Twinele.—The adoption of tunnels in lines of railway has been the subject of much discussion; for the most part apparently arising from individuah who are interested in the execution of certain lines of railway, in which tun nels are excluded. Many timid and ignorant persons have thus been frightened into the apprehension of suffocation from the noxious state of the air, caused by the decomposition of the fuel in the locomotive engines. In order to show to what extent the air in a tunnel is thus contaminated, Mr. Gibbs, in his report already alluded to, observes : " Let us suppose a tunnel one mile in length, to be traversed by a locomotive engine and its train, of a gross weight of 100 tons. The experience of the Liverpool .and Manchester railway has shown that the average consumption of coke is considerably less than half a pound per on for each mile it is carried on a railway; but taking the consumption at half • pound, the whole weight of 100 tons will require the consumption of 50 lbs. of coke. It may be calculated that every ten pounds of coke will evaporate a cubic foot of water; so that the whole 50 ths. will convert into steam five cubic feet of water in the distance of one mile. Now, to convert into steam one cubic foot of water, requires 1950, or say 2000 cubic feet of air; then five feet of water will of course require 10,000 feet, and this will be the whole amount of contaminated air in one mile of tunnel. To determine the proportion of such an amount of foul air, and the whole of the air contained in the tunnel, we may take, for example, a moderate sized tunnel, 30 feet high, having an area of 800 square feet. One mile in length of such a tunnel, will contain 4,224,000 cubic feet; hence the contaminated air will bear to the whole quantity in the tunnel, the ratio of 10,000 to 4,224,000, orit will be as 1 to 422. It will scarcely after this, appear any valid objection to tunnels, to assert that an injurious effect must result from the contaminated air, when we find that the quantity of this description of air produced by the passing of the whole train will be no more than part of the whole quantity in the tunnel." On the preceding page is represented a longi tudinal section of a tunnel, (supposed to be cut through marl, shale, or clay strata,) showing the proportion which an engine with its tender and train of carriages bears to the size of a tunnel. A transverse section of the same tunnel is given in the subjoined figure ; in which the irregular diagonal lines are intended to represent the form of the strata in marl, shale, or plastic clay. In making tunnels through strata of this nature, it is an important consideration that no sprin will en croach ; on the other inverted arches become necessary, as shown in the figure.
In the formation of tunnels through chalk rocks, the expense is less than through any other material ; the excellent stone of which it is usually composed renders artificial side walls unnecessary, while the material will, in some cases, exceed the cost of excavation. " As to the expediency of adopting tunnels at all," Mr. Gibbs observes, I" it is certain that this ought not to be admitted until after much consideration and investiga tion, with a view if possible to avoid them ; yet when, by the introduction of a tunnel a positive good might be effected, such as a shortening of the line, the means of penetrating a difficult summit, or of reaching a Country which might otherwise be shut out from the advantages offered by the railway, it might pog sibly be great injustice, alike to the shareholders in the undertaking, and the surrounding district, to adhere too rigidly to the determination of excluding tunnels."
The engraving on the preceding page represents a view of the Dublin and Kingstown Railway, as seen from. the Black Rock The subjoined engraving is another view of the same railway, passing through Lord Clonclarry's estate, and looking towards Kingstown.
Gradients.—The most important consideration in the construction of railways. is the arrangement of the gradients. To effect this arrangement, two distinct and opposite systems present themselves, each having its advocates ready with arguments in support of their particular theory. In the one system, the rises and falls are distributed over the whole length of the line, in planes of gradual inclination ; while the other proceeds on the principle of concentrating the acclivities in a few points, and thus gaining the summits at once, by short and steep inclined planes, at the same time obtaining levels throughout the rest of the line. To decide which of these systems is the most judicious, an inves tigation of the principles connected with the laws of retardation becomes Gibbs's Report upon the several proposed Lines for the Brighton Railway, this subject has been examined and illustrated with great simplicity and ability : to this Report we therefore with pleasure refer our readers, contenting ourselves with giving the results of his investigations ; which are these : First, That on a series of railway inclinations, thepower required to transport a weight from one given point to another, is precisely the same whatever incli nations are adopted, provided none of these inclinations exceed 21 feet in a mile, which is the limiting slope of a plane, on which the force of gravity becomes equal to, and consequently capable of balancing ; or by any increase, of overcoming the retarding force of friction.
Second, On any series of inclinations, the power required to transport a weight both ways, is exactly equal to the power required to convey the goods on a level plane. This must be clear if we consider that a certain amount of power must be expended in order to overcome the gravity in ascending, in addition to the power employed to overcome the resistance of friction ; and also in descending it is evident that a less quantity of power is necessary than that required to overcome the friction. Now the increased power in the one case is exactly equal to the decreased power in the other case; the gross amount, taking the two together, being equal to that required to overcome fric tion; this, of course, is equal to the power required on a level ; and hence the conclusion, that whatever be the inclinations of a railway, provided none of these inclinations exceed 21 feet in a mile, the same power will work the railway both ways, as would be required to work the same distance on a level railway.
Statistics of Itailways.—The Liverpool and Manchester railway was mainly designed for the transport of goods at the rate of about ten miles per hour, but it was found that treble the required speed was easily attainable by the improvements that were made in the engines; it became, in consequence, a more important object to carry passengers, and the result has been a con tinually increasing amount in their number. The ingenious Dr. Lardner baa been at some pains to discover a statistical law, by which the increase of inter communication is governed. The Doctor has made some very interesting statements on this subject, which we shall endeavour to compress into a small compass.