been a reftraint and impediment to the progrefs of every art and fcience from the highest to the loweft; and to this, in a great degree, it is owing that we are so far from having arrived at perfection. An enlarged and liberal mind will rife fuperior to thofe felfish and unreasonable attachments, which have been fo great an obftruction to every real improvement, whether moral or mechanical.

The Author of these Obfervations appears under this refpectable character, and is well known for his skill in that branch of business, which is his peculiar province. He discovers a confiderable acquaintance with the principles of mechanics, and his obfervations are the refult of his own ftudy and experiments: the publication of which (fays he) is rather intended to make them generally known, in order that they may be confirmed or refuted by the experience of others, than to gratify any vanity I may be fuppofed to entertain in appearing an Author.'

This work is divided into four parts: in the first, the Author treats of the nature and economy of draught, which he defines the giving motion to a carriage at reft, and the preferving the velocity of that motion.' The impediment or refiftance to be overcome in the generation of motion is that which is ufually called friction, and depends on the weight of the body to be moved; so that if a flat fledge be made to flide over the surface of planifhed iron, brafs, or hard wood, it will require a force nearly equal to a third part of its own weight to give it motion; and this force is fuppofed to act at right angles to the direction of the compreffure. The reafon of this refiftance is the imperfection of the polish we give to all cohering furfaces. All furfaces are to be confidered as rafps or fingle-cut files, where teeth are lifted over one another, rubbing against each other's inclined furfaces, as the furfaces of the two files appear to flide or rub horizontally one against the other and therefore the moving power must be fufficient to raise the carriage over the inclined planes of fuch indentions, before it can move. The angle of the elevation of thefe planes is found, by experiment, to be somewhat lefs than 20 degrees; for if any plane be raised to fuch an angle with the horizon, a weight or carriage refting upon it will move downwards by its own gravity: and from this principle we may infer, by a well-known theorem in mechanics, that the power which moves the weight must be greater than in the proportion of one to three. This familiar explication of the nature of friction accounts for the advantage which is derived from the obliquity of the line of traction, though fuch a line has inconveniences which overbalance this real advantage. Thefe inconveniences are very properly ftated and urged in the next fection, where it is fhewn, that the oblique

line of traction, though of use to put carriages at reft into mo tion, is very improper for the mere continuation of that motion, when its velocity is once arrived at its required maximum ; in which cafe the animal fhould continue to move with the leaft poffible contraction or compreffion of its muscles.'

In the fecond part the Author proceeds to examine the ftructure and draught of wheel carriages in particular. The use of wheels in the draught of carriages (he obferves) is two-fold; firft, that of diminishing (as it is popularly and improperly called, but, properly fpeaking, the more eafily overcoming) the refiftance, arifing from the friction of the carriage; and, fecondly, the more readily furmounting fuch adventitious obftacles as, being attached to or preffed against the plane over which it is drawn, form very angular prominences, which must be either depreffed by the weight of the carriage, or require that the carriage, with its load, be lifted over them.' The want of difcriminating (fays this Writer) between these two purpofes to be answered by wheels of different fizes has led those who have written on this fubject into mistakes. For, though it be certain that high wheels have greatly the advantage of low ones in overcoming friction, they have not fuch great advantages as they are fuppofed to have, particularly on inclined planes, and in furmounting prominent obftacles, when thofe obftacles are indepreffible, and the carriage is to be lifted over them.' He then proceeds to eftablifh this general principle, and to correct fome mistaken notions on this fubject. He fhews in what way the fpokes of wheels act as levers in overcoming friction between the wheel and plane, and between the nave and axle, in the two cafes of a wheel moved along any furface, and likewife fupporting a loaded carriage. By the use of wheels the friction, or rubbing, of the outer circumference against the fupporting plane, is only transferred from the under to the upper, or rather divided between those furfaces and the furfaces of the axle and nave. Were the nave nearly as big as the outer furface of the wheel, the axis would flide nearly all the way; as, the wheel being but little bigger than the axis, and its radii, or fpokes, of fcarcely any length at all, there would, in fact, be little or no wheel, and confequently no rolling. The cafe therefore appears fimply to be this, that the motion of the carriage being rectilinear, and parallel to the fupporting plane, the wheel muft flide nearly the whole length of that plane, and therefore rub all the time either at the axle or at the circumference. The more it rubs at the one, the lefs it will rub at the other; the rubbing at the axle increafing as its diameter is diminifhed, and the rubbing at the circumference increafing as the diameter of the axle is increased. It will hence follow, that if a method could be difcovered to make the furfaces both of the

nave and circumference roll all the way, the friction which attends the rubbing of parallel moving furfaces, and increases in proportion to the weight of the upper, will be removed, whatever be the proportion which the diameter of the axle may bear to the circumference of the wheel. Thus, if a carriage were drawn over rollers, conftantly finding a new roller to reit upon, as it progreffively paffed off another, the friction arifing from the neceffity of lifting the upper furface over the teeth of the under, would certainly be annihilated; and perhaps fuch a method is practicable. But the cafe is, that if it were practicable, it would be of little ufe, as the diminution of friction beyond a certain degree is of very little confequence in the draught of wheel carriages.'

The next fections contain fome curious remarks on the benefits to be expected from the various fchemes propofed for diminishing, or more eafily overcoming, the refiftance occafioned by friction; and offers fome experiments to prove, that how much foever we diminish, or find means to overcome, the friction of carriages, there ftill requires a power or active body of a momentum proportional to their weight, to draw them forward with a certain degree of velocity. And that this velocity is effential to the confideration of the comparative excellency of wheel carriages needs hardly be remarked, as expedition feems at present to be the principal object confulted in their improve ment. But to attain this end, it is not the diminution of friction, fo much as the diminution of the weight, that is the great object in view. But this in carriages of burthen cannot be difpenfed with; which, for that reafon, are under a mechanical neceffity of being drawn by proportionably ftrong and heavy teams, and of moving proportionably flow: nor will any contrivance to diminish friction do more for them than to fave the wear and tear of the feveral parts of the carriage; which, however, is always the lefs in proportion to the flowness of its

motion.'

Our Author then proceeds to fhew the ufe of wheels in faci litating the progreffive motion of carriages, together with the advantages and difadvantages attending their various fizes. High wheels, provided that they are not made fo large as to become too heavy or too flight, have the advantage of thofe that are fmaller, both with refpect to friction, and alfo to the more readily furmounting obftacles: but along inclined planes they have the difadvantage of thofe that are fmaller, as they increafe the relative gravity of the load, according to their height and the obliquity of the plane.

The third part treats of the conftruction of various carriages for different purposes; in which there are many pertinent and fenfible obfervations. In the fourth part the Author propofes Some improvements in the Aructure of wheel carriages. The

first of these is a new method for the fhort turning of wheel carriages, which anfwers the end, whilft it faves a great part of the expence, and prevents fome other inconveniences attending the crane necked perch. It is well known that, in the common conftruction, the fore-end of the perch refts on the axle of the fore-wheels, which turns on the iron pin paffing through the perch. In turning fuch a carriage, the larger the wheel the fooner it will ftrike againft the perch. To remedy this inconvenience, it is propofed to fix the pin that paffes through the perch backwards from the center of the axle, fo that the axle with the wheels may turn round this pin; in which cafe the angle, which the wheel makes with the perch, becomes confiderably lefs, before the wheel ftrikes against it: So that a larger wheel in a carriage, thus conftructed, will not touch the perch fo foon as a fmaller wheel in a carriage of the ordinary conftruction; and be the wheels of what fize they may, the carriage will turn fhorter than if conftructed according to the former method, with the center-pin in the center of the axle.' The Author adds, that this mode of conftruction is attended with no additional expence; that it may be applied to all kinds of carriages, and is ftronger than the ufual conftruction.

Another improvement is that of conftructing the wheels with a fingle rim, or with two fellows only: for this the Author has a very curious method peculiar to himfelf: and it is certain many inconveniences are hereby avoided. A number of joints weaken the wheel; and therefore, in order to give it ftrength, the wheel-wright leaves thofe parts higher than the other, fo that the rim of the wheel becomes uneven, and its motion of courfe remains no longer uniform: and, as the fellows are fegments of a circle, fawed or hewn out of ftraight wood, they are thence rendered fo brittle, from the crofs direction of the grain near the joints, that they are with difficulty kept together, even though near twice the quantity of timber be employed as would otherwife be neceflary.' This method faves timber, and at the fame time the wheels are equally strong, or ftronger, and much lighter.

Section 3 contains fome hints with refpect to the proper method of hanging coaches and wheel carriages, fo as to prevent jolting and uneafinefs. In fection 4 we have obfervations on the utility and inconvenience of broad wheels. Our Author advifesto diminish the breadth of the wheels in common ftage waggons to fix inches, and by making the fore and hind axle of different lengths, to cause both wheels to roll the full furface of nine inches, which they might well do, without admitting any ridge or vacuity between them.'

Or, fillies, as commonly written.,

The

The laft fection exposes the abfurdity of a practice among wheel-wrights, of bending the arms of the axletrees downward,. fo as to make the wheels ftand confiderably wider from each other at the top than at the bottom: a practice that occafions a great deal of partial rubbing, and therefore not only wears the parts in contact, but in fome degree retards the progrefs of the carriage.

Thus have we given as full, and yet as compendious, an abftract as poffible of this treatife. It appears in point of style with peculiar advantage, on account of the affiftance which the Author acknowledges to have received from an ingenious friend; and it contains many obfervations that are of confiderable importance on this fubject. Several of them have been repeatedly urged by mechanical writers; fome are new and interefting; and others difputable: but they are all delivered with perfpicuity and cafe, fo that the Reader, we apprehend, will perufe this performance with pleafure and advantage.

ART. III. A Parallel between the English Conflitution and the former Government of Sweden; containing fome Obfervations on the late Revolution in that Kingdɛm, and an Examination of the Caufes that fe cure us against both Aristocracy and abfolute Monarchy. By J. L. D. L. LL. D. 8vo. I s. 6 d. Almon. 1772.

IT

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is very natural for the inhabitants of a free country to pay a ferious attention to the great event which has recently taken place in Sweden. The Author of this pamphlet expreffes his fatisfaction in having obferved that fome of his worthy countrymen, at the first news of it, took up their pens, and endeavoured to spread the alarm in the nation. But while he plauds their zeal, he does not admit the conclufions drawn by fome, who having found that the original and real prerogatives of the Swedish king were much inferior to thofe of the British monarch, have inferred that the conftitution of England, compared with the former government of Sweden, is in a very tottering condition. To obviate the inference which may be raised upon fuch a foundation, we are here prefented with a view of the conftitution of the Swedish government, prior to the late revolution, in order to fhew its great difference from our own.

We cannot follow this Writer through the particulars of his reprefentation, but a fummary of the ftate of Sweden may be feen in the following paragraph:

The government of Sweden, because it had what they called a King, may have been mistaken for a monarchical one, by thofe who judge of things from their outward appearance; or for being popular, becaule even peafants had a feeming fhare in it. But the executive and military powers centering in the nobles, with the reality of the legillative authority, it was

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merely