1 897-] "Weather." 49 



the upper air currents. At the late international meteorological 

 congress held at Paris it was decided to make cloud measure- 

 ments during the coming year. At each international bureau, 

 daily if possible, two observers at the same instant from fixed 

 points, being at the ends of a measured base line, take observa- 

 tions on the same point of a cloud, and within two minutes 

 again on the same point. This is about the longest time that 

 a given point will remain identifiable. 



In another manner the nature of the upper strata of the 

 atmosphere is being explored by means of kites. They are 

 flown tandem, i.e., several kites on one string or rather fine 

 steel wire, as the latter is far more suitable. Self registering 

 instruments are attached to the kites. 



Of the observed phenomena of the dynamics of the atmos- 

 phere up to the last century none impressed itself so much on 

 physicists as being subject to law as the " Trade Winds." Ex- 

 perience had shown navigators that once they got their ships 

 into the paths of these regular winds, they could be depended 

 on with almost as much confidence as we now place in steam. 

 The primary cause of the atmospheric motions is the unequal 

 distribution of the temperature on the earth's surface produced 

 by the solar heat. We know from observations that there is a 

 large, but not constant, difference in the temperatures of the air 

 at the poles and equator, amounting, at an elevation of but a few 

 feet above the earth's surface, to about 81° F. for the average 

 for the entire year. The heating up of the air at the equator 

 causes its expansion, and consequent increase of bulk, but does 

 not increase its weight or pressure at the earth's surface, it does 

 however elevate the successive isobaric surfaces, i.e. surfaces of 

 equal pressure, and this causes differences of level or gradients, 

 which cannot exist in fluids without a motion ensuing, in the 

 direction of the lowest level. The levels of these gradients are 

 measured with our atmospheric sounding rod, the barometer. 



