As might have been expected, a good deal of discussion is going on, as to the cause of the late unfortunate explosion on board the steamboat Shamrock, without, as yet, much light being thrown on the subject, and the comparative safety of high and low-pressure engines. We have lately looked over different works descriptive of these engines; and at last met with a short and explicit account of their construction in a British journal, which may be of use on the present occasion.
There are two distinct kinds of steam-engines, namely, high-pressure or non-condensing, and low-pressure or condensing. The high-pressure engine is of very simple construction. The steam, generated in a boiler, rushes through a tube to the cylinder of the engine. It enters the cylinder at two openings, one at the bottom and the other at the top. The steam which enters at the lower opening drives up the piston to the top, whence it is driven down again to the bottom by the steam which enters at the upper opening. An external rod, from the piston, thus drawn up and down, moves the machinery. The force exerted in the cylinder of this kind of engine, it will be observed, is steam working against steam. The steam in either end, on being driven out, escapes into a waste pipe, which conveys it into the atmosphere, where it appears in puffs of vapour. The degree of force required to be exerted to drive the steam out, is considerable. The weight of the atmosphere upon every square-inch of surface of our earth is fourteen pounds; this weight, therefore, which presses upon the spare steam in its progress outwards, must be overcome. This is accomplished by raising the pressure of the steam in the boiler to about twenty-eight pounds on the square inch, one half of which, or fourteen pounds, is thrown away in overcoming the pressure of the air. In the low-pressure-engine, the steam is supplied to them in the same manner as to those of the high-pressure description, but the motion is of a much more complicated nature. The force is not steam against steam, but steam against a vacuum. As soon as the steam, which rushes in at the lower opening of the cylinder, has driven the piston upwards, it is instantaneously abstracted or withdrawn into a separate vessel called the condensor, where it is condensed by a jet of cold water, and runs off into a cistern; from which cistern, the water, in a warm state, is pumped into the boiler to make new steam. The same process takes place with the steam which drives the piston downwards. The abstraction of the steam into the condenser is affected by an air-pump (wrought by the engine), which, at the proper moment, sucks out the steam from the cylinder, and leaves a vacuum. As this vacuum presents no obstacle to the action of the piston, the steam which rushes in requires to be of comparatively small force. Not having the weight of the atmosphere to overcome, the pressure of the steam in the boiler may be kept at fourteen pounds, instead of twenty-eight pounds, on the square inch. Such, then, is the low-pressure steam-engine, which, from the complexity of its parts, and its tendency to go out of order, as well as the large supply of cold water constantly required to effect the condensation, costs a good deal more money, and is more expensive in working, even though requiring less fuel, than the high-pressure engine. In Great Britain, the small steam-engines are made chiefly on the high-pressure principle; but all large engines, including those of sea-going vessels, are of the low-pressure kind; these being not only more steady and powerful in their action, but infinitely more safe.
On a consideration of the above peculiarities in the construction of steam-engines, it will readily be perceived, (says the journal from which we extract,) that a high pressure of steam on a large scale must be apt to produce extremely dangerous consequences. In order to avert as much as possible the chance of explosion, care is generally taken to lay such a weight on the safety-valves of the boiler, that they will rise and permit the escape of steam before the pressure becomes so severe as to burst the boiler. The plates of the boiler are made so strong that they will endure a pressure of 50, or even 100 lbs. per square inch, and this is a height to which the pressure can never come under ordinary circumstances. Unless, however, the quantity of water in the boiler is regulated by the intensity of the fire, danger will ensue, even although the valves be open, and whatever be the strength of the boiler; in the same manner that a tea-kettle, on getting red-hot from want of water, will burst if water be suddenly put into it, - the steam being generated quicker than it can possibly escape.
Upon the whole, it will readily be seen from this comparative view of the construction of the high-pressure and low-pressure steam-engines, that the latter is by far the safest. Every exertion ought therefore to be made to adopt it wherever possible, especially on the waters of the St. Lawrence, on which so many hundred emigrants are daily conveyed by steamboats at this season of the year. We trust, that during the approaching session of the Legislature, some measure will be adopted on so important a subject, in order that full confidence may be placed in the means of conveyance, for emigrants especially, to every part of a country where their labour and capital are so much in demand, and where they can obtain that competency which is ever the reward of peaceful and industrious habits.