was about to be given up, when it occurred to the proprietor to stereotype it at Edinburgh, and cast two copies of the plates.  This is now done about three weeks before the day of publication—­one set of plates being sent up to London by the mail, an impression is printed off by steam:  the London agent has then time to send packages by the cheapest conveyances to several of the large towns, and other copies go through the booksellers’ parcels to all the smaller towns.  Thus a great saving is effected in the outlay of capital, and 20,000 copies are conveyed from London, as a centre, to all parts of England, whilst there is no difficulty in completing imperfect sets, nor any waste from printing more than the public demand.

334.  The conveyance of letters is another case, in which the importance of saving time would allow of great expense in any new machinery for its accomplishment.  There is a natural limit to the speed of horses, which even the greatest improvements in the breed, aided by an increased perfection in our roads, can never surpass; and from which, perhaps, we are at present not very remote.  When we reflect upon the great expense of time and money which the last refinements of a theory or an art usually require, it is not unreasonable to suppose that the period has arrived in which the substitution of machinery for such purposes ought to be tried.

335.  The post bag despatched every evening by the mail to one of our largest cities, Bristol, usually weighs less than a hundred pounds.  Now, the first reflection which naturally presents itself is, that, in order to transport these letters a hundred and twenty miles, a coach and apparatus, weighing above thirty hundredweight, are put in motion, and also conveyed over the same space. (2*)

It is obvious that, amongst the conditions of machinery for accomplishing such an object, it would be desirable to reduce the weight of matter to be conveyed along with the letters:  it would also be desirable to reduce the velocity of the animal power employed; because the faster a horse is driven, the less weight he can draw.  Amongst the variety of contrivances which might be imagined for this purpose, we will mention one, which, although by no means free from objections, fulfils some of the prescribed conditions; and it is not a purely theoretical speculation, since some few experiments have been made upon it, though on an extremely limited scale.

336.  Let us imagine a series of high pillars erected at frequent intervals, perhaps every hundred feet, and as nearly as possible in a straight line between two post towns.  An iron or steel wire must be stretched over proper supports, fixed on each of these pillars, and terminating at the end of every three or five miles, as may be found expedient, in a very strong support, by which it may be stretched.  At each of these latter points a man ought to reside in a small stationhouse.  A narrow cylindrical tin case, to contain the letters, might be suspended