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so also, it requires a stronger rail to carry it; whilst the rails will wear no longer, in point of traffic, than if the engine were lighter and the rails lighter, as the time they will last is according to the time they will carry the engines, which generally form the greatest load supported upon the same number of wheels in the train, but where powerful locomotive engines are used to assist on inclined planes, the rails require to be made stronger than the other part of the line; that strength is only governed by the weight of the assistant engine, and only requires to be laid on that part where the assistant engine travels.
The next thing to be done, after the system of levels is agreed upon, is to trace a line for the railway, upon a survey of the country or map, in which we proceed in the following manner :
The dry land is that portion of the earth sufficiently elevated to rise above the level of the ocean ; indeed we should naturally suppose that the general face of the country would rise from the sea-shore tothe central parts, by noticing that all rivers flow towards the sea, some descending many feet perpendicularly, as in the case of waterfalls. It also follows that as rivers are nothing more than the drainage of the country through which they flow, they will in all cases occupy the lowest land in the neighbourhood, on account of all the water finding the lowest level. Thus, if we find two rivers whose direction is nearly parallel to each other, we may conclude that we have some high land between them. Also when the course of a river is very circuitous, we may consider that the country through which it flows is hilly. If we find that two rivers commence in the same place but proceed in two opposite directions towards the sea, we shall find rising land from the sea to that part in the course of each river. With the assistance of these principles to guide us, and a good map, some idea may be formed as to the course we should follow to obtain a line of gradients that may suit our road. In the map used, all the rivers, ridges of high hills, brooks, &c., should be accurately marked.
Thus, the situation of the two places having been considered as to their probable difference of level, the country between should next be considered, and, if it is supposed to consist of high land, the lowest level will be indicated by the drainage, and if hilly, a gap in the chain may be found through which we can conduct our line. Should the intervening land be marshy and have high land at the back, the best way would be to cross the marsh with a level embankment, and to ascend to sufficient height to obtain a long level by means of a steep inclined plane, in preference to ascending by a long inclined plane with a small inclination per mile, as I have before described. In this manner we ascertain the general nature of the country through which we have to pass, but as the land is generally undulating, the only way to find the best levels is by means of the levelling instrument in the field, with the assistance of judges and the eye, or where any particular level is required with the least cutting and embanking, the line which that line would follow on a map is found by cross levelling.
Having obtained a line that contains the least difference of level, it must be further reduced by means of embanking and cutting. The base of a railway is the breadth of the top of an embankment, or the bottom of a cutting, exclusive of the drains, and varies from fifteen to thirty feet; embankments and cuttings have their sides sloped, at about the rate of one foot perpendicular to one foot six horizontal. The cuttings have a drain on each side about two feet deep, and the embankments a brick drain at right angles to their length, where occasion may require.
Where a ridge of high land intersects the intended line of railway, that cannot be otherwise avoided, tunnelling is resorted to, and is approached by means of a cutting at each end. Tunnels should always be avoided where it is possible, bo
on account of the expense of constructing them, and the public feeling which is not greatly in favor of them. Where
the railway crosses a brook or stream, a bridge must be constructed for it, for the railway to pass, and where it intersects a turnpike road, an arch is constructed either over or under the road, as it is exceedingly inconvenient that they should cross upon the same level.
Such, my young friends, are a few remarks concerning railways-sufficient, I hope, to make you acquainted with the subject.
The Industrial Arts and Manufactures of
THE ART OF STAINING AND PAINTING GLASS.
HE art of colouring glass is nearly as ancient
as that of its manufacture. Strabo, Seneca, and Pliny, all give evidence of the fact of glass being tinged with various colours in early
times. From the Egyptian mummies, we have various specimens of it, as well as in the composition of mosaic decorations at a much later period; and from the chemical examination of many ancient specimens by Klaproth, it appears that ancient glass was indebted to metallic oxides for its various hues, as it is at the present day. The metallic oxides used for the various colours have been already alluded to.
The ancients employed methods of converting coloured glass into representations of natural objects, which were extremely beautiful; but the manner of producing this is now lost. They had a method of blending together, in one piece, numerous parts of an object, such as of a bird or flower, with such perfect assimilation as to be almost equal in beauty to