CHAPTER V, 



CIRCULATION AND CONVECTION IN LIQUIDS AND GASES. 



Circulation and Convection of Heat Hot- Water Heating Systems Ocean Currents 

 Convection in Gases Convection Currents in the Atmosphere Winds Land 

 and Sea Breezes Trade-Winds Water-Vapour Aids Convection Currents 

 Weather Forecasting in the Case of Cyclones Convection in Chimneys and 

 Hot- Air Heating Systems. 



Convection Of Fluids. Owing to the ease with which one portion 

 of a fluid can glide past adjacent portions, any local change in density 

 due to expansion by heat, at once s-^. 



results in motion. If, for example, jf( ^ ^ 

 a flask be heated from below, as in (( n>> ty 

 Fig. 39, and a little bran be put in 

 the water to show the direction of 

 motion, it is very soon seen that the 

 heated, and therefore lighter, water is 

 rising up from the bottom, its place 

 being taken by a down-current of the 

 colder water from the top. Usually 

 the down-current is along the side ; 

 but if the flask be heated at one side 

 only, it is easy to establish the circu- 

 lation up that side and down the oppo- 

 tion of Water in s ^ e colder side. The circulation may 

 a Heated Flask, also be seen through the varying re- 

 fraction of the up and down currents, 

 which give an apparently shimmering motion to any 

 object looked at through the water. This process of 

 circulation through expansion by heat carries the heat 

 from one part of the vessel to another, and this car- 

 riage of heat by motion of the heated matter is termed 

 convection. Convection obviously expedites the com- 

 munication of heat to the liquid as a whole, for not only 

 are fresh portions of liquid being continually brought 

 into contact with the heating surface, but also the heated 

 liquid is continually coming into contact with colder 

 surroundings, with which it shares its heat much more 

 rapidly than with surroundings nearer to it in tempera- 

 ture. As an illustration of this, we may compare the 

 method of boiling water in a test tube by applying heat 

 at the bottom as in Fig. 40 with that represented in Fig. 41. In the 

 latter case the circulation is very local, and the hot water, being the 



S3 



IG ^Q Boiling 



Water by Con- 

 vection. 



