B. TERRESTRIAL PHYSICS AND METEOROLOGY. 81 



shown by Hann, Mohn, and others. In general, the difference 

 of temperature between the upper and lower stations is 

 greater in clear than in cloudy weather. Zeitschrift fur 

 Meteorologie, Vienna^ X., 296. 



METEOROLOGY IN THE 17tH CENTUKY. 



Caspari gives, in the Revue 3Taritime et Coloniale^ an in- 

 teresting review of a work published at the Hague by Vlacq 

 in 1663. The work in question is by Vossius, " De Motu 

 Marium et Ventorum." In this book the theory of the tides 

 takes an important place, and after it the winds are treated 

 upon in a manner showing the state of physics and chemis- 

 try in that century. The fable of the four elements is re- 

 jected by Vossius ; the air is considered by him as being 

 simply rarefied water, the principal source of which is found 

 in the ocean. In reference to atmospheric theorems, the 

 author indicates that observation has shown to him a re- 

 markable correlation between the movements of the barome- 

 ter and the gusts of wind. "For," says Vossius, "if the 

 barometer is low and tends to rise, the danger is past. It is 

 when it is very high that we know, on seeing it fall, that the 

 tempest is coming. The amplitude of the movements of the 

 mercury is in proportion to the violence of the wind." Our 

 author, therefore, at so early a date as this, proposed to em- 

 ploy the barometer on board of a vessel for the prediction 

 of the weather. In 1660, Otto von Guericke had remarked 

 that a considerable barometrical depression corresponded 

 generally with an atmospheric disturbance ; but the invent- 

 or of the air-pump did not thoroughly nnderstand the theory 

 of the movements of this instrument ; and we are forced to 

 believe that practically Isaac Vossius should have the credit 

 of first formulating this useful law, viz., that it is the di- 

 rection of the movements at any instant, and not the simple 

 readini^s of the heisfht of the barometer, which should 2:uide 

 us in our predictions of the weather. Revue Maritime et 

 Coloniale^ 1875,330. 



THE RESISTANCE OF THE AIR TO THE MOVEMENTS OF 



PROJECTILES. 



An important subject in artillery practice is the resistance 

 of the air to the movement of the projectile a question that 



D -1 



