land by means of their additional manure; the only difficulty is, how to maintain an increased number without increafing the expence. This, he afferts, may be done by the use of falt; and advances the three follow=ing propofitions: 1. That falt, given with the food of cattle, augments the nourishment of that food. 2. That, in proportion to the quantity of falt eaten by cattle, the effects of that augmentation will be perceivable. 3. That no ill confequence will follow from excess of falt eaten by cattle, even though it should be given them without ftint. These propofitions he endeavours to fupport by unquestionable facts. In the jurifdiction of Arles, in the county of Provence, there is, he fays, a district called the Crau, extending in length about fix leagues, and in breadth about three, the whole furface of which is covered with small rough ftones, and not a tree or bufh is to be feen in the whole district, except here and there on the borders; yet on this fpot, fo feemingly fterile, by the free use of falt, more numerous flocks of fheep are bred and reared, than upon any other common of equal extent throughout the whole kingdom; and what is no lefs remarkable, the fheep are healthier, and hardier, and endure the feverity of the winter with lefs lofs, though they have fewer sheep-cots for covering, than thofe bred and fed on more copious paftures, and that have, befides, the advantage of more convenient fhelter. Add to this, that the wool of the flocks bred and brought up in the Crau, is not only the finest in the whole country, bat bears the highest price of any in France. From hence he concludes, that it is to the unlimited use of falt that these furprizing effects are to be afcribed; for it frequently happens that the Crau is fo burnt up in the fummer,. that the poor animals are forced to turn up the very ftones to come at the few blades of grafs that grow round them: and yet none perish for want of food. Let every excellence, therefore, that can reafonably be fuppofed inherent in the herbage, be allowed to it, yet the quantity of it is fo fmall, that, without the abundant use of salt, a fourth part of the fheep kept in the Crau, could not fubfift in it. But, as a ftill further demonftration, that this aftonishing effect is folely to be attributed to falt, we have, fays the writer, in Languedoc, on the borders of the Rhone, a fpot of the fame kind of ftony land, in every refpect fimilar to that of the Crau; yet, for want of the free ufe of falt, that of Languedoc does not maintain a tenth part of the number of sheep that are brought up in the Crau, though in other refpects it is no ways inferior, the wines and other fruits produced on the borders of both being, in their goodness and other effential qualities, equal. Having proved his first propofi-` tion incontrovertibly, he proceeds, in proof of the fecond, to recommend an eafy experiment, which it is in every farmer's power to make; and that is, to give to one half of his cattle falt, and to the other half none. By this fimple trial,` he fays, in less than a month, the difference will be difcernible. The cattle to whom the falt is given will fhew it in their looks, in the fleckness of their coats, in their H 4 growth, growth, and in their ftrength and fitnefs for labour. He adds, that with little more than half their ufual food all these effects will be produced. To establish his third propofition, he appeals to the practice about Arles, where the cattle have as much falt as they will eat, and none are fo healthy, or thrive fo faft, as thofe that eat the most of it. T From thefe obfervations, there I cannot but be extremely pleaf cannot remain a doubt of the good effects of falt in the feeding and fattening of cattle; but it is much to be regretted, that the writer is totally flent with refpect to the method of giving the falt to the labouring cattle. He has, indeed, informed his readers, that in 8 days his flock of 300 fheep eat 15 lb. of falt, being one pound to every score; and it fhould feem by his manner of expreffing himself, that he gave them the whole quantity in one day, as he cautions the farmer against fuffering his sheep to drink on the day the falt is administered, apprizing him at the fame time how much it fharpens their appetite; and that he had feen them not only browse upon ftubbs after eating the falt, but even gnaw pieces of wood of a furprising bigness. As the fubject of the above Memoir appeared to me of importance, I have only to request of you, Mr. Urban, the immediate infertion of the few hints which are here extracted from it; as, during the prefent fcarcity of hay, it may be interefting to many; and as it has, in its confequences, a tendency to lower the price of provifions, it is to be hoped, that a difcovery that promifcs fo much benefit to the ing to a devout mind, to contemplate the extreme difference between the productions of human art, and thofe of the God of nature, when obferved with exactnefs, in point of elegance and truth of workmanship. The dif gufting irregularity that appears in the finest needle, when examined by a microscope, has been compared with the wonderful accuracy of the fting of a bee or a wasp; and the unequal contexture of the moft delicate cambrick, when compared with fome natural productions, has been obferved, and devoutly acknowledged; but I do not recollect that I have any where feen a comparison inftituted, between the fubjects I am now propofing to confideration, which yet well deferve our notice, and may perhaps be as amufing as any of them. Few are totally unacquainted with what is meant by Mofaic work. "It is a kind of Paint " ing," fays Dr. Johnson, in his dictionary, "in fmall pebbles, 66 cockles, and fhells of fundry co"lours." This is not the most happy of his defcriptions; many other materials are used, and some more frequently than thofe he has mentioned, particularly pieces of marble, of burnt clay, and of glafs. glafs. In truth, Mofaic work means in general, the forming flourishes and figures with fmall bodies of different colours. 1 Beautiful pavements of antient Mofaic, are fometimes found in our country, the remains of old Roman magnificence in this ifland. The Antiquarian fociety has publifhed plates of feveral; as the Royal fociety, before them, gave an account of others, in some of their firft" volumes. appear to be a finger's breadth, "but the more elegant pieces con"fit of glass pins (if that term "might be made ufe of) not "thicker than a common fewing "needle, fo that a portrait of four "feet square fhall take up two mil"lions of fuch pins." The diameter of the, fquare teffellæ of a pavement in Suffex,, we are told by Dr. Tabor, was about of an inch. I fuppofe the diagonal was intended, fince we are informed immediately after, that the longeft fide of thofe that were oblong at the head, little exceeded an inch t. If the diagonal" of the fquare teffella was meant, then the fides of the fquare must be in courfe about of an inch; and if of an inch, then 100 teffellæ would cover nine fquare inches, and confequently eleven teffella would nearly cover one inch fquare. But the moderns have greatly improved the art of making Mofaic work. They form pictures, it feems, for altar-pieces moft amazingly fine. I do not know any author that has defcribed this fort of work more minutely, or has carried it to an higher degree of perfection, than Keyfler. The fubftance of what that celebrated traveller has faid, is as follows: "That the materials ufed by the "moderns in these works are small He goes on to inform us, " that thefe pins are fo closely joined together, being fixed in a fort "of paste which he defcribes, "that after the piece is polished, "(which is done in the fame man"ner as looking-glaffes) it can "hardly be difcerned to be an ar 66 rangement of an infinite number "of particles of glafs, but rather "looks like a picture painted with "the finest colours, with crystal "placed before it........That it 66 may be easily feen how much "this curious art has been improved during the two laft cen"turies, by comparing fome of "the old cupolas of the chapels "in St. Peter's church †, with the "other pieces lately erected there. "The ftuds in the old works are "made of clay burnt, and the "furface only tinctured with va"rious colours; but that they This word means the little pieces of ftone, brick, &c. of which Mofaic work is compofed. + Phil. Tranf. Abrid. Vol. 5. part 2. chap. 2. art. 22. 1 At Rome. Keyfler adds, "that a piece of 10 fquare feet, if performed with tolerable care and delicacy, would employ eight artists the space of two years." Confequently it will take an artift a twelvemonth, to make five fquare feet of this curious kind of Mofaic. Such is the fubftance of Keyfler's account; let us now compare this very great improvement, and thefe high exertions of human genius and induftry, with the divine Mofaic of the wing of a butterfly. I took a piece of paper, and cut a quarter of an inch fquare out of it, as exactly as I could. I afterwards faftened this fmall fquare, with pafte, to one of the wings of a common feacock ✶ butterfly. I then cut out a portion of the wing (it was one of the upper ones) as near as could be a quarter of an inch fquare, by cutting clofe to the edge of the paper to which it was faftened. I told in this, by the help of the third magnifier of an opaque microfcope, 70 rows of fcales, and about 90 fcales in a Tow. I found, confequently, that there were 6300 fcales on one file of this small portion of the creature's wing: for 70 +90=6300. To be more fure, I afterwards took a piece of paper of a confiderable fize, on which I drew lines, nearly dividing it into fall fquares, and applying the piece of the wing, thus fattened to the frit-mentioned fmall paper fquare, to the large paper divided by thofe lines, at first gently, and afterwards more forcibly, I told the fcales, that I found sticking to the large paper from time to time, which I could pretty easily do by the help of a fmall eye-glafs, as they were divided into fmall parcels by the lines I had drawn. Upon repeating this operation 6 or 7 times, I found the fcales, that were detached from the piece of wing to be about 5752 in number, which added to thofe that were left (for several remained after rubbing it pretty hard), which I found to be 544 at leaft, make 6296, and comes very near the number found by telling the rows and the fcales in each row, and multiplying them together. If now we multiply 626 by 16, the number of fmall fquares contained in an inch square, of the fize of this piece of the wing, the product will be found to be 100736, which will be the number of fcales placed on a fquare inch of one fide of the wing of this kind of butterfly. But in the curious Mofaic pictures which Keyfler defcribes, there were but about 870 pins in an inch fquare. For he fays a picture of four feet Iquare will take up two millions of pins. Now fuch a picture contains 16 fquare feet, and there being 144 fquare inches in a fquare foot, fuch a picture contains 23 4 fquare inches. If therefore we divide two millions (the number of pins in fuch a picture) by 2304, the number of fquare inches in it, the quotient will be the number of pins in a fquare inch; So called from its having an eye, in each of its wings, a good deal refembling thofe in the tail of a peacock. The infect is extremely common. ROW now 2.000.000 2304 =68 and fome-. thing more: if therefore we fet the number of glafs pins at 870, in a fquare inch, it will make the number in the whole picture 2.004.480, which is 4480 more than Keyfler fuppofed would be ufed in fach a picture; but, if a few more should be wanted, 4480 must be thought fofficient. The coarseness of fuch a picture compared with the Mofaic on the wing of this infect, is in the proportion of 115 at least to one. That is, fuch a picture is 115 times coarfer than this natural Mofaic. In truth, almost 116 times: for 100.736, the number of fcales in a fquare inch, divided by 870, the fuppofed number of glafs pins, gives 115 with a large remainder. An admired picture then of this kind, compared with this butterfly's wing, is proportionably much coarfer, than a teffellated pavement, compofed of pebbles, fmall bricks, or pieces of marble, of an inch fquare at the top, is coarfer than fuch a modern picture, one of the highest productions of human industry and genius: for there being, as I obferved at the beginning of this account, 100 fuch teffellæ in 9 fquare inches, 25.600 teffella would be required to cover the 2304 fquare inches of a picture four feet fquare. But fuch a pavement would not be 79 times coarfer than fuch a picture *, whereas the picture is above 115 times coarfer than the Molaic of the animal, * For 2.004.480 25.600 This, however, is by no means placing the matter in fo ftrong a point of light as it deferves. The wings of a butterfly, when it first breaks out of the integument of its Aurelia + ftate, are much smaller than afterward. It is true, the wings expand themfelves, and even come to their full growth in a few minutes; but, if they are watched with care, they will be found to be very fmall at first, yet all the teffella are then placed in them, Nay, they may be difcerned, in fome butterflies, through the fufficiently tranfparent integument, to be placed in due order fome days before their quitting the Aurelia ftate; and confequently, they may be examined then with the greatest ease, by opening the cafe in which they are inclofed, and taking them out of it. This will fhew that they poffefs a much greater fuperiority ftill in point of fineness of workmanship, to the moft celebrated Mofaic pictures. To make this examination in the fureft and eafieft manner, I purfued the following method. By means of paste, I faftened a piece of catgut, extended as evenly as I could, to a card, in which I had cut an hole near 2 inches long and about 1 wide. This catgut, 1 found, had 1792 mefhes in a fquare inch, having 56 meshes in the length of an inch one way, and 32 the other. Upon meafaring an upper and under wing of this peacock butterfly, which was pretty large, by the meshes of this catgut, as carefully as I 7680 25.600. is equal but to 78 The Aurelia flate means the middle state between the creature's being a caterpillar and a butterfly. For 56 X 32 1792. well |