HEATING BY HOT WATER. 179 
Saddle-boilers are made in both wrought and cast iron, 
and those of the former class may be had either welded 
or rivetted. 
Boilers of the horizontal tubular type are now extensively 
employed by market gardeners and nurserymen for heating 
long ranges of piping. These consist chiefly of cast-iron 
pipes, but in this form the metal is not nearly liable to 
give way suddenly as in large castings, besides which 
these boilers are now so constructed as to enable a defective 
tube to be removed and replaced in a very short time and 
with comparatively little trouble. 
But even where these, or indeed any boilers of large 
size are employed for important work, it is always 
advisable to fix them in pairs or trios, connected together 
with valves, etc., so that either or both can be put to work 
the whole of the piping, or be disconnected entirely at 
any time, for repairs or the like, without affecting the 
temperature of the houses to any extent. Thus, supposing 
there is say 2,500ft. of piping to be heated, if only one 
boiler is put in this ought to be capable of working at 
least 3,000ft., but instead of this it is much better and 
safer to fix wo, each sufficiently powerful to heat 2000ft. 
or so. Then, in mild weather, when less heat is required, 
or one or two houses may be cut off, either boiler will 
do the work alone, while when coupled together they 
will maintain the maximum temperature in severe weather 
easily. Lastly, should one break down, the other will 
suffice to maintain at any rate a moderate temperature 
while the other is being repaired or replaced. 
The heating power of all hot-water boilers is calculated 
by reckoning the number of square feet of surface exposed 
n 2 
