228 PROGRESS OF METEOROLOGY IN 1889. 



"Nowlet tbebirdcbaugebiscouisejturningobliquely upward auclpass- 

 iuginto tbe uppercurreiit. His velocity witb refereuce to tbe air iu wbich 

 beisimmersedisatonceiucreasedfrom Fto F+«. Next let tbe bird wbeel, 

 to tbe rigbt or to tbe left, uutil tbe dircctiou of bis motion is coincident 

 witb tbatof tbe wind. His velocity witb reference to tbe upper curreut is 

 still F+ i, but tbe reversal of bis direction bas cbanged bis relation to 

 tbe currents. He is passing tbe lower and slower current more rapidly 

 tban be passes tbe upper, and bis velocity witb reference to tbe lower 

 curreut is greater by tbeir difference; it is V + 2i. Now let bim de- 

 scend obliquely and enter tbe lower current. His velocity is uot af- 

 fected by tbe transfer. Finally let bim wbeel in tbe lower current until 

 bis direction is once more directly opposed to tbat of tbe wind. Tbe 

 cycle of evolutions leaves him witb tbe velocity V + 2i, referred to tbe 

 lower current, in tbe place of bis initial velocity V, referred to tbe same 

 datum. He bas gained a velocity of 2i, or double tbe velocity of one 

 air current referred to tbe otber, and be bas resumed bis original rela- 

 tion to tbe currents. Manifestly be can repeat tbe process indefinitely. 



"Add now tbat velocity tbus gained is tbe required compensation 

 for tbe velocity lost by friction, and tbe essence of tbe tbeory is stated." 



Wben tbe orbit of tbe bird is circular and lies in an inclined plane 

 rising toward tbe wind, and wben tbe borizontal velocity of tbe air di- 

 minisbes uniformly from the highest i)oint to the lowest i)oint of tbe 

 orbit, the velocity gained by the bird in making the circuit is equal 

 to tbe differential velocity of the highest and lowest layers of air 

 traversed, multiplied by I into the cosine of tbe angle of inclination of 

 tbe idane of the orbit. 



IV.— PHYSICAL PROPERTIES OF THE ATMOSPHERE AND THE OCEAN. 



Height of the atmosphere. — M. Soret criticises the assumption made 

 by Liais in bis method for determining tbe height of tbe atmosphere. 

 Liais found tbat tbe light of tbe sky is still polarized iu a pla-ue passing 

 tiirougb the sun when the sun is 18^ below the borizon, and concluded 

 from this thai tbe suu must have directly illuminated the air particles 

 iu the zenith of the observer. Soret shows tbat by diffusion of the 

 second order a mass of air in the shade may be illuminated by the par- 

 ticles in the sunlight. Mathematical analysis shows tbat iu this case 

 also, tbe light diffused by the mass in sbade is polarized in tbe plane de- 

 termined by the point considered and tbe eye of the observer. Hence 

 tbe results of Liais are subject to tbe error of bis assumption. {Comptes 

 Rendns, cii, p. 103.) 



Specific volume of aqueous vapor. — Dr. Dieterici has presented to the 

 Physical Society of Berlin tbe results of bis researches on the determina- 

 tion of tbe specific volume of saturated vapor at 0° C. His new method is 

 to measure tbe amount of water that must be converted into vapor at 

 0° 0. in order to fill completely a known space with saturated vapor by 





