Alterations in the Light of the Sun* 271 



siderable work, that the experimentalist should first do a 

 small piece, and let it stand with the top only covered for a, 

 winter at least. 



It has before been observed that the excellence of the 

 work depends on its having due compression, as well as be- 

 ing of proper soil. If the compression be not perfect, aU 

 though the soil be good, the walls will be unsound; and 

 unfortunately it so happens, that when a wall is built and 

 badly rammed, its imperfection cannot readily be observed, 

 and further, the defect is likely only to be found but by 

 its failure : and hence arises the greatest bar to its general 

 introduction ; for, as it requires considerable labour to build 

 a wall, it requires exertion to do it in proper season ; and if 

 the labourer be employed to do the work by task, it be- 

 comes his interest to get on and do it slightly, and if done 

 by day, it will not advance so rapidly : consequently, in 

 either way, it will require great attention from a careful 1 

 overlooker. 



From the foregoing comparative statement of pise against 

 brickwork, persons unacquainted with building are inclined 

 to suppose that the whole expense of the building will be in 

 proportion thereto : contrary to this, it only affects the wall- 

 ing, — the roof, floor, &c. remaining the same as before, 

 excepting as it may reduce the quantity of bond timber and 

 lime used in plastering the inside ; this latter is less than 

 when plastered on brickwork, the face of the wall being so 

 much truer than brickwork. 



A working drawing, on a scale of one inch to a foot, is 

 left with the Society, for the inspection of any person in- 

 clined to construct the apparatus. 



LII. Memoir on the Alterations which the Light of the 

 Sun undergoes on passing through the Atmosphere. Bif 

 M. Hassknfratz. Read to the Class of Mathematics* 

 and Fhijsics of the French Institute, 20th October 1806\* v 



J. HE sun presents different colours to our eyes : its disk 

 appears white, yellow, orange, or red, according to the 

 purity of the air, the height of the orb in the horizon, the 

 latitude of the places where we observe it, and their eleva- 

 tion above the level of the sea. 



In the torrid zone, the disk of the sun is always white, when 

 the air is pure, and when it is at the zenith of the place. 



In the irigid zone, the disk of the rising or setting sun is 

 always red in the shortest days of the year.. 



* Annates de Chiinie, tome btvi. p. 54./ 



On 



