322 
TlMEHRl. 
The bottom line of this science is the bottom line of an 
old pot over a fire, which is the best heating surface in 
the world ; there is water upon one side of the piece of 
metal, and heat on the other : one square foot of metal 
will transmit through it a given number of units of heat 
into the water, at a given temperature, in a given time ; 
2 square feet twice as many, and 3 square feet three 
times as many, and so on : put a cover on the pot and 
seal it tight, leave an orifice for the steam, and that is 
the steam boiler with all its working mysteries. The 
points to be considered in the selection of a new boiler 
are type, design, materials, workmanship, safety, effi- 
ciency, and economy. 
Type. The particular circumstances, and require- 
ments of the steam user, should always be carefully 
considered before deciding as to what type of boiler 
ought to be adopted. In large towns where space is a 
matter of importance, boilers of the multitubular class, 
such as locomotive, marine &c, will be found very suita- 
ble, and as a cheap supply of pure feed water can now 
be obtained in most towns, and also a good furnace 
draught from the necessary tall chimneys, the principal 
objeftions to the use of these boilers has been removed. 
Some few years back I remember seeing some boilers of 
this type that had been working with scaling water in a 
sugar estate in Australia, the furnace crown stays were 
simply matted, or scaled together with limestone most 
beautifully stratified, as may be seen from a scarf-pin I 
am wearing, which is far more useful, and ornamental, to 
me, than it ever was to the boiler. The freeness of water 
in Demerara from carbonate of lime, chalk, gypsum 
and other incrusting solutions, coupled with the great 
