ORIGIN OF THE MISSISSIPPI VALLEY RAINFALL. 105 



the Rocky Mountains, and in running along their eastern slopes 

 are deflected to tlie southeast, and become the northwest winds of 

 the valley of the Mississippi. . . . These cool winds meet the sur- 

 plusage of the moist return trade winds, and by their coolness 

 condense still more the latter's vapor, which descends in rain- 

 storms that are sometimes quite violent, but furnish water for the 

 head streams of the Missouri and its branches" (Prof. Guyot, 

 Earth and Man, p. 100). 



It has been suggested that this warm air, thus saturated with 

 vapor, loses the latter when it floats aloft, because of the cold in 

 the higher regions of the atmosphere, and consequently such air, 

 floating north, could not deposit moisture when it reached the 

 earth. That theory is not consistent with the fact that vapor 

 often becomes visible in the form of clouds, which frequently 

 float higher than the altitude of the Sierra Madre. In this special 

 case it is worthy of notice that the plateau of Mexico is five thou- 

 sand feet above sea-level, and it is also under a tropical sun, and 

 therefore the incumbent air is so much the more heated. In such 

 circumstances the vapor-loaded winds would not be likely to lose 

 so much of their warmth and moisture as under conditions where- 

 in there was no similar elevation. The great valley being free 

 from mountain barriers at both ends, the winds flowing either 

 way are unobstructed. In consequence, the comparatively warmer 

 and vapor-loaded winds off the equatorial current meeting those 

 coming from the north that are nearer the surface and also cooler 

 and drier, the moisture of the former is condensed into mists and 

 clouds, and finally descends to the earth in copious rains. 



One of the most perfectly adapted pieces of machinery for handling heavy 

 weights is the modern "rapid-transit elevated railway traveling crane," which 

 has been found highly useful in manufactories of locomotives and other ponder- 

 ous machinery. Before it was introduced, heavy weights were moved from one 

 part of the shop to another by means of jib cranes, the arms of which swung in 

 arcs of a circle. A series of them occupied the middle of the floor. The weight 

 to be moved was swung upon one of them and borne round to the next, when it 

 was changed; and so on, till it reached its destination. These machines cum- 

 bered the floor, and were otherwise inconvenient. The traveling crane requires 

 no floor room, but is wholly poised above. Jt consists of four essential parts: 

 (1) the elevated tracks, which are supported by iron columns or built into the walls 

 and run parallel with the walls from one end of the building to the other; (2) the 

 traveling bridge, which is constructed of two parallel plate girders extending 

 from rail to rail, spanning in mid-air the breadth of the building and mounted on 

 wheels; and (3) heavy steel tracks laid between the girders, bearing (4) a trolley 

 car, which runs back and forth, carrying the hoisting mechanism. By the longi- 

 tudinal motion of the bridge and the cross-motion of the trolley, every square 

 foot of available space in the building can be covered, and the position of a steam 

 boiler or of a locomotive engine changed at will. 



