Polmaise Method of Heating Greenhouses, <^c. 483 



Now let us suppose heat to be applied to the boiler, A ; a 

 dilatation of the volume of the water takes place, and it 

 becomes lighter ; the heated particles rising upwards through 

 the colder ones, which sink to the bottom by their greater 

 specific gravity, and they in their turn become heated, and 

 expand like the others. This intestine motion continues 

 until all the particles in the volume become equally heated, 

 and have received as much heat as the fuel can impart to 

 them. But as soon as the water in the boiler. A, begins to 

 acquire heat, and to become lighter than that which is at 

 the opposite end, B, the water which is in the lower hori- 

 zontal pipe, (/, is pressed by a greater weight at c than at /, 

 and it therefore moves towards A, with a velocity and force 

 equal to the difference in pressure (or weight of the two 

 columns,) at c than at /. The water in the vertical pipe, B, 

 would now fall to a lower level were it not that the pipe e 

 furnishes a fresh supply from the boiler, to replenish the 

 deficiency. By means of this unequal pressure on the lower, 

 or what is called the return, pipe, the water is forced to cir- 

 culate through the apparatus, and it continues to do so as 

 long as the water at B is colder than that in the boiler. 

 And as the water in the pipes is constantly parting with its 

 heat, both by radiation and conduction, while that in the 

 boiler is continually receiving additional heat from the fire, 

 an equality of temperature in the water contained in the 

 whole apparatus can never occur, while the apparatus is at 

 work, for if it did, circulation would cease. 



Contrary, therefore, to what is supposed by many, we find 

 that circulation commences in the loicer, or return pipe first. 

 The first motion among the particles being at/, in the fore- 

 going cut, and that this motion is caused by the superior 

 weight of the column of water in the vertical pipe, B. To 

 a person unacquainted with this astonishing principle in 

 hydrostatics, the theory of circulation here given may proba- 

 bly appear erroneous, because the quantity of water con- 

 tained in the boiler. A, is so much greater than that in the 

 vertical pipe, B. It is, however, one of the first laws of 

 hydrostatics, that the pressure of fluids depends for its 



