NATURAL PHILOSOPHY. 209 



the solar spectrum, the rays of terrestrial origin, are much more nu- 

 merous and important than the rays of solar origin. He mentions, 

 also, an experiment made on the Lake of Geneva, by which he deter- 

 mined the presence of these bands in the flame of a (irwood fire at a 

 distance of about L'> miles. This ilame gave no band when exam- 

 ined near. M. Janssen also states, that his experiments prove that 

 the vapor of water dissolved in air, that is, in the state of an clas- 

 tic fluid, performs an important part in the production of the phe- 

 nomena ; and he considers that the results of the researches of Huggins 

 and Miller confirm his own ideas. 



DURATION OF TWILIGHT AND HIGIIT OF THE ATMOSPHERE. 



Hcrr Julius Schmidt, of the Athens Observatory, communicates to 

 Astronomisfhe Nachriehteii a series of observations to determine the du- 

 ration of twilight, and the hight of the atmosphere. He arrives at the 

 conclusion that the minimum hight of the atmosphere ranges from 7 '5 to 

 10'5 geographical miles (15 to an equatorial degree?), and that it 

 varies with the time of year, being highest in November, December, 

 January, and lowest in May, June, July. The depression of the sun 

 necessary for the close of twilight is not constant, the ordinary reck- 

 oning of 18 being correct only in extreme cases. 



NOVEL USE OF POLARIZED LIGHT. 



M. Chacornac the French astronomer states, that while recently 

 watching small cumulus clouds that formed themselves soon after sun- 

 set, he observed them appear as bright spots seen on a dark ground, 

 or as dark spots on a bright ground, according to whether he shut out 

 the polarized rays, or let them reach his eye. From this it appeared 

 that the proportion of polarized light was sufficiently great, that dark 

 spots became luminous through the action of the analysing prism, 

 and thus, passing from the condition of negative to that of positive 

 vision,* could be seen further off. The region of maximum atmospheric 

 polarization he states to be near the horizon about 78 from the sun, 

 it there amounts to about -3-fa of the total light which the atmosphere 

 reflects. He suggests that by adding an analysing prism to the eye- 

 piece of a telescope, distant capes at sea, and other objects, might 

 become visible, just as he was able by such means to see alpine sum- 

 mits after they have been completely screened from ordinary vision 

 by a light fog. 



SIMPLIFICATION OF THE COMMON PUMP. 



A modification of the ordinary suction pump has recently been pa- 

 tented in France, which seems, for many purposes at least, to recom- 

 mend itself strongly by its simplicity, and other good qualities. The 

 common pump consists, as is well-known, of a barrel, a piston, and two 

 valves opening upwards ; the pump to which we direct attention, of 

 only a barrel, a solid plunger, and a single valve. The former pump 

 is exposed to great wear and tear, and very considerable friction, from 

 the necessity for the sucker fitting the barrel exactly ; the latter is 



* Negative vision is when a dark object is seen, not by its own light, but by con- 

 trast with the li^ht surrounding it. Positive vision is when an object is visible 

 by the liffht which it emits or reflects. 



18* 



