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when he has affirmed, that the vague and unsatisfactory speculations of Hartley have thrown as much light on the nature of man, as the reasonings of Sir Isaac Newton did on the nature of body, he can scarcely be allowed to understand in what true philosophy consists. *As to his theology, it is enough to say that he denies the immateriality of the soul, though he contends for its immortality, and arranges himself on the side of Christianity. These inconsistencies and absurdities will perhaps deprive him of the name of a philosopher, but he will still merit the name of a useful and diligent experimenter.”
When the peace of Europe had been completely re-established by the victory of Waterloo, Mr. Playfair undertook a journey into France, Switzerland, and Italy, with the view of collecting materials for a new edition of his “ Illustrations of the Huttonian Theory," a work which he unfortunately did not live to accomplish. Of the facts which he noted and determined, in the course of his travels, we may perhaps give a short account hereafter; meantime we proceed to lay before our readers an abridged description of the Slide of Alpnach, one of the most surprising mechanical contrivances that reward the ingenuity of the present age.
From the reports of the Chamois hunters employed in the Swiss canton of Unterwalden, it was ascertained that there were immense forests of the finest timber spread out on the mountains, particularly on the south side of Pilatus; but in a situation which the height, the steepness, and the ruggedness of the ground, seemed to render quite inaccessible. Mr. Rupp, a native of Wirtemberg, and a very skilful engineer, then resident in the canton of Schwytz, was induced to visit the locality in question; and he was so much struck with the appearance of the forest, that he conceived the bold project of bringing down the trees by no other force than their own weight into the lake of Lucerne, from which the conveyance to the German ocean would be easy and expeditious. The medium height of the forest is about 2500 feet; and the horizontal distance which the trees had to be conveyed, from the spot where they grew to the lake into which they were to be launched, was eight miles and about three furlonys. The declivity is therefore one foot in 17.68: the medium angle of elevation 3° 14' 20''.
This declivity, though so moderate on the whole, is, we are told, in many places very rapid: at the beginning the inclination is about one-fourth of a right angle, or 22° 30'; in many places it is 20°; but no where greater than the angle first mentioned, 22° 30'. The inclination continues of this quantity for 500 feet, after which the way is less steep, and often considerably circuitous, according to the direction which the ruggedness of the ground forces it to take.
The Slide in question consists of a sort of trough, built after the form of a cradle, and extending from the forest to the edge of the lake. Three trees squared, and laid side by side, form the bottom of the trough; the tree in the middle having its surface hollowed, so that a rill of water received from distance to distance may be conveyed along the bottom and preserve it moist. Adjoining to the central part of the trough, other trees also squared are laid parallel to the former, in such a manner as to form a trough rounded in the interior, and of such dimensions as to allow the largest trees to lie or to move along quite readily. In general it is from five to six feet wide at top, and from three to four in depth, varying, however, in different places according to circumstances. In all it contains about thirty
thirty thousand trees; crosses in its way three great ravines, one at the height of 64 feet, another at the height of 103, and the third, where it goes along the face of a rock, at the height of 157; and in two places it is conveyed under ground.
The trees which descend by this conveyance are spruce firs, very straight, and of great size. All the branches are lopped off; the bark is stripped away, and the surface of course made tolerably smooth. The logs, too, of which the trough is composed, are dressed with the axe, so as to remove all considerable inequalities, and to facilitate the passage of the trees; which, being placed in it with the root end foremost, are launched off in the direction of the lake. As the declivity of the trough at the upper part is very great, the tree in a few seconds acquires such a velocity as enables it to reach the water in the short space of six minutes ; a result, as Mr. Playfair observes, altogether astonishing, when it is considered that the distance is more than eight miles, that the average declivity is but one foot in seventeen, and that the route which the trees have to follow is often circuitous, and in some places almost horizontal.
“We saw five trees come down; the place where we stood was near the lower end, and the declivity was inconsiderable, (the bottom of the Slide nearly resting on the surface of the ground,) yet the trees passed with astonishing rapidity. The greatest of them was a spruce fir, a hundred feet, four feet in diameter at the lower end, and one foot at the upper. The greatest trees are those which descend with the greatest rapidity; and the velocity as well as the roaring of this one was evidently greater than of the rest., A tree must be very large to descend at all in this manner: a tree, Mr. Rupp informed us, that was only half the dimensions of the preceding, and therefore only an eighth part of its weight, would not be able to make its way from the top to the bottom. One of the trees that we saw, broke by some accident into two; the lighter part stopped almost immediately, and the remaining part came to rest soon after. This is a valuable fact: it appears from it that the friction is not in proportion to the weight, but becomes relatively less as the weight increases, contrary to the opinion that is generally received.
“In viewing the descent of the trees, my nephew and I stood quite close to the edge of the trough, not being more interested about any thing than to experience the impression which the near view of so singular an object must make on a spectator. The noise, the rapidity of the motion, the magnitude of the moving body, and the force with which it seemed to shake the trough as it passed, were alto. gether very formidable, and conveyed an idea of danger much greater than the reality. Our guide refused to partake of our amusement; he retreated behind a tree at some distance, where he had the consolation to be assured by Mr. Rupp, that he was no safer than we were, as a tree when it happened to bolt from the trough, would often cut the standing trees clear over. During the whole time the Slide has existed, there have been three or four fatal accidents, and one instance was the consequence of excessive temerity.”
The trees thus brought down into the lake of Lucerne, are formed into rafts, and floated down the very rapid stream of the Reuss, by which the lake discharges its waters first into the Aar, and afterwards into the Rhine. By this conveyance, which is all of it in streams of great rapidity, the trees sometimes reach Basle in a few days after they have left Lucerne; and there the immediate concern of the Alpnach company terminates. They are afterwards navigated down the Rhine in rafts to Holland, and are afloat in the German ocean in less. than a month from the time they descended from the side of Pilatus, a very inland mountain, not less than a thousand miles distant. We know not the amount of success which, in a pecuniary point of view,
has attended the speculation of Mr. Rupp; but, at one time, Bonaparte contracted for all the timber which he could send to the Rhine, and thereby prevented at least a stagnation of the commodity.
There are seyeral scientific considerations connected with the facts now detailed, which seem to have puzzled Mr. Playfair not a little, and which, indeed, weighed with him so far as to make him refuse his consent to have his paper inserted in the Transactions of the Society, before whom it was read. The rapidity of the descent, so much greater than could possibly have been anticipated, is not he thought easily to be reconciled with the notions concerning friction, that are usually received even in the scientific world. It appears, however, that Professor Playfair was not familiarly acquainted with the most recent notions concerning friction entertained among practical engineers, and particularly with the fine experiments of the French writer, Coulomb: and consequently, whilst reasoning on the subject, hazarding his conjectures, and proposing his solutions, he was not aware that what appeared to him perfectly new, had been long received as established principles among men engaged in practical mechanics. It is pleasing, at the same time, to observe, that the inferences which the Professor draws from the facts before him, by means of mathematical reasoning, are substantially the same with those which experience has pointed out to less scientific persons; for the conclusions at which he has arrived, through a process of deep calculation, are found to coincide astonishingly well with the practical maxims of the ship-builder, when he launches a vessel from the slips, and with the operations of the engineer, in the movement of very heavy bodies on an inclined plane. In short, it seems to be now perfectly established, that heavy bodies when put in motion on an inclined plane, are relatively less retarded by friction than lighter ones are: and secondly, that friction, in all cases, is diminished in proportion as the velocity of the sliding is increased. What the precise ratio is at which the friction is lessened relatively to the augmented velocity is, we believe, a point not yet clearly determined; but in regard to the fact itself, so little doubt is now entertained, that we are only astonished Mr. Playfair should have esteemed the announcement of it as a novelty in mechanics.
It is very true, however, as the Professor remarks, that we have here a strong instance of the danger of concluding in the researches of mechanics, from experiments made on a small scale, in regard to what should be the practice when applying the result to a large scale. When our experiments lead to the knowledge of a fact and not of a principle, there is the utmost caution requisite in extending the conclusions beyond the limits by which the experiments have been confined. And this is particularly the case with the experiments on friction, where we know only facts and have no principle to guide us; that is, we have not been able to connect the facts with of the known and measurable properties of bodies.
“ That friction belongs to the cases in which great caution is necessary in extending the conclusions of experiments, is indeed most strongly evinced by the operations that have now been described; the result of which is such as could not have been anticipated from these experiments. The danger here, however, is quite of an opposite kind from that which commonly takes place in such instances. The experiments on the small scale, usually represent the thing as more easy than it is upon the great, and engage us in attempts that prove abortive, and
are followed by disappointment and even ruin. In the present case, the experiments on the small scale represent the thing as more difficult than when tried on the great one it is found to be; and would lead us by an error the direct opposite of the last, to conclude things to be impracticable that may be carried into effect with ease. Had the ingenious inventor of the slide at Alpnach, been bet. ter acquainted with the received theories of friction, or the experiments on which they are founded, even those that are the best and on the greatest scale, such as those of another most skilful engineer, M. Coulomb, or had he placed more faith in them, he never would have attempted the great work, in which he has so eminently succeeded.”
It appears, however, in fact, that Mr. Rupp, the inventor of the slide at Alpnach, was much better acquainted than Professor Playfair with the received theories of friction, and in particular, we may be allowed to suppose, with the improved views derived from the ingenious experiments of Coulomb; and, moreover, that it was because he had faith in the received theories, so modified and confirmed, that he engaged in the immense enterprise which is likely to hand down his name to posterity, as one of the most enlightened engineers of the nineteenth century.
The volumes now given to the world as the works of Mr. Playfair, contain his “ Illustrations of the Huttonian Theory;" the “ Dissertation on Physical Science,” published in the Supplement to the Enclyclopædia Britannica; a variety of papers originally printed in the Transactions of the Royal Societies of London and Edinburgh; and, lastly, a selection from the articles which he contributed to the Edinburgh Review. The “Dissertation exhibiting a general View of Mathematical and Physical Science since the Revival of Letters in Europe,” is worthy of the author's name; but unfortunately, owing to his death before the materials could be finally prepared for the press, it remains in an unfinished state, and must for ever continue in the shape of a mere fragment.
One of the last things Mr. Playfair wrote, and with which he seems to have amused himself during part of the time he was confined with illness, is a Memoir relating to Naval Tactics, as improved by the late John Clerk of Eldin.
It is a singular incident in the history of human affairs, that a person who had never been at sea in his life, should have introduced into military seamanship the most important improvement which that difficult art has received in modern times. From his early youth, a fortunate instinct seems to have directed his mind to this line of study.
“I had,” says he, in a document referred to by Mr. Playfair, “ acquired a strong passion for nautical affairs when a mere child. At ten years old, before I had seen a ship, or even the sea at a less distance than four or five miles, I formed an acquaintance at school with some boys who had come from a distant sea-port, who instructed me in the different parts of a ship, from a model which they had procured. I had afterwards frequent opportunities of seeing and examining ships at the neighbouring port of Leith, which increased my passion for the subject; and I was soon in possession of a number of models, many of them of my own construction, which I used to sail on a piece of water in my father's pleasure grounds, where there was also a boat with sails, which furnished me with much employment. I had studied Robinson Crusoe, and I read all the sea voyages I could procure."
Upon the commencement of the American war, Mr. Clerk, who continued to pay the utmost attention to the subject of naval tactics, and derived all the knowledge he could possibly acquire from read
ing and conversation, and particularly from studying the details of the several actions which took place between the belligerents, saw more and more reason to suspect that there was something very erroneous in the method heretofore pursued by the British admirals, for bringing their fleets into battle. He perceived, that while nothing could exceed the skill with which the ships individually were worked and maneuvred, the plan followed in bringing a whole fleet to meet the enemy was extremely uncertain and precarious: and, in a word, he was convinced from the conduct of our bravest and most skilful admirals, that an expedient for forcing their antagonists to fight, on equal terms, was an addition to the art of naval warfare that remained still to be discovered.
It had usually happened, that the British fleet was eager to engage, and that the enemy was unwilling to risk a general action; the object of our commanders, therefore, had almost always been to gain the weather gage, as it is called, of the enemy, or to place themselves to the windward of his fleet. When that fleet was drawn out in line, in the manner necessary for allowing every ship its share in the action, the British fleet bore down from the windward upon the enemy; who was so placed as to have his whole line, and also the broadside of each individual ship, nearly at right angles to the direction of the wind. In such circumstances, the British had usually adopted one of the two following methods, in order to make the attack. They either formed their feet into a line parallel and directly opposite to that of the enemy, whence each ship bore down upon that which was immediately opposed to it; or, sailing on the tack opposite to that on which the enemy stood, ran along parallel to their line, and within fighting distance, till the whole of the one line was abreast of the other, and each ship ready to engage her antagonist.
If the former of these methods was pursued, each ship on coming down had to sustain a destructive fire from the broadside of the one immediately opposed to her in the enemy's line, which she could only return very ineffectually from the few guns mounted in her bows. The rigging, consequently, which presented the best mark, when the ship was moving end on before the wind, was in general so dreadfully cut by the enemy's shot, that the vessel was always much disabled, and sometimes rendered totally unmanageable, before she arrived within fighting distance.
If the second method was pursued, the headmost ship had to endure the fire of the whole line before she arrived in her place; the next, the fire of all but one; the third had to sustain the broadsides of all but two, and so on; so that it was very improbable that any, except the sternmost ships, could reach their station in the line without having received material damage. This mode of fighting, it requires not to be observed, would give to the enemy who remained quietly on the defensive, a great advantage over the attacking squadron, and enabled him almost to a certainty to maim his antagonist's fleet, with very little loss to himself, or even to gain a victory without exposing to any great hazard either his men or his ships.
“ Mr. Clerk had the merit of pointing out the evils now enumerated, in a man. ner most clear and demonstrative, and of describing a method by which the attack might be made, without incurring any of the disadvantages that have been mentioned, and almost with a certainty of success. As the evil arose from an enVol. I. No. 3.-Museum.