over 25, instead of a Httle 

 less than 5 millions of square miles. 



280. The state of our knowledge concerning the actual amount 

 Daily rate of evapo- of evaporation that is daily going on at sea has, not- 

 than on land-ob- withstauduig the activity in the fields of physical 

 servations wanted, roscarch, becu but little improved. Kecords as to 

 the amount of water daily evaporated from a plate or dish on shore 

 iifibrd us no means of judging as to what is going on even in the 

 same latitude at sea. Sea-water is salt, and does not throw off its 

 vapour as freely as fresh water. Moreover, the wind that blows 

 over the evaporating dish on shore is often dry and fresh. It comes 

 from the mountains, or over the plains where it found little or no 

 water to drink up ; therefore it reaches the observer's dish as thh^sty 

 wind, and drinl^ up vapour from it greedily. Now had the same 

 dish been placed on the sea, the air would come to it over the water, 

 diinking as it comes, and arriving already quite or nearly saturated 

 with moistm^e; consequently, the observations of the amount of 

 evaporation on shore give no idea of it at sea. 



281. There is no physical question of the day which is more 

 Kivers are gauges for worthy of attention than the amount of effective 



the amount of effect- "^ . n , • -i -i • • ii i-* 



ive evaporation. cvaporatiou that IS daily gomg on m the sea. 13y 

 effective I mean the amount of water that, in the shape of vapom-, is 

 daily transferred from the sea to the land. The volume discharged 

 by the rivers into the sea expresses (§ 270) that quantity ; and it 

 may be ascertained with considerable accuracy by gauging the 

 other great rivers as I procm^ed the Mississippi to be gauged at 

 Memphis in 1849. 



282. The monsoons supply rains to feed the rivers of India, as 

 Importance of rain the north-cast and south-east trade-winds of the 

 -and river gauges. Atlantic supply raius to feed the rivers of Central 

 .and South America. Now rain-gauges which wiU give us the 

 mean annual rain-fall on these water-sheds, and river-gauges which 

 would give us the mean annual discharge of the principal water- 

 <jourses, would afford data for an excellent determination as to the 

 amount of evaporation from some parts of the ocean at least, espe- 



