6Their circuit being completed, it behoves us to consider the manner of disposing of the area of the space enclosed within the walls, and the proper directions and aspects of the streets and lanes. They should be so planned as to exclude the winds: these, if cold, are unpleasant; if hot, are hurtful; if damp, destructive. A fault in this respect must be therefore avoided, and care taken to prevent that which occurs in so many cities. For instance; in the island of Lesbos, the town of Mytilene is magnificently and elegantly designed, and well built, but imprudently placed. When the south wind prevails in it, the inhabitants fall sick; the north-west wind affects them with coughs; and the north wind restores them to health: but the intensity of the cold therein is so great, that no one can stand about in the streets and lanes.
2Wind is a floating wave of air, whose undulation continually varies. It is generated by the action of heat upon moisture, the rarefaction thereby produced creating a continued rush of wind. That such is the case, may be satisfactorily proved by observations on brazen æolipylæ, which clearly shew that an attentive examination of human inventions often leads to a knowledge of the general laws of nature. Æolipylæ are hollow brazen vessels, which have an opening or mouth of small size, by means of which they can be filled with water. Previous to the water being heated over the fire, but little wind is emitted, as soon, however, as the water begins to boil, a violent wind issues forth. Thus a simple experiment enables us to ascertain and determine the causes and effects of the great operations of the heavens and the winds.
3In a place sheltered from the winds, those who are in health preserve it, those who are ill soon convalesce, though in other, even healthy places, they would require different treatment, and this entirely on account of their shelter from the winds. The disorders difficult to cure in exposed situations are colds, the gout, coughs, phthisis, pleurisy, spitting of blood, and those diseases which are treated by replenishment instead of exhaustion of the natural forces. Such disorders are cured with difficulty. First, because they are the effect of cold; secondly, because the strength of the patient being greatly diminished by the disorder, the air agitated by the action of the winds becomes poor and exhausts the body’s moisture, tending to make it low and feeble; whereas, that air which from its soft and thick nature is not liable to great agitation, nourishes and refreshes its strength.
4According to some, there are but four winds, namely, Solanus, the east wind, Auster, the south wind, Favonius, the west wind, and Septentrio, the north wind. But those who are more curious in these matters reckon eight winds; among such was Andronicus Cyrrhestes, who, to exemplify the theory, built at Athens an octagonal marble tower, on each side of which was sculptured a figure representing the wind blowing from the quarter opposite thereto. On the top of the roof of this tower a brazen Triton with a rod in his right hand moved on a pivot, and pointed to the figure of the quarter in which the wind lay.
5The other winds not above named are Eurus, the south-east wind, Africus, the south-west wind, Caurus, by many called Corus, the north-west wind, and Aquilo the north-east wind. Thus are expressed the number and names of the winds and the points whence they blow. To find and lay down their situation we proceed as follows:
6let a marble slab be fixed level in the centre of the space enclosed by the walls, or let the ground be smoothed and levelled, so that the slab may not be necessary. In the centre of this plane, for the purpose of marking the shadow correctly, a brazen gnomon must be erected. The Greeks call this gnomon σκιαθήρας. The shadow cast by the gnomon is to be marked about the fifth ante-meridianal hour, and the extreme point of the shadow accurately determined. From the central point of the space whereon the gnomon stands, as a centre, with a distance equal to the length of shadow just observed, describe a circle. After the sun has passed the meridian, watch the shadow which the gnomon continues to cast till the moment when its extremity again touches the circle which has been described.
7From the two points thus obtained in the circumference of the circle describe two arcs intersecting each other, and through their intersection and the centre of the circle first described draw a line to its extremity: this line will indicate the north and south points. One-sixteenth part of the circumference of the whole circle is to be set out to the right and left of the north and south points, and drawing lines from the points thus obtained to the centre of the circle, we have one-eighth part of the circumference for the region of the north, and another eighth part for the region of the south. Divide the remainders of the circumference on each side into three equal parts, and the divisions or regions of the eight winds will be then obtained: then let the directions of the streets and lanes be determined by the tendency of the lines which separate the different regions of the winds.
8Thus will their force be broken and turned away from the houses and public ways; for if the directions of the streets be parallel to those of the winds, the latter will rush through them with greater violence, since from occupying the whole space of the surrounding country they will be forced up through a narrow pass. Streets or public ways ought therefore to be so set out, that when the winds blow hard their violence may be broken against the angles of the different divisions of the city, and thus dissipated.
9Those who are accustomed to the names of so many winds, will perhaps be surprised at our division of them into eight only; but if they reflect that the circuit of the earth was ascertained by Eratosthenes of Cyrene, from mathematical calculations, founded on the sun’s course, the shadow of an equinoctial gnomon, and the obliquity of the heavens, and was discovered to be equal to two hundred and fifty-two thousand stadia or thirty one millions and five hundred thousand paces, an eighth part whereof, as occupied by each wind, being three millions nine hundred and thirty-seven thousand five hundred paces, their surprise will cease, because of the number of impediments and reverberations it must naturally be subject to in travelling such distance through such varied space.
10To the right and left of the south wind blow respectively Euronotus and Altanus. On the sides of Africus, the south-west wind, Libonotus southward and Subvesperus northward. On the southern side of Favonius, the west wind, Argestes, and on its northern side Etesiæ. On the western side of Caurus, the north-west wind, Circius, on its northern side Corus. On the western and eastern sides respectively of Septentrio, the north wind, Thrascias and Gallicus. From the northern side of Aquilo, the north-east wind, blows Supernas, from its southern side Boreas. Solanus, the east wind, has Carbas on its northern side, and Ornithiæ on its southern side. Eurus, the south-east wind, has Cæcias and Vulturnus on its eastern and southern sides respectively. Many other names, deduced from particular places, rivers, or mountain storms, are given to the winds.
11There are also the morning breezes, which the sun rising from his subterranean regions, and acting violently on the humidity of the air collected during the night, extracts from the morning vapours. These remain after sunrise, and are classed among the east winds, and hence receive the name of εὐρος given by the Greeks to that wind, so also from the morning breezes they called the morrow αὔριον. Some deny that Eratosthenes was correct in his measure of the earth, whether with propriety or otherwise, is of no consequence in tracing the regions whence the winds blow:
12for it is clear there is a great difference between the forces with which the several winds act. Inasmuch as the brevity with which the foregoing rules are laid down may prevent their being clearly understood, I have thought it right to add for the clearer understanding thereof two figures, or as the Greeks call them σχήματα, at end of this book. The first shews the precise regions whence the different winds blow. The second, the method of disposing the streets in such a manner as to dissipate the violence of the winds and render them innoxious. Let A be the centre of a perfectly level and plane tablet whereon a gnomon is erected. The ante-meridianal shadow of the gnomon being marked at B, from A, as a centre with the distance AB, describe a complete circle. Then replacing the gnomon correctly, watch its increasing shadow, which after the sun has passed his meridian, will gradually lengthen till it become exactly equal to the shadow made in the forenoon, then again touching the circle at the point C. From the points B and C, as centres, describe two arcs cutting each other in D. From the point D, through the centre of the circle, draw the line EF, which will give the north and south points.
13Divide the whole circle into sixteen parts. From the point E, at which the southern end of the meridian line touches the circle, set off at G and H to the right and left a distance equal to one of the said sixteen parts, and in the same manner on the north side, placing one foot of the compasses on the point F, mark on each side the points I and K, and with lines drawn through the centre of the circle join the points GK and HI, so that the space from G to H will be given to the south wind and its region; that from I to K to the north wind. The remaining spaces on the right and left are each to be divided into three equal parts; the extreme points of the dividing lines on the east sides, to be designated by the letters L and M; those on the west by the letters NO; from M to O and from L to N draw lines crossing each other: and thus the whole circumference will be divided into eight equal spaces for the winds. The figure thus described will be furnished with a letter at each angle of the octagon. Thus, beginning at the south, between the regions of Eurus and Auster, will be the letter G; between those of Auster and Africus, H; between Africus and Favonius, N; between that and Caurus, O; K between Caurus and Septentrio; between Septentrio and Aquilo, I; between Aquilo and Solanus, L; and between that and Eurus, M. Thus adjusted, let a bevel gauge be applied to the different angles of the octagon, to determine the directions of the different streets and lanes.