THE PEOBLEM OF FALLING BODIES. 319 



non with attention and awakened senses ; and then we find, 

 as soon as we put a tube to the mouth to raise a liquid, that 

 the operation is at first quite easy, but that afterwards it re- 

 quires an amount of exertion which rapidly increases as the 

 column of liquid becomes higher. Is there, perchance, an 

 ascertainable limit to the action of suction ? As soon as we 

 once begin to experiment in this direction, it can no longer 

 escape us that there is a barometric height, and that it attains 

 to about thirty inches. This number is a second chief pillar 

 in the edifice of human knowledge. 



Question now follows question, and answer, answer. We 

 have learned that the pressure exerted by a column of fluid is 

 proportional to its height and to the specific gravity of the 

 fluid ; we have thus determined the specific gravity of the at- 

 mosphere, and by this investigation we are led to carry up 

 our measuring-instrument, the barometer, from the plain to 

 the mountains, and to express numerically the effect produced 

 by elevation above the sea-level upon the height of the mer- 

 cury-column. Such experiments suggest the question, Whether 

 the laws of falling bodies, with which we have become 

 acquainted at the surface of the earth, do not likewise un- 

 dergo modification at greater distances from the ground. 

 And if, as d priori we cannot but expect, this should be really 

 the case, the further question arises, In what manner is the 

 number already found modified by distance from the earth? 

 We have thus come upon a problem the solution of which is 

 attended with many difficulties ; for what has now to be ac- 

 complished, is to make observations and carry out measure- 

 ments in places where no human foot can tread. History, 

 however, teaches that the same man who put the question 

 was also able to furnish the answer. Truly he could do so 

 only through a rich treasure of astronomical knowledge. But 

 how is this knowledge to be attained by us ? 



Astronomy is, without question, even in its first principles, 

 the most difficult of all sciences. We have here to deal with 



