HOT-WATER BOILERS AND PIPES. 189 



the boiler, which we have frequently proved, it is, therefore, 

 evident that there can be no advantage by making the pipe to 

 incline from the horizontal level ; for whether the water descends 

 through a vertical or through an inclined tube, the force of 

 gravity will only be equal to the perpendicular height ; there 

 must, therefore, be an equality of pressure on the boiler under 

 all circumstances, whether the pipe entering the boiler be on a 

 level, or inclined from its junction with the flow-pipe. 



When it is necessary to sink the return-pipe below the level 

 of the boiler, there must be a sufficient weight of water in the 

 pipes, above its level, to overcome the perpendicular column that 

 exists below the level of the boiler, otherwise the tendency of 

 the lower column will be to a retrograde motion. The only way 

 is to raise the pipe sufficiently to afford a perpendicular return- 

 ing column of sufficient pressure to raise the water in the per- 

 pendicular pipe attached to the boiler. 



If the flow-pipe be carried on a horizontal level with the boiler, 

 and the return-pipe carried below the level of the boiler, it is 

 scarcely possible to obtain any circulation ; and if this depth be 

 much, no circulation at all can be obtained. We have seen some 

 costly apparatuses completely useless on this account ; and those 

 erectors of heating apparatus, unacquainted with the principles 

 of hydrodynamics, are very apt to commit similar mistakes. 

 The velocity of circulation in such apparatus will be just in 

 proportion to the difference of weight between the columns 

 above and below the boiler. 



It must not be supposed that water will not circulate in pipes 

 below the level of the boiler; and much trouble and expense 

 have frequently been incurred in consequence of being ignorant 

 of this position. All that is necessary is to give the upper section 

 of pipe a sufficient preponderance to raise the water in the lower 

 one, allowing for the superior density of the water in the lower 

 pipe. It, however, requires considerable judgment in adopting 

 any such forms of apparatus as this, for many concurring cir- 

 cumstances are essential to complete success. It should, there- 

 fore, never be adopted when a common horizontal working 

 apparatus can be introduced. 



