D I A 



Diaphauo- 

 meter. 



New di."- 

 phanome 

 ters sug- 

 gested. 



M. cle Saussure selected, for his experiments, a 

 straight road or plain of about 12 or 1500 feet in cir- 

 cumference, which towards the north was bounded by 

 trees or an ascent. Those who repeat them, however, 

 must pay attention to the following remarks : — When 

 a person retires backwards, keeping his eye constantly 

 fixed on the pasteboard, the eye becomes fatigued, and 

 soon ceases to perceive the circle ; as soon, therefore, 

 as it ceases to be distinguishable, you must suffer your 

 eyes to rest ; not, however, by shutting them, for they 

 would when again opened be dazzled by the light, but 

 by turning them gradually to some less illuminated ob- 

 ject in the horizon. When you have done this for about 

 half a minute, and again directed your eyes to the paste- 

 board, the circle will be again visible, and you must 

 continue to recede till it disappear once more. You 

 must then let your eyes rest a second time, in order to 

 look at the circle again, and continue in this manner 

 till the circle becomes actually invisible. 



If you wish to find an accurate expression for the 

 want of transparency, you must employ a number of 

 circles, the diameters of which increase according to a 

 certain progression; and a comparison of the distances 

 at which they disappear, will give the law according to 

 which the transparency of the atmosphere decreases at 

 different distances. If you wish to compare the trans- 

 parency of the atmosphere on two days, or in two dif- 

 ferent places, two circles will be sufficient for the expe- 

 riment. 



According to these principles, M. de Saussure cau- 

 sed to be prepared a piece of white linen cloth eight 

 feet square. In the middle of this square he sewed a 

 perfect circle, two feet in diameter, of beautiful black 

 wool : Around this circle he left a white ring two feet 

 in breadth, and the rest of the square was covered with 

 pale green. In the like manner, and of the same mate- 

 rials, he prepared another square ; which was, however, 

 equal to only T X T of the size of the former, so that each 

 side of it was 8 inches ; the black circle in the middle 

 was 2 inches in diameter, and the white space around 

 the circle was 2 inches also. 



If two squares of this kind be suspended vertically 

 and parallel to each other, so that they may be both 

 illuminated in an equal degree by the sun, and if the 

 atmosphere, at 'the moment when the experiment is 

 made, be perfectly transparent, the circle of the large 

 square, which is twelve times the size of the other, 

 must be seen at twelve times the distance. In M. de 

 Saussure's experiments, the small circle disappeared at 

 the distance of 314 feet, and the larger one at the dis- 

 tance of 3588 feet, whereas it should have disappear- 

 ed at the distance of 3768. The atmosphere, there- 

 fore, was not perfectly transparent. This arose from 

 the thin vapours which at that time were floating in 

 it." 



The writer of this article, several years ago, made 

 numerous experiments both with the diaphanometer of 

 Saussure, and with other contrivances, but they were 

 not attended with any very satisfactory results. One 

 of the instruments which he employed, was a telescope 

 with a variable magnifying power, produced by the mo- 

 tion of a second object-glass along the axis of the instru- 

 ment. By this means, he got rid of the error arising 

 from the adjustment of the eye to different distances ; 

 and that organ did not need the successive refreshments 

 . '.ich Saussure found it to require. The magnifying 

 power being known with the greatest accuracy from a 

 scale on the moveable tubes, we had only to shift the 

 moveable object-glass till the object vanished, and the 



728 D I A 



a magnifying power, indicated by the scale, became a Diaphragm 

 relative measure of the transparency of the atmosphere. D ; a rbekr. 



By this means we are enabled to extend our experi- 

 ments to a much larger portion of atmosphere. 



Another contrivance which we adopted, was a tele- 

 scope having tubes twice the length of what was ne- 

 cessary for ordinary purposes, in order to allow the eye- 

 piece to be pulled out a great way beyond the focal 

 point of the object-glass, and to be pushed as far with- 

 in it. When the telescope was adjusted to distinct vision, 

 the object was seen with perfect clearness; but when 

 indistinct vision was produced by pushing in or pull- 

 ing out the eye-piece, the object of course vanished, 

 and the distance of the eye-piece from the focal posi- 

 tion, was obviously a measure of the transparency of 

 the atmosphere. For farther information on this and 

 analogous subjects, see Mem. Acad. Turin, vol. iv. 

 where Saussure first published an account of his inven- 

 tion ; Gren's Neue Journal cler Physik, vol. iv. which 

 contains Murhard's Paper; Tilloch's Phil. Magazine, 

 vol. iii. p. 377 ; Bouguer's Traile d'Optique sur la gra- 

 dation de la Lumiere, livre iii. 1 760 ; and Lambert's 

 Photomelria, the last of which we have not seen, but it 

 is referred to by Mnrhard. (tt) 

 DIAPHRAGM. See Anatomy. 

 DIARBEKR, or Diarbek, from Dhyar a duke, and 

 behr a country, is the name of one of the pashaliks of 

 the province of Armenia and the Ottoman empire. This 

 pashalik, which, next to that of Erzeroon, is one of the 

 largest in Armenia, is situated between the Tigris and 

 the Euphrates, and is separated from the dependencies 

 of Merdin by a small river, and by a branch of Mount 

 Masius. The whole of the pashalik is difficult of ac- 

 cess, from the number of mountains which it contains. 

 It is interspersed with narrow and fei-tile valleys, and 

 abounds in the most beautiful and romantic scenery. 

 Diarbekr was formerly the name of an extensive pro- 

 vince, which is now divided into different pashaliks. 

 See Macdonald Kinneir's Geographical Memoirs of the 

 Persian Empire, (t) 



DIARBEKR, or Kara- Amid, the capital of the pa- 

 shalik of the same name, and the Amida of the ancients. 

 It is situated in a delightful plain or rather table land, at 

 the vertex of a triangle formed by one of the inflexions 

 of the Tigris, which encompasses it on the east. It is 

 encircled with a prodigious Avail, flanked with seventy- 

 two towers. This wall is built of black stone, from 

 which circumstance the city derived the name of Kara- 

 amid, or the Black Amid. This wall, which is now in 

 a ruinous condition, is described by Mr Kinneir as far 

 superior, both in height and solidity, to any thing which 

 he had seen either in Europe or Asia. He thinks that 

 there is no doubt of its having been built by the Ro- 

 mans, and attributes the mistake of those travellers, who 

 ascribe it to the Arabs, to the number of Kufic inscrip- 

 tions which have been inserted into different parts 

 of the walls and towers at a later period. The stile of 

 architecture has no resemblance to that of the Arabs, 

 and similar Arabic inscriptions are often found in the 

 ruins of Persepolis, the castle of Shuster, and the Rock 

 of Besitoon. The houses, which are built of stone, have 

 a respectable appearance, but the streets, though well 

 paved, are narrow and dirty. In this city there are two 

 or three stately piazzas, well stored with corn and pro- 

 visions, and all kinds of merchandise; and there is a large 

 magnificent mosque, which was formerly a church be- 

 longing to the Christians. The castle, which is on the 

 north side of the town, is encompassed with a strong 

 wall. It has many courts and elegant buildings, in 



