152 GREENHOUSE CONSTRUCTION AND HEATING. 
used, but oil stoves are fitted with the usual broad flat or 
circular wicks. The only disadvantages are that the 
commoner and cheaper forms of boiler are apt to be very 
extravagant in fuel, in proportion to the heat obtained, an 
excessive proportion of the caloric generated by the flame 
passing away up the flue and being wasted. Again, unless 
a wind-proof cap is fixed on the top of the flue pipe there 
is always a risk of the flame being blown out by a sudden 
down-draught in stormy weather, and though with oil such 
an occurrence leads to no more serious harm than the loss 
of heat, if the flame is thus extinguished during the night 
where gas is burnt, this of course continues to escape until 
turned off, and a serious explosion may take place. The 
better types of boiler, however, utilize a very high 
percentage of the heat generated, and are consequently 
fairly economical in working, while one of the patent 
wind-proof caps fixed on the flue-pipes .will effectually 
prevent all down-draught, and render any accident of 
the kind referred to impossible, 
