482 Pohnaise Method of Heating Greenhouses, 5fc. 



point, and generating steam. It is this circulation which 

 causes all the water in the apparatus to pass successively 

 through the boiler, — conveying the heat to the parts most 

 distant from it, by which it is again cooled, and again returns 

 to the boiler to absorb heat and carry it away. Without this 

 circulation those parts of the apparatus which are remote 

 from the source of heat, would receive little or no heat at all. 

 From this, then, it will be evident that circulation forms one 

 of the most important of our present considerations. 



Our first object, then, is to consider the nature of this 

 circulation, and the power that produces it, so that we may 

 have a clearer perception of the causes of its obstruction. 



The cause of circulation in hotwater pipes is in conse- 

 quence of the unequal pressure of water on the lower pipe, 

 not of any alteration in the level of the water in the pipes, 

 as some erroneously suppose. Some persons imagine that if 

 the pipes be inclined so as to allow a gradual fall of the water 

 in its return to the boiler, circulation is obtained, or increased. 

 This, at first, appears very plausible, particularly with regard 

 to the forms of apparatus generally used in heating hothouses ; 

 but the principle is entirely erroneous, and appears to arise 

 from regarding the subject as a simple question of hydraulics, 

 instead of a compound result of hydrodynamics. If the 

 question were only as regards a fluid of uniform temperature, 

 then the greatest effect would be obtained by inclining the 

 pipe towards the boiler ; but the fluid contained in the pipes, 

 when motion is necessary, is of varying density and temper- 

 ature, which materially alters the conditional results. 



In order to explain this more clearly, let us illustrate one of 

 the most simple forms of apparatus, {Jig. 29,) such as are used 



T 



A 



Fig. 29. Hotwater Apparatus, showing the principle of Circulation. 



in some of the most extensive forcing establishments in Eng- 

 land, the principle of which is represented in the above cut. 



