Hawthorn 



( 400 ) 



Heating 



Keiuwardtii, Ivs. U" to 

 2" long, V' broad. 



retusa, Ivg. 1" to 1" long, 

 J" broad (>//. Aloe 

 retusa of Botanical 

 Jfti'/ici/if 455). 



rigida, Ivs. 1" to 1.;" 

 long, warted, br., red. 



tortuosa, Ivs. li" to 2" 



loug, dark grn., upper 

 surface hollow (*//. 

 Aloe rigida of Sotamoai 

 Muguziur 1337). 

 viscosa. Ivs. 1" to H" 

 loiiK,uppersurfacemuch 

 hollowed out, dark grn. 

 (y. Aloe viscosa of 

 Jio/anicalJfagt<:i>ie 8H). 



SYSTEM or HOT WATER CIRCULATION. 



I, boiler. 



'2, tire bars. 



3, ash box. 



4, brickwork. 



5, 5, flow pipe. 

 (>, air pipe. 



7, tap. 



8, 8, return pipe. 

 ;i, junction. 



10, supply pipe. 



II, cisterns. 



a, 6, o, upward flow. 



d, e. f, g h, downward flow. 



A, fuel space. 



m, n, level of water in cistern. 



x y, vertical linn showing depth of water column. 



y z, horizontal line showing the rise of the pipes. 



HAWTHORN. 



The popular name of the flower of the White- 

 thorn, Cratsegus Oxyacantha, and its varieties. 

 Also called " May," in which month it usually 

 appears in an ordinary season. (For culture, species, 

 and varieties, see CRA'MSGUS.) 



HAZEL. 



The popular name of the wild Nut, Corylus 

 Avellana. (tee CORYLUS and NUTS.) Hazel is 

 much planted for covert purposes, being cut down 

 at intervals of six or seven years. The cut-back 

 stools soon break again, unless they are very roughly 

 handled. 



HEATH (.tee ERICA). 



HEATING. 



Much of the success that attends gardening 

 operations under glass depends upon the efficiency 

 or otherwise of the heating apparatus which may 

 be in use. Briefly, the systems of artificial heating 

 may be summed up as (1) by hot air, and (2) by 

 hot water. To the former belong the antiquated 

 .system of heating by flues, now almost entirely 

 discarded in favour of the newer and more scien- 

 tific methods of heating by hot. water ; heating by 



Hazel, Witch (see HamameUs). 



Heartsease (see Viola tricolor). 



Jli'ath, St. Dabeof's (nee Daboecia polifolia). 



Jfeatli, Sea {see Frankenia). 



Heatlier {see Calluna ndyaris). 



Heathtvorts {see Erica). 



fermenting materials, as in the case of hotbeds, 

 and of some propagating frames : and heating by 

 oil and gas lamps, without the medium of water. 



By Hot Water. When we note that the artificial 

 heat used in plant and fruit houses should be moist 

 and equable, and not given to violent extremes, we 

 see how excellent a medium hot water is, when 

 conveyed by a proper system of piping, for keeping 

 up the temperature of such houses when natural 

 heat fails. The makes of boilers are almost in- 

 numerable, and include various methods of fixing 

 the pipes, so that we have at the start a number of 

 apparently totally distinct apparatuses. Yet all, 

 from the'big Duplex Upright Tubular, or Keith's 

 Saddle, capable of driving 20,000' of 4" piping, to 

 the little Independent boiler with scarcely 100' 

 to its credit, work upon a common principle the 

 circulation of the water. The fire being lighted, 

 the water lying close to the walls of the boiler 

 begins to get warm, its specific gravity becomes 

 less, the colder water, with the greater specific 

 gravity, finds its level, and the warmer and lighter 

 water is pushed upwards, the cold water taking its 

 place. Thus it goes on as long as the fire is alight, 

 a constant stream of cold water from the " return" 

 pipes pushing upwards the warm water. In this 

 way is set up the circulation of water, and if 

 the apparatus is to be a success this circulation 

 must be regular and constant. . Any hitch or 

 stoppage means cold pipes and frosted plants in 

 one house, and perhaps a burst from over-pressure 

 in another part. This is technically termed the 

 " low pressure system of heating by hot water." 

 There is also a "high pressure " system, but it is 

 almost exclusively employed for the heating of 

 dwelling houses and public buildings, and does not 

 come within the scope of a gardening dictionary. 



Boilers. {Xee remarks under BOILER.) Generally 

 speaking, cast iron boilers are more satisfactory 

 than wrought iron ones, because greater uniformity 

 in the thickness of the walls is obtained. More- 



4.\ 5. 



Hoi WATER PIPE CONNECTIONS. 



1, double T 



2, T piece. 



3, H piece. 



4, inside and outside ell>o\v. 



5, Y piece. 



0, double elbow. 



over, wrought iron heaters are liable to scale off in 

 flakes and to wear unequally. 



Gas and Oil Heaters. Small hot water appar- 

 atuses are procurable, the heating medium of which 

 is gas or oil, and they are much superior to the oil 

 lamps which heat by hot air. Oil and gns have 

 the advantage over coke and coal in that the 



