26 THE POPULAR SCIENCE MONTHLY 



a new series of researches opens up ; we descend to the study of details 

 and try to unravel all the complexities of natural phenomena. To 

 enforce this point we may say that in our atmosphere every local weather 

 phenomenon results from the interaction of the following seven forces : 



1. The diurnal rotation of the earth on its axis. 



2. The annual revolution of the earth in its orbit. 



3. The attraction of gravitation holding the atmosphere to the 

 earth. 



4. The centrifugal force resulting from the rotation of the earth on 

 its axis, and due to the inertia of the moving masses of air. 



5. The molecular forces known as heat, light, chemism, electricity 

 and radiant energy received by radiation from the sun with all the varia- 

 tions depending upon latitude, diurnal rotation and annual revolution. 



6. The loss of heat by radiation from the earth and atmosphere. 



7. The irregular expansions due to the irregular distribution of heat 

 in the atmosphere which depends on the distribution of continents and 

 oceans, and the presence of an easily condensable vapor like steam mixed 

 with the permanent gases, nitrogen and oxygen that form the great 

 mass of the atmosphere. 



You will see from this brief and partial enumeration that we have 

 to do with very complex combinations of phenomena, and that the re- 

 sults must vary with every slight variation in any one of these forces. 

 Worse than this, we have not yet been able to observe or investigate the 

 boundary between our atmosphere and the illimitable planetary space 

 beyond. "We know not whether gaseous particles are being added to and 

 removed from this outer boundary. We know not whether our outer 

 layer of atmosphere experiences any resistance from the cosmic ether 

 as the earth rushes along in space. There are many speculations as to 

 the origin of the earth's atmosphere ; not only do these belong to geology, 

 cosmic physics or cosmology, but they also lie at the foundation of 

 meteorology. In the present state of our knowledge these are merely 

 speculations; dynamic meteorology passes them by in silence and as- 

 sumes that the atmosphere is now unchangeable as to its composition 

 and mass. But who knows how soon the day will come when we shall 

 have to recognize that a change has taken place ! From this point of 

 view I should say that in logical order the first problem for future study 

 bears on the condition of the outer layer of our atmosphere, and in fact, 

 my predecessors in this course of lectures, Professor E. S. Woodward, of 

 Washington, and Professor J. H. Jeans, of Princeton, have already 

 touched upon this question. More than that, Dr. C. C. Trowbridge, of 

 Columbia University, has brought together many interesting facts re- 

 lating to the trains left behind by meteors or shooting stars as they rush 

 through the upper air. These meteors become visible at altitudes as 

 high as 120 miles, showing that at that elevation there are obstacles of a 



