HEATING BY HOT WATER. 197 
when opened wide, admits a quantity of cold air to the 
base of the smoke-pipe, and thus reduces the draught 
and consequently the amount of heat generated also, 
while when closed the draught is rendered stronger and 
more heat is obtained. The makers claim for this form 
Fig. 136. 
of boiler that the fire will keep alight from twelve to twenty 
hours without attention, and that a maximum of heat is 
obtained with a minimum consumption of fuel, while the 
temperature can be regulated with ease and great precision. 
