PLANTS, WITH GLASS ROOFS. 213 



mended, though sometimes convenient. When the ten-inch boiler is em- 

 ployed to small quantities of pipe, it must he fitted with a reservoir, as in fig. 

 152. In this manner it may be made to work as low as fifteen or twenty feet 

 of four-inch pipe. Four-inch pipe is taken as a standard, because each foot 

 of it contains about one square foot of radiating surface. Of three-inch, one 

 third more ; and of two-inch, double the quantity, may be considered as the 

 equivalents of the above amounts. 



These boilers are so constructed that they can be cleaned out ; and, if ne- 

 cessary, they can be taken to pieces, to remove any calcareous deposit which 

 may in time take place in them. It is, however, particularly desirable, in 

 these, as in all hot-water apparatus, that nothing but pure rain or pond water 

 should be employed. Where the boilers are employed for steaming, this 

 precaution is particularly important, otherwise calcareous incrustation must 

 take place. To prevent leaves, dirt, &c., getting down the pipe of the supply 

 cistern, it should be guarded by a double cap of pierced zinc ; one moveable, 

 that the gardener may cleanse it if clogged, and the other fixed. 



The advantages of these conical boilers are no longer matter of speculation 

 or experiment. Very many have been erected in the course of 1839 and 1840, 

 and are highly approved ; although few of them possess all the advantages 

 which experience has since combined in the form now described. They are 

 peculiarly adapted for those purposes where perpetual heat is required ; for 

 plant-stoves, pineries, and forcing-frames ; also for small propagating-houses, 

 or preserving-pits. To pits in general, from their small size, and from the 

 small expense incurred in setting them, a recommendation not heretofore 

 noticed, they are peculiarly applicable, and have been extensively applied. 

 Card. Mag., 1840, p. 139.* 



505. Rain-water should, as we have just seen, always be used in hot- 

 water apparatus ; for hard- water deposits a sediment or incrustation, which 

 if not removed, will form a coating of several inches in thickness, which 

 acting as a powerful non-conductor, will allow the bottom of the boiler to 

 become red-hot without sufficiently heating the water it contains ; and ulti- 

 mately, from the cracking of the deposit in consequence of the greater 

 expansion of the red-hot iron, the water comes in contact with the red hot 

 metal, and an explosion takes place. (See Gard. Mag. vol. ix. p. 206.) 

 Hence the necessity of having all boilers where hard- water is to be used 

 constructed so as to admit of being readily cleaned out. As the deposit con- 

 sists of calcareous matter, it may be removed by a weak solution of muriatic 

 acid aided with a slight mechanical agitation : but it is much better to 

 prevent its taking place by using only soft water. 



506. To prevent the water in the apparatus from freezing, salt may be 

 added to it ; but this may be rendered unnecessary in the case of horizontal 

 pipes by drawing off a portion of the water, so that they shall not be quite 

 full, because in that case the water has room for that expansion which takes 

 place when it passes into ice. The quantity of salt put into water to keep 

 it from freezing, Mr. Hood observes, may vary from 3^ per cent., the quan- 

 tity contained in sea- water, which will not freeze when it is above 28, to 

 35 per cent., the greatest amount of common salt which water will hold in 

 solution. With 4.3 of salt, water freezes at 27^ ; with 6.6 per cent, of salt, 

 at 25 J ; and at 11.1 per cent., 21^. The effect which would be produced 

 in cast-iron pipes and boilers by any of these quantities of salt, Mr. Hood 

 states, would not be of much importance. As salt does not evaporate when 



