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



those ingenious instruments which have been devised by Pouillet*. 

 The construction of the direct Pyrheliometer (Plate I. fig. 1) is 

 as follows : — The vessel (aa) is a shallow metallic cylinder, with 

 a blackened surface (b). The diameter of the cylinder is 1 deci- 

 metre, and its height 1*5 centimetre; it can therefore contain 

 150 grammes of water. To the bottom of the vessel a cylinder 

 is appended, within which is a thermometer held fast by a cork ; 

 on the outside of this cylinder is a screw (c), by means of which 

 the instrument may be fastened to a vertical stick. [In this 

 way the stand recommended by Pouillet was superseded.] The 

 double motion of the screw in horizontal and vertical direction 

 enabled us to place the instrument so that the sun's rays always 

 fell perpendicularly upon the blackened upper surface. Of this 

 we convinced ourselves by fixing a pasteboard disc (e) of the same 

 diameter as the cylindrical vessel (aa), near the end of the ther- 

 mometer. When the entire disc was shaded by the vessel at the 

 other end, we might be sure that the rays fell perpendicularly 

 upon the latter ; and by this arrangement the thermometer-tube 

 is protected from the direct action of the sun. The bulb of 

 the thermometer is contained within the cylinder (aa), that is to 

 say, it is directed upwards. Were the air completely removed 

 from the thermometer, on placing it in this position the mercury 

 would flow downwards, and thus render the reading of the instru- 

 ment impossible ; but a small quantity of air, left intentionally 

 in the tube during its preparation, hindered this descent of the 

 mercury, without however invading the exactitude of the results. 

 To impart a uniform temperature to the entire mass of water, 

 the pyrheliometer, during the experiment, is turned round its 

 longer axis ; to permit of this, the screw (c) must not be made 

 too fast. In the filling of the instrument great care must be 

 taken that no air remains in the cylinder, as this would spread 

 itself between the water and the upper metallic plate, and thus 

 modify the quantity of heat received by the water. 



During its exposure to the action of the sun, the instrument 

 loses a quantity of heat by contemporaneous radiation from its 

 surface. This source of disturbance cannot indeed be avoided, 

 but its magnitude may be determined, by observation, for each 

 experiment. The procedure is as follows : — The vessel is first 

 filled with water, to which time is given to assume the tempe- 

 rature of the surrounding air. The instrument is then brought 

 into the vicinity of the spot where it is intended to be exposed 

 to the action of the sun, and so placed in the shade that its 



* Memoire sur la ebalenr solaire, sur les pouvoirs rayonnants et ab- 

 aoxbants de l'air atinospberiqucet sur la temperature de l'espace. — Comptes 

 Rendus, 1838, vol. vii. p. 24-65. Compare ako Hersehel's actinonietcr, 

 in Ksemtz's Treatise on Mineralogy, vol. iii. p. 14. 



