" The effects of the atmosphere in the two cases are not so different as to be of much moment. On no malleable iron railway has oxydization or rusting, taken place to any important extent.
" I am inclined to think that this effect is prevented, on the bearing surfaces of much used railways, by the pressure upon them. To account for their extraordi nary freedom from rust, it is almost necessary to suppose that some diminution takes place in the chemical affinity of the iron for the oxygen or carbonic acid. The continual smoothness in which they are kept by the contact of the wheels, has the usual effect of polish, in presenting to the destroying influence a smaller surface to act upon. The black oxyde or cruet, which always remains upon rolled iron, appears to act as a defence against the oxydizing power of the atmosphere, or water. This is the reason why the rail does not rust on its sides." According to Mr. Wood, practice seems to have established the fact since the above was written, that there is no waste or destruction from oxydation or exfo liation, and that the wear is less than in cast-iron, subjected to the same action.
A more severe test of comparison in the wear of wrought and cat-iron, exists in wheels made of the two materials ; locomotive engine-wheels of the latter material generally become, by wear, unfit for use in nine months, while the wrought-iron tires have worn in some cases three years, and are not yet unfit for use.
One phenomenon in the in the tendency to rust, between wrought iron laid down as rails, and subjected to continual motion by the passage of the carriage over them ; and bars of the same material, either standing upright or laid down, without being used at all, is very extraordinary.
A railway bar of wrought-iron, laid carelessly upon the ground alongside of one in the railway in use, shows the effects of rusting in a very distinct manner. The former will be continually throwing off scales of oxidated iron ; while the latter is scarcely at ell affected.
The first cast-iron rails were by far too weak. Scarcely any of the rails laid down twenty years ago are in existence ; this is partly owing to the increased weight now carried upon the rails, and partly to the mistaken policy in the saving by the lightness of rails, to keep the cost below that of the wooden way.
It seems necessary that the rails should be made considerably stronger than merely to support the weight they have to carry. The blows they are subjected to, from the unevenness of the road, transferring the weight alternately from one side of the carriage to the other, and the side shocks from projections upon the sides of the rails, all have a tendency to snap in two the cast-non, or bend the malleable iron rails. We shall have occasion to introduce some more remarks on this part of our subject, but as they have relation to a more advanced stage of improvement than had been attained at this period of time, we shall here resume our chronological narration of the progress of invention.
Many ingenious contrivances have been devised to enable (what has been termed) a carriage to carry its own railway. The generality of these inventions have been turned to very little account ; partly, in some cases, from their inhe rent defects ; and partly, in others, from their being only useful under circum stances which rarely occur, in countries like our„ own, wherein mechanical skill and industry have done so much to mend our ways. Nevertheless some of these contrivances exhibit such admirable combinations of parte, that they are ultimately rendered subservient to other uses than those which their inven tors designed them for. It not unfrequently happens that the general benefit is more advanced by an original clever invention, that has failed in accomplishing the object intended, than in one of the little every-day ameliorations which per fectly succeeds. Original combinations of genius, founded upon correct scientific knowledge, we are disposed to venerate as the result of a power that has been bestowed upon us by the beneficent Author of nature, to imitate, for our parti cular uses, his glorious works. We are therefore indisposed to pass by unnoticed such inventions, because they might have failed in their object on first application ; and believing, with the late Sir Humphry Davy, that " a history of failures invariably shortens the road to success, in mechanics as well as' chemistry, we hesitate not to give the matter insertion without more apology.