OF COAL AND RATE OF EVAPORATION, 39 
disposition to adhere to the boiler, unless cemented by the 
sulphate of lime. 
It is found accordingly, that no scale of much conse- 
quence will be formed on engine boilers, with even such 
hard water as is used, if 100 gallons of water daily, and 200 
additional gallons on Saturdays be run off through the usual 
mud machine, and if the boiler on every sixth Saturday be 
further entirely emptied and swept out. ‘There is little 
loss incurred by proceeding in this manner, as the chief 
discharge may take place at the close of each day, while a 
large profit accrues by the saving of coals and the increased 
durability of the boiler, with security from danger to all 
concerned with it. 
25. In the five boilers most used in these experiments, 
the highest results were obtained when all the air required 
to consume the coals was introduced into the fire-place 
through the fire-bars. The introduction of additional air 
elsewhere uniformly occasioned loss, whether it was ad- 
mitted by means of the furnace door during two minutes 
after firing, or for a longer or shorter period, or through a 
regulating slit above the furnace door, or through apertures 
of larger or less size and more or less numerous, in the 
sides of the furnace, in the front of the bridge, or on the 
top or back of the bridge. 
26. Perfect combustion of black smoke was never ar- 
rived at, without the introduction of additional air. The 
consequent loss was a little over 1 per cent. 
This loss, by the addition of air over the fire, will be 
uuderstood when we consider the smallness of the weight 
of black smoke. The following experiments may convey 
some information on this pomt. I had in view the possi- 
bility of working bleaching keirs with hot air from the 
flue of the furnaces instead of steam. The apparatus used 
is shown in the drawing : — 
