178 GREENHOUSE CONSTRUCTION AND HEATING. 
being merely to heat the water as it passes through them, 
and not to boil it—of various kinds on the market, all 
of which possess their good and bad points, while most 
of them are more or less suitable for different purposes. 
The majority are made in various sizes, according to 
the amount of piping they are intended to heat, but some 
forms are better suited for large quantities of piping, 
and others for smaller lengths. 
The best ‘boilers’ are made of stout sheets of wrought- 
iron, welded or rivetted together, the usual thicknesses 
employed being ,§in. and gin. Cast-iron boilers are 
also employed to some extent, but though these are at 
once much less expensive, and resist the combined action 
of fire and water better than the wrought metal, they 
Fic, 118. 
are unfortunately liable to crack at any time. This usually 
occurs in frosty weather, particularly if the furnace door 
is opened suddenly, though it may take place without 
any apparent cause, and occasionally happens the very 
first time the fire is lit. Hence they are scarcely depend- 
able enough for work of any importance or magnitude, 
though at the same time a cast-iron boiler will frequently 
last and do excellent work for the best part of a lifetime. 
The saddle-shaped boiler is one of the most popular 
and useful types for all ordinary purposes, and for heating 
lengths of from 100ft. or 200ft. to as many thousand 
feet of 4in. piping either a plain saddle or one of the 
rather numerous forms of this boiler can scarcely be beaten. 
