14. A great deal has been written on the relative qualities of wheels which have broad or narrow felloes, and opinions are generally given in favour of the former. The reasons assigned are, that the narrow wheels have a tendency to cut up and materially injure the roads; whereas, on the contrary, the broad wheels, acting like a roller, smooth down, consolidate, and harden them. This, as a general principle, is undoubtedly true; but the question still remains, What is the most advantageous breadth? It is not very easy to answer this question satisfactorily. By giving a considerable breadth to the wheels, they no doubt act as a roller, and are advantageous to the roads; but, at the same time, they become heavier by the additional wood and iron, supposing the hoops nearly of the same thickness, and they cannot well be reduced in thickness inversely as their breadth, otherwise they would be too thin to afford proper support to the felloes, and would require a frequent changing of the iron, which would be both troublesome and expensive. This would, in some degree, counterbalance the advantage to the roads, by the disadvantage of trouble, expense, and the additional load upon the cattle employed as a moving power. The difficulty appears, then, to be to combine the greatest advantage with the least disadvantage, and this will be accomplished by making them neither extravagantly broad nor too narrow, according to the size and nature of the cart or carriage, and the roads on which it will generally be employed. From 2 to 5 inches may be the limits within which they ought to be kept, except in extraordinary cases. We now proceed to the second section proposed.

II. Of the agents generally employed as a moving power. 1. The agents generally employed to move wheel-carriages are animals; and, in this country, chiefly the horse. The powers and capacity of the horse for this purpose are so well known, that it appears almost unnecessary to point them out. The power of a horse, or the exertion he is capable of making, according to different writers on this subject, differs in some degree from one another.

A horse, when treading in a mill-path, at the rate of 21 miles an hour, will, on an average, raise about 150 lb. by a cord hanging or passing over a pulley capable of easy motion, which is equi.

valent to 33,000 lb. 1 foot high in a minute. This serves as a scale to measure the effective exertion of a horse, and is generally denominated a horse-power. Messrs Boulton and Watt adopt a result somewhat less, or 32,000 lb., in estimating the effect of their steam-engines, and Tredgold only 27,500. In different varieties of the horse, the power is doubtless different, and perhaps 30,000 lb. may be a fair approximate value, in round numbers; and, when compared with the strength of a man, the effect of a horse will be, in ordinary circumstances, nearly equivalent to that of five or six men.

A horse generally works with advantage about eight hours a-day only, while a man commonly can work, without injury, at least ten. The action of a horse is indeed greatly reduced by the length of time during which he works, though the rate of decrease is not perhaps well known. Writers on this subject have advanced different hypotheses, and given formulæ to ascertain the effects under various circumstances of load and velocity, and some of them are at least tolerably good, by affording approximations somewhat useful. Thus, according to Euler, if ƒ be the force which an animal exerts, measured by the weight with which it is actually loaded, in the manner just mentioned, F the utmost effort which the animal can exert, or the resistance which, at a dead pull, it is just able to overcome, v the velocity with which the same animal is moving with a given load, V the velocity with which the power of drawing or carrying a load entirely ceases, then

This formula, originally proposed by Euler, from theoretical considerations, has been shown by M. Schulze to be pretty near the truth, by direct experiments.

The effect of animal strength, or quantity of work done in a given time, will be proportional to fx v, or as the product of the force employed multiplied by the velocity of the animal's motion. It is also measured by E, the effect, which will be

This, by the principles of the differential calculus, becomes a maximum when v= , and f; that is, the effect will

V

3

=

be greatest when the animal moves with one-third of the speed

with which, at its greatest velocity, it is only able to move itself, without carrying any load, and at the same time is loaded with four-ninths of the load it is capable of moving. It also appears that the greatest effect produced will be represented by the product of the two last, or

fxv= × FX V

27

(C);

that is, multiply the greatest weight which a horse can move at a dead pull, by the greatest velocity with which he can travel without being able to move any weight, then ths of this product will be the greatest effect; or the product of the weight which he ought to exert to move a load, and the velocity at which he ought to travel. There is considerable difficulty in estimating correctly the values of F and V. Euler supposed, from his own experience, which has in some degree been confirmed by Schulze, that F might be taken, for men, at about 72 lb. and V at 6 feet per second, which reduces formula (A)

Thus, when standing still, he can pull with a force of 72 lb., but if he walk at the rate of two miles an hour, he will be able to exert a force of traction of 19 lb. only; if at three miles an hour, it will amount to 5 lb.; and, at four miles an hour, or about 6 feet per second, he will be able to exert no force, but be enabled to carry himself forward only, which is pretty conformable to our experience, four miles an hour being the quickest rate at which a man in general, for some length of time, can travel, without exerting any force in pulling.

If v be assumed at a certain number of miles an hour, in this case,

a formula which will give the value of ƒ in lb when v is taken in miles.

=

Thus let v 2 miles, then ƒ 18 lb. Again, let v = 3 miles, and ƒ = 4 lb., agreeing very nearly with those stated above, derived strictly from Euler.

f

(To be continued.)

ESSAYS ON THE ORIGIN AND NATURAL HISTORY OF DOMESTIC

ANIMALS. BY JAMES WILSON, Esq. F.R.S. E. M. W.S. &c.

ESSAY IV.

ON THE ORIGIN AND NATURAL HISTORY OF THE SHEEP AND GOAT.

(Continued from page 376.)

DOMESTIC RACES.

THE natural history of sheep, or such particulars as relate to these animals in their unreclaimed condition, was discussed in the preceding Number of this Journal. We shall now proceed to consider the history and characteristic properties of the more remarkable of the domestic breeds.

The milk of sheep yields but little cream, and produces butter of a quality greatly inferior to that of the cow; but it has been remarked, that the same measure of ewe's milk will yield double the quantity of curd which cow's milk affords. The milk of sheep is thicker than that of cows. Its taste is strong, and not very agreeable. The cheese made from it is rich, and, though of a peculiar flavour, is by many highly esteemed. The skins of these animals are in great demand by bookbinders and saddlers; and those essential parts of stringed musical instruments, commonly called catgut, are manufactured from their intestines.

In this country, seldom more than two are produced at a birth, more frequently only one. But, in the Netherlands, there is a large breed which produces three, four, and occasionally even five young at a time. Sheep rarely survive beyond their fourteenth or fifteenth year.

THE LONG-LEGGED SHEEP OF AFRICA.

Ovis aries guinensis, Linn. GMEL. Ovis aries longipes, DESM. Ovis guinensis seu angolensis, MARCGRAVE. Belier et brebis des Indes, BUFFON. Le Morvan, Ejusd. Supplt. Mouton à longues jambes, F. CUVIER.

Of this variety the chaffron is arched, the ears pendant, the neck short, and the legs remarkably long. The body is covered with hair, which, on the nape of the neck and shoulders, as

The

sumes the appearance of a heavy mane hanging down to the breast. There are portions of wool on the croup, back, and flanks. The tail reaches to the heels. The horns are generally short, forming not quite an entire circle round the ears. skin of the throat is frequently pendant, and the colour varies from white to brown, black, and pied. Its length is 4 feet, and its height above 3 feet at the shoulders.

This is probably the largest and the tallest of all the domestic breeds. It resembles the wild musmon in the outline of its countenance, and its general deficiency of wool. It appears to have been first introduced into Europe by the Dutch, who naturalized it in East Friesland. Crossed with the comnion breed, it has produced the Mouton flandrin or Texel sheep (of which the wool, with a certain degree of fineness, is of great length), remarkable for always producing several lambs every year. Though ascribed by Buffon to India, its true country is Africa, more particularly the coast of Guinea. A specimen from Fezzan was sent to M. F. Cuvier by the French Consul at Tunis.

Several subordinate varieties may be referred to this race,such as the Morocco breed with long wool, the hair on the neck short, undulating, and of a rufous-brown colour, the ears small and horizontal, the horns small, turning spirally outwards, the scrotum forming two separate sacks, and the general colour white, marked with liver-coloured brown*: also the Congo breed, of which the proportions are similar to those from Guinea, but with hollower flanks, and the chaffron greatly arched; they are meagre and powerless, and, instead of wool, are covered with loose hair, slightly lengthened beneath the throat; the horns are short, turned back upon the neck; the ears are pendulous; the throat wattled or pouched, and the tail very long, slender, and almost naked. The general colours of the fur are composed of large clouds of rufous coloured brown upon a white ground: The females are hornless.

The Angola breed are better proportioned. The chaffron is scarcely arched, the horns are small, and the wool, at least on those called Coquos, finer and more abundant: the others are

A specimen of this kind was formerly in the possession of Sir Joseph Banks.