The above method supposes that it is a matter of importance to the two termini towns to take in the traffic of the two intermediate towns, and of indifference to turn the line of railway to the extent needed, out of its direct course. Such in practice is actually the case, when the termini are far apart, or the centre of commerce of the intermediate towns not far out of the direct line; for to turn a little to the right or left, increases the distance so triflingly as to render it immaterial.

If there is a third town E, whose traffic it is needful to include, a point e should be found distant from E, in proportion to one of the previous distances D b, as the commercial importance of the denominating town D is to that of E; which, on the same principle, will be another point through which the line ought to pass, giving to E its due participation in the railway. In the same way, may a point f be found, for a fourth town F, and so on for others to any number.

Were it to happen, that one or more of the points e, for instance, so computed, would bend the direct line from b to B, out of its course concave to F, it is obvious the method ought to be abandoned. The line under such circumstances should be permitted to follow the straight course, and F left to enjoy the advantages nature has given it of having the railway a little nearer to it than its relative importance has a right to claim.

If instead of single towns E or F, there should happen to be a pair of them F, G, whose line of communication is transverse to the grand line of railway, then it will be advisable to proceed with finding the centre of commerce g of them, independently of the others, in the same manner as we did with b, that of the towns C, D. This new point will be that instead of f, through which the railway should pass.

Presuming the ground to be everywhere equally favourable, this I apprehend, would be the way to lay out a line of railway, so as to give to each town its due and proportional share in the benefits of the railway; and preserving to commerce and the public, the maximum advantage to which they are entitled. If the ground is not equally favourable, then it is manifest that these points should be determined first, and the line kept as near to the direction they would have given, as the ground will permit.

In a similar way should we deal with towns of mere population. They might very fairly be compared with one another, though we would not admit of a comparison between them and the strong holds of trade. In these cases the importance of the towns must be measured by their population.

ON CROSSING ROADS, LONG TRACTS OF LOW COUNTRY, AND NAVIGABLE RIVERS.

6. A turnpike road should on no account whatever, be permitted to be crossed with a rail-road on a level.

The same will hold good, but less forcibly with any public path or highway. To this we may add that it would be preferable for the

railway to be as free as possible from inclines, at about the crossings. If it could, it would likewise be better for the rail-road to pass over rather than under any public road; for it would then be more out of the reach of passengers, and therefore be safer for the trains. But there are places in which this would be unadvisable, or perhaps impracticable.

7. Long tracts of low flat country should be avoided as unfavourable to railways, not only on account of their dampness and interruptions to the good working of the trains, but more particularly if crossed by navigable rivers or canals, as generally more expensive, less convenient, and less safe for a rail-road than a country gently undulating. 8. Whenever a rail-road has to cross a navigable river, it is always advisable to do it at such an elevation as to avoid a swivel bridge.

If it be considered that a train going at a great velocity cannot be pulled up at once like a stage coach, and that it frequently takes near half a mile to arrest it, we shall immediately see the danger and impropriety of admitting these bridges. It would in all cases be much better if the legislature would compel the vessels passing to strike their top masts, and let the company allow some trifle for it, rather than to run the risk, particularly in foggy weather, of having the whole train hurled into the water. Who would think of intersecting any public highway with a dangerous chasm or ditch? And what are rail roads but a new kind of turnpike road, the child indeed of necessity?

But probably there may be no actual need for a swivel bridge, or to make the vessels strike their mast. According to the velocity which the engineer can command on either side of the river, he may elevate his rail road over it, by reasonably abrupt ascents of from 10 to 30 feet above the general level of his line on each side, and ascend by the force of impetus, the engine merely doing the work on a level. This subject will be more largely handled when I come to treat of railway gradients.

TUNNELLING.

9. Tunnels ought to be especially avoided.

It is not merely on account of the great cost of their construction, the expense of gas and other establishments, afterwards entailed on them, but the excessive nuisances which it is morally impossible to prevent, and the danger to delicate constitutions of travelling through such holes of impurity. Let any one of a tender habitand thousands of such 'must and do travel daily-who doubts this, go through the Thames and Medway tunnel, or the Canterbury and Whitstable, and he will at once be convinced not only of the nuisance but of the danger of them. The latter is so proverbially in the neighbourhood the sink of colds, &c., that I have been informed very few will venture into it at all, and the railway consequently carries scarcely any passengers. As to the former, a gentleman and myself went down last August the 3d, a light fine day, on purpose

to see it. We commenced at the Medway end, with a determination to walk through it. The tunnel being large and cut in chalk, we at first found it light and not uncomfortable, but when we had reached about 500 yards, the darkness became so intense, that we could neither see the path we trod, nor distinguish it from the water beside us. At the same time the atmosphere grew so oppressive, that it was with difficulty the gentleman with me could walk from a giddiness in the head, while I felt full, hard pains in my chest. We were therefore both obliged to abandon our intention, and return. A similar account was given to me of another party of several who went on a like excursion. If such then be the effect on persons not naturally very delicate, and when no locomotive engine is travelling, what would be the consequence on tender constitutions, and when the nuisance is accumulated by locomotive, after locomotive?

But it may be asked, "Suppose there are two towns of great traffic which cannot have a railroad communication without a tunnel, what would you do, not make a railroad, while all the rest of the country is enjoying them?" The answer is obvious, "Our rule is general not universal;" try every means first-try an inclined plane with a fixed or assistant engine, and when every thing else has failed, but not one moment before, have recourse to your tunnel, not as desirable, nor a matter of choice, but of stern necessity.

I should indeed prefer an inclined plane, even if the initial and annual expense were much greater than with a tunnel; for I am in great hopes that a few years will shew us locomotives working up those inclinations with their loads, which now they dare not attempt. Besides, the passenger trade may so increase, as to reduce year after year the relative expense of the fixed or assistant engines; while I fear, through a tunnel, this trade, which is the most profitable of all, would rather decrease, the more the nuisance becomes known.

While I am on this subject, I do not know that I can do better than reprint the observations, with some additions, which I published some time ago on tunnels, in a work not likely to come under the notice of the respectable readers of the RAILWAY MAGAZINE.

OF THE LIGHTING OF TUNNELS.

It has been gravely talked of lighting tunnels artificially, so as to supersede the necessity of day-light. How or by what means this is to be done, remains a secret. To philosophers and practical men, the hopelessness of approaching the solar by any artificial light is well known. Sir John Leslie computes, from experiments, that a piece of the sun's matter less than half an inch diameter would give more light than 12,000 wax candles; and every light we know of, even the hydro-oxygen light, I am informed, the most intense by far yet discovered, sinks into a dark spot when held up before the sun's disk. It has been computed by Bouguer, that the light sent us by the sun exceeds that by the full moon 300,000 times. Coarse as our optical nerves are in judging of degrees of light, it would, therefore, be impossible to have a sudden transition from solar to lunar light without producing the sensation of great darkness; and it is only to the slow and insensible gradations, first of the descending sun, and then of twilight, occupying at least an hour, that we are enabled to bear so great a difference.

But what would be the time of passing from broad daylight, with a train in full speed, to the intense darkness of a tunnel-a single second! Not more if the velocity was thirty miles an hour; for in that space the train would go forty-four feet. Any one, therefore, may judge for himself of the effects of so instantaneous a transition from the full light of day to that of the full moon, could such a light, and so diffused, be introduced into a tunnel, which I apprehend to be impossible.

But it is not the transition from light to darkness which is anything so bad as the contrary-from the intense tenebrosity of a tunnel to the full broad glare of day-light. Let any one, with his face towards a strong light, as a window in a room, look at his eyes reflected from a mirror, and he will perceive the pupils of them contract to very small dimensions. Let him now turn to the darker parts, and the pupils will in a little time expand to four, six, or perhaps ten times their previous area. If he once more turn sharply to the stronger light, he will observe his pupils contract to their former dimensions pretty rapidly, but by no means instantaneously. This contraction will be for the purpose of excluding so much of the strong light as is unnecessary for the distinct visibility of objects, and which would distress and injure the optic nerves. In young and healthy persons the contraction and expansion will be much more marked than in old, debilitated. Now let us suppose, from having been shut up within a coach in a tunnel for a few seconds, the pupils of the eyes had attained their utmost distension; and let us for one moment consider, in the state of this unusual distension to nine or ten times their natural size, that a light from 500,000 to probably 1,000,000 times greater than that of a mould candle, was all at once to burst upon so delicate an organ! I appeal with confidence to any medical man to answer the question of the effect-particularly if often repeated-on -on a tender constitution and sight.

It is well known that any object, not illumined directly by the sun, depends for its visibility on the indirect light falling on it from all visible points, especially from that portion of the sun's rays intercepted and diffused in every direction by the particles of the atmosphere; and that the more this indirect light is diminished, the darker the object grows. Hence it is, that in descending an uncovered well or pit, the indirect light is gradually curtailed until it is too weak on the eye to resist the feeble light of the stars, which become visible, though that indirect light comes from a column of atmosphere thirty miles high illumined by the sun. On the same principle, too, it is, that when the eye is protected from the indirect light by a long tube, darkness so increases on it that the stars may be seen as well as in a pit. So also on the Andes, the brighter planets and stars may be detected in the day, which happens on account of almost half the indirect light being there lost by these lofty mountains rising above nearly half the actual substance of the atmosphere, though to scarcely a tenth of its altitude.

If, therefore, the visibility of bodies depending on Nature's most powerful light, diffused by the whole extent of the atmosphere, is so very easily destroyed, how is it possible in the contracted limits of a tunnel, boxed up besides in a coach, any artificial light, far inferior to the sun's, can cause that luminous diffusion needful to produce general visibility of objects? The idea is too absurd to be entertained by any one not interested in propagating it. That a tunnel can be so lighted that a person may walk in it, is obvious; but in carriages it would be light where the lights fell, and intense shade where they did not. I have myself walked up and down the Liverpool short tunnel, about 300 yards, without much inconvenience; but when I entered it in a train, impenetrable darkness ensued, disturbed now and then by the gas, whose sickly and jaundiced hue seemed calculated only to render the horrors of the place more apparent.

It has been hinted to me that Mr. Stephenson, the engineer, intends to light his tunnels so as to do away with the objections to the want of daylight.* I should rejoice to see it; but I cannot believe that Mr. Stephenson, who is often styled "the Pope of Engineers," could give rise to so silly a report. That Mr. S. is an industrious, ingenious man, no one can doubt; but to say that he claims to be able to do that which men much more conversant with the subject judge to be impossible, is to throw an odium on him for arrogance and presumption, which I cannot imagine he merits. I would as soon believe Mr. Stephenson had declared his possession of the Laputan philosopher's desideratum, namely, a method of bottling up the summer sun's rays to grow cucumbers with in winter.

And

What has been said, presupposes the atmosphere of the tunnels perfectly clear and transparent; but the moment an engine enters, the confined air will be intermingled with a vast volume of dense steam and smoke. since each succeeding engine will add to the nuisance of its predecessor, I should be glad to learn how a tunnel of any length is to be cleared, if there be much traffic on the line, and by what legerdemain any one will undertake to send a light through such an atmosphere. Would it not savour more of sober reason, at the end of each tunnel to provide the means of ablution from the filth the poor passengers had contracted, with an antidote of some kind to neutralize the smoke and steam they had swallowed, and a cordial to revive their exhausted spirits?

OF THE PERMANENCE OF TEMPERATURE IN TUNNELS.

About eighty feet under the surface experiments have shewn that the temperature varies seldom more than a degree all the year round. This, therefore, presents another serious objection to tunnels. For as tunnels are seldom made unless the superposed ground exceeds eighty feet, it follows that the temperature of them cannot differ much from the mean temperature of the soil, or 512, at any season of the year. A person, therefore, leaving the external air of a hot summer's day, may change an open dry air for a damp cold one, perhaps 30°, 40°, or 50° lower. Now, if any time was spent in the tunnel, or if accident should detain the train, this would be no trifling change so suddenly made to any constitution. A miniature effect of it may be felt in St. Paul's in a hot day, by a cold current of air about the ankles. But I have heard that in the Thames tunnel, which Mr. Canning very significantly denominated “a great bore," the sensation is very chilly and unpleasant. However, as the air is a weak conductor of heat in respect of quantity, though it transmits it rapidly, that is, at the rate of 1100 feet per second, it is possible in the passage of quick trains the change within the carriages would not be so much felt. But there is another point in which the constancy of temperature in tunnels would have a very powerful, and, I fear a very pernicious effect. I allude to the

VENTILATING OF TUNNELS.

Tunnels for locomotive engines must evidently be horizontal, or so nearly so that they could never ventilate themselves. Omitting the perpetual liberation of vapour from the sides and ground of the tunnel, which, if it was only to render the confined air thoroughly damp, is bad enough; the

One of the methods I have heard, is to make the carriages carry lights. Success would be more certain, to carry off all the earth above, and on the sides, and make a glass tunnel, and do away with the locomotive engine too. If a diffused light can be thrown into a tunnel, Mr. S. may take a hint from this, and have glass panels, &c. to the carriages,