known in England before the reign of Henry VIII. But most of those wonderful pieces of mechanism, which surpass even the experienced hand, in the delicacy of their operations and excellence of their products, are of very recent invention.

No longer ago than the middle of the eighteenth century, there was hardly a species of manufacture which did not depend principally on manual labor for the performance of most of its operations. Cloths of every description were made almost wholly by human force. The fibre was taken from the animal or plant by hand, and the tedious operations of carding, spinning, weaving, and shearing were all performed by hand. The finer kinds of chain or mesh-work were made without any machinery. Coining was divided into many laborious operations. Rolling and slitting mills, although known, were hardly used, from the imperfection of the machinery by which they were driven. Nails were drawn out under the handhammer. The art of pottery, in Europe at least, furnished no wares, which, in their texture or ornaments, exhibited either strength or beauty, but at prices which would now be considered enormous. The quadrant, it is true, was invented before the period above alluded to, but practical mechanics had furnished no machine for dividing its arc, so that great skill and much time were required to arrive at any tolerable approximation to accuracy in this part of the instrument. The necessary consequence was, that the cost of a good instrument placed it beyond the reach of the common mariner, whose life was constantly depending on observations made with it.

It has been the fate of practical mechanics, until within a few years, to be considered beneath the attention of those men who were best qualified to improve them. It is true that. mathematicians have now and then demonstrated some truth, or given a useful formula to guide the engineer, yet the advantages, which the arts have received from this source, have not been so considerable as is generally supposed. Rather than abstract mathematical learning, the useful arts have required actual experiments directed by acute and well ordered minds. They have wanted the presence of the philosopher, in the manufactory itself, to supply them with machinery, and direct the labors of the workmen.

It is even not unlikely that general knowledge may have deterred its possessors from attempting mechanical inventions, inasmuch as it made them not only acquainted with the diffi

culties and perplexities to be encountered, but with the small chance of reward awaiting such pursuits. To undertake the production of a considerable machine requires a degree of ardor and enthusiasm rarely found united with much science, or indeed experience; for the history of invention presents us at every page with instances of years of drudgery, ending in the ruin of the adventurer. Unacquainted with what has been done by those who have gone before him, he often goes on making experiments, which have before been found to give no advantageous results; or new methods are tried, and new devices wrought upon, until, with his patience exhausted and his substance consumed, he abandons his designs forever. In those instances, which have been attended with success, it has not been until after years have been passed in anxious labor, and great sums of money been expended. So true is it, that a machine is never the result of a chance hit or a lucky thought. There were expended on Arkwright's cotton machinery twenty thousand pounds, before any return was made by it. The nail machinery in the United States was not perfected under ten years, nor at a less expense than one hundred thousand dollars. One individual in England has expended three thousand pounds merely for letters patent to secure the property of his inventions in the manufacture of lace. But this is not all; the completion of a machine is often far from terminating the labors and difficulties of its inventor. On the contrary, it seldom happens that his invention is not pirated. This opens to him fourteen years of disaster in the law, when his monopoly expires, and he is left with his reputation as the only reward of his labors. Yet here and there a great fortune has been acquired, which has drawn on crowds to these pursuits, who have possessed none of the gifts necessary to command success, except enthusiasm and confidence. But discouraging as the circumstances may seem; if the instances have been rare of men of extensive learning laboring in practical mechanics, yet of late years such numbers of judicious as well as ingenious men have been allured, either by a true admiration of the subject, or, as has often no doubt been the case, the hope of reward, that the lights of improvement have been carried into the darkest recesses of the arts, and machines have been contrived no less useful in their products than astonishing in their operations.

The last half of the eighteenth century has been marked by

the great improvements of Mr Watt in the steam-engine; an invention which, if it had only enabled the English to keep possession of their mines, would have been of incalculable ad vantage. But when we see it driving every species of machinery in the vast establishments of England, and its power propelling vessels through the course of every river and bay in our own country, we cannot but exult at the superiority of this age over those which have preceded it.

Mr Watt's improvements are so generally known, as hardly to need a recapitulation. They consist in condensing the steam in a vessel apart from the cylinder, or steam-vessel, as he calls it, the temperature of which could therefore always be kept above the boiling point; in driving the piston both ways through the cylinder with steam, by which the engine could be carried to a force beyond the weight of the atmosphere, to which it was before limited; in surrounding the cylinder with wood or some slow conductor of heat; and in the invention of the sun and planet wheels, by which the reciprocating motion of the engine was converted into a rotatory one, for the more convenient application of it to various machinery.

Since these early inventions of Mr Watt, several curious improvements have been made, as the laws of the expansive force of steam have become better understood. As these may not be generally known to our readers, the high pressure engine being very much used in this country, we shall be excused for giving an account of one or two of them. The first began with Mr Watt himself. This gentleman had discovered, in the course of his experiments, that steam, which possesses an expansive force but very little superior to the pressure of the atmosphere, when suffered to enlarge itself to several times its former volume still possesses an expansive force equal to that pressure; or, in more general terms, that the force of steam does not diminish in the same ratio that its volume increases. From this fact, Mr Watt drew the conclusion, that there will be a great economy in using steam of a high temperature at a low pressure; and to apply this principle, he constructed his engine, so that the communication between the boiler and steam-vessel or cylinder should be cut off, after the piston had passed but part way through the cylinder, and the stroke of the piston be completed simply by the expansion of the steam. Mr Hornblower soon after constructed an engine with two cylinders of different sizes, which he suppos

ed better adapted to take advantage of the above law. But this being at the time that Mr Watt possessed the monopoly of the condenser and some other parts of the steam-engine, which were indispensable to carry Mr Hornblower's invention into effect, he was never able to put it to the test of fair experiment. Subsequently Mr Arthur Woolf, who has pursued his inquiries into this subject with considerable success, has constructed an engine with two cylinders, which seems to embrace all the laws of the expansive force of steam, in the most complete manner. The large cylinder has no direct communication with the boiler, while the small cylinder communicates both with the large cylinder and the boiler. The steam of an high temperature is as it were measured off by the small cylinder into the large. The piston of the small cylinder is driven with a force which effectively is constantly increasing. While the large piston being driven by a quantity of steam, which enlarges itself during the stroke from the volume of the small to equal the volume of the large cylinder, is constantly diminishing in force. Thus the two piston-rods, being fixed to the same end of the lever-beam, regulate each other, and drivethe beam with a force nearly constant through the whole stroke. Hornblower's engine effected this equalization of the force very well, while in Mr Watt's no provision is made for it. His engine having but one steam cylinder, the stroke is given with a constantly diminishing force.

The high pressure engine, as it is called, is a great favorite with our adventurous countrymen. Mr Oliver Evans, well known for his numerous improvements in flour-mills, constructed a great many engines of this kind, and it was principally through his influence that they became so generally used. Indeed Mr Evans says, that the discovery of a curious fact in regard to steam, of which we have not yet spoken, and from which these engines derive their great power, was made by himself more than forty years ago. This fact, as he states it, is, that an increase in the heat of the water, by arithmetical progression, will increase the elasticity and power of the steam by a geometrical progression. This statement, however, is not exactly true, if we consider the degrees of Fahrenheit's thermometer, as marking equal increments of temperature. For although the addition of equal portions of heat, as measured by this thermometer, augments the expansive force of steam in quantities constantly increasing as the mercury ascends the scale, yet this augmentation

does not hold that exact relation, to the increase of temperature, which the expression of Mr Evans would seem to imply. Thus, the addition of 151° of heat to steam of the temperature of 212°, increases its expansive force five pounds per square inch. But the addition of 151° of heat to steam, already at the temperature of 259°, increases its expansive force upwards of twelve pounds on the same area. Mr Dalton, however, is of opinion, that Fahrenheit's thermometer does not measure equal increments of heat; but, on the contrary, that the expansion of mercury is as the square of the temperature, and that the force of all elastic fluids increases in geometrical progression, if the temperature be increased by equal increments. On the assumption of this fact, he constructed his thermometer. This agrees perfectly with Mr Evans' opinion, and is clearly supported by general analogy, though it does not seem to admit of more absolute proof. The fact, however, that steam of a high temperature increases in force much faster than that of a low temperature, on the addition of an equal quantity of heat, as measured by any thermometer, or even by the consumption of fuel, is sufficient to render the use of high steam much more economical than that of low, could it be equally well condensed. But this is not in practice so easily performed. On account of this, the condensation is wholly omitted in the high pressure engine; which, therefore, is unprovided with the condensing apparatus. This is the only material respect in which it differs from the low pressure engines. The steam, therefore, is freely let off to the atmosphere after the stroke of the piston is completed, and however great the pressure may have been, it will be immediately perceived, that after this communication is open, it cannot resist the return of the piston in a degree above the weight of the atmosphere, and when steam of the force of fifty or sixty pounds per inch is used, more than an equivalent for the amount of loss from the piston acting against a resistance equal to the pressure of the atmosphere, is obtained from the small quantity of heat required to render steam, which already exerts a force of twenty or thirty pounds per inch, capable of exercising a force of the above magnitude. The high pressure engine is cheaper and more compact than any other. This last fact would render it peculiarly advantageous for the purposes of navigation, were it equally safe. But in this important particular it is very questionable, whether it will bear a comparison with other en