10 Dr. H. Schlagintweit's Observations in the Alps 



racteristic localities. The difference between the temperature of 

 the air and a thermometer placed in long grass is assumed to be 

 1000; the difference between the temperature of the air and the 

 radiation thermometer in the remaining positions is expressed in 

 parts of the above number. The thermometer used to determine 

 the temperature of the air was placed four feet above the ground. 



Relative Powers of Radiation of different Bodies*. 



2 feet above the points of the "I 



Long grass 1000 



Short grass , 8/0 



An inch above the ground. 



Covered with grass 209 



Onthe ground under longgrass 66 

 On the ground under short grass 200 



1 inch f above the points of the) /-yi 

 grass / 



2 inches above the points of \ r-Q 

 the grass J 



3 inches above the points of \ ,-7 

 the grass J 



6 inches above the points of\go 

 the grass J" 



1 foot above the points of the \ , 9 g 

 grass J 



r 86 



grass J 



4 feet above the points of the "1 ™ 



grass J 



6 feet above the points of the "I rn 



grass J 



8 feet above the points of the ) -i « 



grass / 



12 feet above the points of the 1 , . 



grass } 



Garden earth 472 



Gravel 288 



River sand 454 



On stone 3!)0 



White sheep's wool 821 



In the focus of a parabolic me- \ aro 



tallic mirror / 



Transparency. 



The absorption of the rays of light by the atmosphere varies 

 also with the ratification of the latter. For a luminous point in 

 the zenith seen from the earth's surface, the absorption amounts 

 to - 80 of the brightness which would reach us did no absorp- 

 tion by the atmosphere take place J. Many investigations on 

 the brightness of the stars can be regarded as excellent determi- 

 nations for the transparency of the atmosphere §; in general, 



from various bodies placed on or near the surface of the Earth." By James 

 Glaisher, Esq., of the Royal Observatory, Greenwich. Phil. Trans. Lond. 

 1847, part 2, pp. 11.9,217. 



* Glaisher in the place cited, tab. 45, p. 147- 



t Inches and feet are here English measure. 



X This number is the mean of the results of Bouguer and Seidel : "Erste 

 Resultate Photometrischer Messungen am Sternhimmel." — Miincher Gel. 

 Anzeig, July 2, 1846, No. 131, p. 18. The two measurements were made 

 by quite different methods ; their close approximation therefore renders 

 them the more deserving of confidence. Seidel found 0'78, Bouguer - 81. 



§ Compare Humboldt's astrometer and his experiments therewith. — Voy. 

 ed. 8vo, vol. iv. p. 32 and 287- Steinheil's measurements of brightness. — 

 Denkschriften der Miincher Acad., 1837, p. 1-142. Herschel in Schuma- 

 cher's Astr. Nachr., vol. xvi. No. 372, p. 190. Seidel, as cited above. 

 Compare also the earlier experiments of Bouguer, Optici de diversis lu- 

 minis gradibus dimetiendis 4. Vienna?, 1762; and Lambert's Photometria. 

 An elaborate collection of the proper instruments and methods of observa- 

 tion in Herschel on Light; II. Photometry, § 17-87. 



