Processes in the Useful Arts. 169 



as the steam presses on the surface of the water in the same ratio as the 

 water increases in temperature, it only boils without rising, as when at 

 atmospheric pressure ; but if the steam should be drawn off faster than it 

 is generated, this artificial pressure would be taken off, and the water 

 would rise with the steam in proportion to the suddenness and rapidity of 

 its escape. The water and steam in this mixed state thus filling every 

 part of the boiler, the excess of caloric in the supercharged steam, as well 

 as the extra heat from the boiler, will be instantly taken up by the water 

 which rises with the steam, by which means the steam becomes sufficient- 

 ly dense (or powerful) to produce the fatal effects too often experienced, 

 not only from high but from low pressure boilers. If, for instance, the 

 water, (as has before been noticed,) which is exposed to the fire, should 

 be suffered to get below any part of the boiler, the steam will soon become 

 supercharged with heat. If a boiler thus circumstanced should have the 

 weight taken from the safety valve, * or a small rent effected in the boiler 

 from its giving way by the pressure of the steam, an explosion will be sure 

 to follow. A remedy for this kind of explosion, which appears to be the 

 only serious one, is that of not allowing the water to subside below any 

 part of the boiler which is exposed to the fire. In case the water should 

 settle, it may be known by having a tube, with its upper end trumpet- 

 mouthed, and its lower end fixed in the boiler, entering a few inches be- 

 low the surface of the water ; then, as soon as it subsides sufficiently to 

 allow the steam to blow off, the blast will give warning that no time should 

 be lost in supplying water or checking the fire, f When highly super- 



P * It was stated in evidence at the coroner's inquest taken at the Humber, in the 

 case of an explosion on board of the Graham steam-boat, that, just before the explo- 

 sion took place, twenty pounds were taken off the safety valve. Now, if the steam 

 iu this boiler had been properly generated, the relief given to the safety valve could 

 not have produced explosion ; but if the w ater had got low in the boiler (as was 

 probably the case,) and the steam supercharged with heat, the ready way to pro- 

 duce explosion was to allow the steam to escape faster than it was generating, when 

 kept in the lower part of the boiler by the pressure of the confined steam. 



Several instances have occurred when there has been sufficient warning, by the 

 rushing of the steam from a rent or fracture, for the bystanders to escape from in- 

 jury before the explosion took place. There has been at least one case where the 

 boiler was raised from its bed into the air by the force of the steam issuing from the 

 rent (upon the principle of the rocket) before the water had sufficiently expanded, 

 by the removal of the steam, caused by the rent or fracture, to take up the heat of 

 the boiler and the supercharged steam, when an explosion took place after the boiler 

 had been raised many feet in the atmosphere, and a separation took place with great 

 report, one part rising still higher, while the other dashed with great force on the 

 ground. It is, I believe, a fact, that more persons have been killed by low than by 

 high pressure boilers. It is but about twelve months since sixteen persons were 

 killed by the bursting of a low pressure boiler in Flintshire. High pressure boilers 

 have since been substituted. Some of the most dreadful accidents from exploaoos 

 which have taken place in America have occurred from low pressure boilers. 



1-|- This will apply only to low pressure boilers, on account of the height of the 

 column which would be required to balance the pressure of the steam. The high 

 prgssurc engines, as used in Cornwall, would require a column varying from 60 



