484 Pohnaise Method of Heating Greenhouses, S^c. 



amount, on the height of the coUimn only, wholly irrespective 

 of the bulk, or actual quantity of the fluid contained in the 

 column. Therefore a pipe, which is not larger than a quill, 

 will transmit the same amount of pressure as if it were a foot 

 or a yard in diameter, provided the height in both cases be 

 alike. On this law, therefore, depends the circulation of the 

 water in the forms of hot water apparatus generally used in 

 horticultural structures. 



From the foregoing cut it will be seen that the height of 

 the column, B, is just equal to the distance between the 

 apertures of the two pipes, / and g. The effective pressure 

 is the same, whether the return pipe be carried on the level 

 from c to/, or according to the dotted lines, as is frequently 

 done. But suppose that the return pipe be carried on an 

 inclination from B to /, as is frequently done, under the 

 impression of gaining additional power in the circulation. 

 Without having recourse to abstruse calculations, I think it 

 will be evident enough that this inclination of the pipe tends 

 to reduce the amount of pressure instead of increasing it ; 

 for, though the height of the column be in both cases the 

 same, the effective pressure of the column is reduced by its 

 horizontal position, in consequence of the friction on the 

 sides of the pipe, which is not compensated in any manner. 

 Moreover, as there is actually more matter in a pipe filled 

 with cold water, than in one filled with hot water, the 

 gravitating force will be inversely proportional to the tem- 

 perature ; that is, it will be less in proportion as the tempera- 

 ture of the water is greater. There must, therefore, under 

 all circumstances, be a positive loss of effective pressure by 

 inclining the pipe in the manner stated. 



If we are right in the conclusion that the power which 

 produces the circulation in the pipes is the unequal pressure 

 on the extremities of the return pipe, then the result will be 

 precisely similar, whether this force acts on a pipe ten feet, 

 or on one a hundred feet in length, and it is so. 



This has always occurred to me one of the most simple 

 and efficient forms of apparatus for heating hot-houses, — and 

 many fine structures are so heated. As an instance, I might 



