Polytechnic Association Proceedings. 541 



the great pyramid. 

 Mr. C. Piazzi Smyth's new work in three volumes, contamhig au 

 account of his operations in Egypt during the first four months of 

 1865, just published, has again directed attention to ttie pyramids. 

 Herodotus, Strabo, Pliny and the Arabian wi-iters all concurred in 

 the statement that the great pyramid was originally covered with 

 a white casing. For many years this statement was not believed, 

 because no traces could be found. However, in 1837, Col. Vyse 

 found two casing stones in situ. They were of white limestone, 

 from the Mokattam hills. As early as the year 1000, the Kalifs 

 of Egypt began to strip off this casing. The blocks, measuring 

 eight feet by five, were used in building palaces, mosques and 

 bridges- Those which remained were near the foundation, and 

 covered with sand at the time of the devastation alluded to. They 

 were cut so true that a cement no thicker than ordinary paper was 

 required to join them. The true angle of the pyramid's shape 

 was determined by means of the remaining casing; as measured by 

 Prof. Smyth, it is 51^ 51' 14.3". The base of the pyramid is a 

 square, each side being seven hundred and sixty-four feet, and the 

 original height of the pyramid was four hundred and eio-hty-sir 

 feet. Mr. John Taylor was the first to discover that th^hei-ht 

 bore the same relation to the sum of two sides of the base as the 

 diameter to the circumference of a circle, carrying the fractions to 

 only two figures, the proportion being thus expressed: 

 486:1528 :: 1 : 3,14. 



LIGHT-HOUSES AT PARIS. 



In the park of the International Exposition is a first class lio-ht- 

 house, and there k another on the banks of the Seine, just a])ove 

 the bridge of Jena. The largest is about one hundred and sixty 

 feet high, and stands on an island in a small lake or pond. The 

 structure is of iron, and contains a staircase with two hundred and 

 fifty steps. The lantern is six feet in diameter and eight and a " 

 half feet high. Its light could be seen at sea from a distance of 

 thirty-six miles. It is to be finally placed on an elevated point of 

 the Roches Douvres, an island between the Isle de Br6hat and 

 Guernsey, about twelve leagues from the port of Portrieux. The 

 mode of illuminating the pharos now generally used, is the dioptric 

 system of the celebrated Augustin Fresnel, who first demonstrated 

 that the undulations, producing light, move in a transverse direc- 

 tion to the line of the ray. This system consists of zones or 



(Inst.] 4I 



