Numerous alterations have been successively made upon the Stanhope press by the manufacturers, who magnify them to the public as being vast improvements, as increasing the productive power of the press in a duplicate and even triplicate ratio; but our mechanical readers will at once perceive the impossibility of the correctness of such statements: that if there be a loss often or fifteen per cent. of the power applied to the Stanhope press, arising from friction, etc, no modification whatever of the six mechanical agents can save the whole of such loss. The press may be rendered more convenient and handy, and the minor arrangements and appendages may be also improved; indeed, we doubt not that such ameliorations have been and will continue to be introduced; but they become perfectly insignificant and trifling when compared with the beautiful invention of the patriotic Stanhope. Amongst the ablest manufacturers of the present day of iron presses, we may mention Messrs. Ruthven, Medhurst, Cope, Sherwin, Clymer; there are many others, we doubt not, of equal ability, who have not succeeded in making themselves as well known.

All the presses that we have from time to time seen, and especially those of the manufacturers we have named, possess some peculiar points of excellence as well as defects in their mechanism, to describe and discuss which would take up much time and space. In justice, however, to the two first-named gentlemen, whose inventions possess great originality and simplicity, we must afford room for a compendious notice of the peculiar contrivances which distinguish them from all others.

In 1813 Mr. Ruthven, of Edinburgh, took out his patent, which term having expired, the invention is public property. Instead of placing the types, as was the case in all previous inventions, upon a movable carriage, they are fixed upon a stationary table, and the platten and tympans are drawn over it, and the impression is effected by a system of levers, the action of which the annexed diagram will serve to explain. a c b is an angular lever, whose longer arm a c is in the form of a winch, to which the workman applies his power; while the shorter arm c b acts upon the extremity of the connecting rod d, by which its efficacy is transmitted to the point e of the lever ef, whose fulcrum is at g; this lever is connected by the rod h to the extremities of the levers k l m, whose fulcra are m m. The rods nn are connected with the levers k l m at ll, while their upper ends act upon the support of the platten by means of a species of hooks. Now if the lever or winch a c be turned in the direction of the dotted line a o, the shorter arm c b will push the rod d in the direction be; consequently the point e of the lever efg will move in the direction of the dotted line ep; and as the point/will describe a similar arch, the rod h will depress the ends k of the levers klm; therefore the rods n n will be drawn down, bringing with them the platten.

The same regulation with regard to the angular positions of the levers is observed in this beautiful arrangement as in the Stanhope, so that their greatest efficacy is exerted at the moment of impact.

Mr. Medhurst's press, except in the mechanism by which the power is communicated to the platten, resembles those in general use; but in that respect it forms a very remarkable exception; no screw is used: but the spindle to which the platten is made fast is swelled out at its upper end into a broad stout collar, as shown at a in the following cut, into which the lever or handle b o. the press is inserted. At equal distances apart on the upper side of this circular coflar are turned out of the solid two steps or cups, which receive the ends o. two inclined bolts c c, which bolts are supported at their upper ends by the points of two screw-bolts d d, that pass through the head e, and enter sockets made in the heads of c c. When the platten is up, the rods c e lean in the inclined position, as shown; but when the spindle is turned a quarter of a revolution, the bolts c c take a vertical position, and as the head e is immovable, the collar a on the spindle is forced down, and with it the platten to which it is attached.

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Prior to the introduction of printing machines, the press department was one of great labour, whenever extraordinary expedition was required. It was particularly the case with newspapers, of which, with the utmost exertions, scarcely ever more than 750 copies could be obtained in an hour: the consequence was, that in newspaper offices where the circulation was extensive, it was found necessary, in order to get the paper published in time, to compose two or more copies; so that, by going to press at the same time, the demands of the public might be complied with, thus occasioning an enormous increase of expenditure both in the compositors' and press department. In a newspaper circulating 7 or 8000 copies, this expense amounted annually to at least 2000/., all of which has been saved by the introduction of machines.

In the 3d vol. of the Quarterly Journal of Science (new series) is inserted a communication "on the recent improvements in the art of printing," by Mr. Cowper, a gentleman of extensive information upon every thing relating to the subject, who has invented many important improvements in the mechanism and process of the art, both individually, and in conjunction with his partner, Mr. Applegath, and who is therefore eminently qualified to give a correct statement of the facts, which we shall subjoin, slightly abbreviated from the original. The little diagrams that are inserted in the body of the text serve to explain, in a very clear and concise manner, the leading principle or arrangements of the successive inventions described, respecting which it is also necessary to observe, that -