434 Abstract Report of Agricultural Discussions. 
"boiler there is no danger. During the time tlae steam is being 
got np, the fire is in an excited state, and the irregularity of the 
llamc playing upon the boiler has a tendency to keep the water in 
a state of ebullition ; the danger of the water attaining heat withotit 
throwing oli' steam is when there is a steady coat of fire upon it. 
AVhen the steam is being got np, and the fire is in that state of 
levity, the opening of the valve is of no use, except to ascertain 
that the valve is in order. 
Mr. Api'Dld said he believed tlie freer the water is from air, the 
more likely is the boiler to explode. He had taken water, got rid 
of the air, and heated the water up to 240° instead of 212°. One 
day he left it in his room with a thermometer in it, and soon after- 
wards it exploded, the steam blowing the thermometer out of the 
flask, so that there must have been a considerably greater pressure 
than 240°, up to which point he had watched it ; from that lio 
arrived at the conclusion that the more free the water is from air, 
the greater is the danger of accidents. In the case of agricultural 
engines, on shutting oii' the water altogether, and allowing a small 
jet of steam to escape from the boiler, the whole of the air will bo 
got rid of. For a time, if the valve closes, the water thus exhausted 
of air may lio still, but the moment the pressure is removed, how- 
ever little, it boils again, and an explosion may follow. If when 
the engine is at work the pump is kept going so as to pump the air 
in, there will be no chance of the water getting bej'ond boiling point. 
Professor W'lLSOX expressed his concurrence with the remarks 
of Mr. Appold, and said the cause of explosions may probably 
be traced to the fact that, after water has boiled for a time all the 
contained air is expelled. AVhen a jiortion of pure water has 
passed off in the form of vapour, the remaining volume of water 
is denser than before, because charged with more than its own 
original share of impurity. "When the fire is stopped for dinner 
or other purposes, not only is the pressure of steam lowered, but 
the external pressure is increased relatively to the pressure on 
the valve. Assuming the valve to have been weighted — say to 
50 lbs., and the engine to have been working at that amount of 
pressure ; during the cessation of work the pressure on the boiler 
may have gone down to 30 lbs. or 35 lbs. : relativelj', therefore, 
there will be a greater degree of external j'lressure than before. 
If, under siich circumstances, the fire be increased suddenly, in the 
anxiety of the engineman to get the steam up as quickly as possible, 
the water, being free from air and charged with impurities, and 
thereby rendered dense, will require to be acted on by a higher 
temperature before it will generate steam. These circumstances 
would tend to induce a sudden and tumultuous discharge or burst 
of steam (as was shown by Mr. Holland's experiments) which, if 
greater than the valve, probably now not working so freely as at 
starting, could instantly relieve, would show its effect by bursting 
through any portion of the boiler not cqiial to the suddenly increased 
pressure on its surface. This appeared to him to be the more common 
cause of the bursting of boilers, which generally may be noticed as 
