20 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 51 



more and more as it draws near it, and finally as it gathers about 

 the central spot all its forces are resolved into a simple whirl. 

 Ring after ring succeeds, and the whirling action is permanent. 

 The deflecting power thus applied . is not small. The rotary- 

 motion of the earth varies as the cosine of the latitude, and the 

 differences of velocity for any differences of latitude are easily 

 computed. The following are samples; being differences of velocity 

 for i° or 6o£ miles of latitude. 



Miles 

 per hour. 



Between lat. 2° and 3 diff. of velocity 0.79 



Between lat. 3 and 4 diff. of velocity 1.11 



Between lat. io° and n° diff. of velocity 3.31 



Between lat. 23 and 24 diff. of velocity 7.25 



Between lat. 42 and 43 diff. of velocity 12.28 



The differences of velocity for one mile, or 51.84" of latitude 

 are as follows: 



Difference of velocity 

 for 1 mile north. 

 Latitude. Feet per minute. 



io° 4 



23° 9 



42° 15-4 



43° *5-7 



45° 16.3 



The deflection of easterly and westerly breezes by reason of 

 the spherical form of the earth, also, can be computed; and it is 

 obviously no less important than the deflection produced in merid- 

 ional winds. The angle between the courses north and east, at 

 any point, is a right angle; and if two points in the same latitude 

 are taken, it is evident that the obliquity of the north courses 

 from the two points equals the obliquity of the east courses from 

 the same points. 



These results show that in the northern states a fire large enough 

 to affect the atmosphere over a few acres may possess the essential 

 force for generating a whirlwind, and may produce it in fact if 

 the day be calm. A large storm, covering the whole country with 

 its centripetal currents, must produce a vortex about the center, 

 which will combine the principal energies of the storm. The tor- 

 nado and water-spout must revolve with terrific violence. 



The necessary condition, centripetal motion, may arise when- 

 ever a central spot subjected to intense heat is surrounded by a 

 cool atmosphere. This state of things, on a small scale, may occur 

 on a summer day, upon a ploughed field surrounded by extensive 



