incontestably, that the earth is really flattened at the poles. The degree of Lapland proved longer than that of Peru, by a quantity much too considerable to be reasonably attributed to uncertainty of observation. Nevertheless, the compression calculated from these measures, still left reason to suspect, that one of them, at least, was not exact, for the amount of the result was more than is adapted even to the supposition of homogeneous structure, while from theory it ought, at most, to be equal to this term, if the earth were homogeneous, and less if it were not. This was also indicated by the observations of the seconds pendulum, made by the two commissioners at Peru and Lapland, for they give as the measure of compression the fraction, which differs but little from that which we find at the present day, from much more precise operations. A new measure of a degree in Lapland, made since by Mr. Swanberg, an able Swedish astronomer, has proved that the error was on this side, and reckoning from his conclusions, the compression calculated from the degrees measured at the polar circle, and at the equator, agree with the result of the experiments upon the pendulum. It is not improbable, that the error of the northern commission arose from a want of verification, arising from too great confidence in the instruments employed; while on the other hand, in the operations in Peru, Bouguer took infinite pains to study the faults of his apparatus, to enable him to correct them, or to prevent their effects by a happy choice of circumstances the most favourable to the observations; so that with instruments inferior in construction, but proved and rectified by his dexterity, he obtained results which can certainly only be affected by very trifling errors. In general, in such delicate operations, the most perfect instruments ought to be employed with distrust; and their very perfection may become a snare to him who trusts too implicitly to them.

The doubts of which I have been speaking, were not yet removed when the Academy of Sciences conceived the design and the hope of equalizing all the measures commonly employed in France by the introduction of a standard common to all the parts of that great kingdom. It was agreed on all hands that this standard, to be invariable, ought to be taken

from some natural phenomenon, and after having for some time balanced between the length of the seconds pendulum, and the measure of the earth, the last was fixed upon. Such was the origin of those great geodetic operations which have been undertaken since 1790, by the French philosophers in France and Spain, and which now united with the operations of the same kind undertaken in England under the direction of Col. Mudge, will comprehend from Formentera to Unst, an arc of 22 terrestrial degrees, situated almost exactly under the same terrestrial meridian. The length of the second's pendulum has been measured at the same time as the degrees themselves throughout the whole extent of this great arc, with apparatus calculated to show its smallest variations. I shall describe in another letter what there is remarkable in the processes by which these different kinds of observations have been made; I shall endeavour then to show the degree of confidence to be placed in the conclusions derived from them; I shall point out with what certainty we may be enabled to derive a standard of universal and invariable measures from them; and I shall explain the reasons which have determined the French philosophers to choose this standard from the measure of the earth, rather than from the length of the simple pendulum.

I have the honour to be,

&c. &c. &c.

Note upon the knowledge which the ancient Egyptians appear to have had of the Figure of the Earth.

[This note is extracted from an unpublished Memoir, which was honoured with a prize by the Academy of Inscriptions and Belles Lettres of the Institute of France. It was communicated by the Author himself, who is at this day a member of the Academy.]

IN examining the ancient system of measures made use of in Egypt, and in comparing it with the geography of that country, and with the dimensions of such monuments as are still in existence, we discover some traces of the attempts which have formerly been made by the Egyptians to acquire VOL. V.

S

a knowledge of the measure of the earth, and we can acquire some notion of the degree of exactness which they had been able to arrive at.

The schænus, a measure of length made use of amongst the Egyptians, contained 12000 cubits, or 40 stadia of 300 cubits each. These stadia of 40 to the schænus were, according to Pliny, those of which Eratosthenes made use. They were contained 252,000 times in the circumference of the earth, or 700 times in a degree. The Egyptian cubit, found upon the nilometer at Elephanta, was 0,527 parts of the metre. The schane was therefore equal to 12000 times .527, or 6324 metres; and the stadium which was its 40th part, was equal to 9 or 158.1 metres. So that as the last was said to be contained 700 times in a degree, it follows that the Egyptians gave to a degree a length equal to 700 times 158.1, or 110670 metres. According to the tables of M. Dehambre, the mean degree of Egypt, that is to say, between 24° and 31° of latitude is 110796 metres; the difference therefore is 126 metres, and is equivalent to an error of 4 seconds in latitude. The Egyp tians, therefore, had acquired a knowledge of the terrestrial degree within 4 or 5 seconds, that is to say, or Tooo part,

and nearly of its real quantity.

If we now apply to the territory, the measure in schæni or in stadia, which the ancients prescribed to Egypt; we shall find that that country must have been trigonometrically surveyed with sufficient exactness. In fact, such of the partial distances by means of which the ancients, and amongst others, Herodotus, have given the complete length of Egypt, are nearly taken in a straight line; so that modern observations give to Egypt between Pelusis and Sienna, a length of 7° 38 15", while the ancient measures prescribe to the same interval a distance of 7° 37′ 7′′, the difference being 1' 8", or about 1. How did the Egyptians arrive at such a degree of precision? It is not known. We are at liberty, however, to review the means which they had at their disposal. All antiquity attests, and the nature of their country demonstrates, that they were often under the necessity of recommencing the division of their lands. The multitude of operations supplying

what was wanting on the side of precision, they might arrive, after the lapse of ages, at a sufficiently exact knowledge of the measure of their country. Moreover, they knew how to draw a meridian line, as the placing of the faces of the great pyramid to the four cardinal points, sufficiently proves. They were therefore competent to take the azimuth of a triangle. Lastly, they knew how to make use of the gnomon; and after their taste for every thing colossal, there is no doubt but that they erected some very large ones, which might have given the proportion of the shadows with so much the more exactness, as it was possible to multiply and combine observations. Nothing more is wanting, to find with all the precision that the Egyptians have attained to; 1st, the difference in latitude of two points; 2dly. their distance expressed in the measure of the country.

ART. VI. Experiments and Observations relative to Vision; by Marshall Hall, M. D. of Nottingham, formerly Senior President of the Royal Medical Society of Edinburgh.

THE following detail is nearly confined to a series of obser

vations and experiments made by myself. I have been induced to adopt this plan partly from the difficulty of meeting with persons sufficiently interested in their results to prosecute experiments at once nice and difficult in themselves, and requiring a certain degree of the power and habit of abstraction, for their performance; but principally from a peculiarity in my own vision, by which I am enabled to give the subject of this paper a peculiar illustration.

The peculiarity of vision to which I allude, consists in an ability to adapt the left eye for distinct vision at shorter distances than the right, and in an incapacity for adapting the left eye for distinct vision at great distances, whilst the right eye possesses the power of adaptation for distinct vision at very considerable distances. The nearest distance at which a

bright point is distinctly seen by the right eye is 4 inches; but by the left eye the point is seen with perfect distinctness at the distance of 3 inches. The same point is seen distinctly by the right eye at the distance of 17 inches; by the left it is seen indistinctly at any distance beyond 14 inches. With the right eye I distinguish each small branch and each leaf on a tree planted about thirty yards from my window; with the left eye these objects are seen in the most indistinct and confused manner. A distant light seen distinctly, or as a point nearly by the right eye, appears magnified into à large star to the left. The distant object seen thus indistinctly by the left eye, immediately acquires distinctness by the use of a concave lens.

A number of experiments have convinced me that, in myself at least, ordinary vision is performed principally by one eye alone, the left eye being chiefly employed and adapted for distinct vision at short, and the right eye at long distances; whilst the axis of the other eye is merely directed to the object, in order to prevent the confusion and double vision which would arise from the different direction of the two eyes. In proof of this observation I may observe, that when the eyes are directed to a distant object, as the tree before my window just mentioned, any intervening object, placed within certain limits with respect to distance, and seen of course double, appears indistinct to the right eye, but perfectly distinct and with a well-defined outline, to the left. On the contrary, when the characters on a printed page placed at the distance of about eight inches from the eye, are observed, whilst the point of a penknife placed at the distance of six inches is seen single and distinctly by both eyes, each word and line is of course seen double, and the right part of the double image, or that seen by the right eye, appears distinct, whilst the left side of this image, or that arising from vision by the left eye, is seen indistinctly and obscurely.

When the eyes are fixed on a distant object, and an intervening object placed also at a considerable, although at less distance, is observed, it is seen nearly distinctly by the right eye, and less distinctly by the left; and there is a particular