Dr. J. R. Mayer on the Mechanical Equivalent of Heat 495 



In carrying out this experimental investigation, various diffi- 

 culties have to be contended with ; but these must and can be 

 overcome; and then the truth is arrived at, that for every body 

 a fall of 16 feet, or a time of descent of one second, corresponds 

 to a final velocity of 32 feet per second. 



A second phenomenon of daily occurrence, which is in appa- 

 rent contradiction to the laws of falling bodies, is the ascent of 

 liquids in tubes by suction. Here, again, the rule applies, not 

 to allow the maxim, velle rerum cognoscere causas, to lead us into 

 error through useless and therefore harmful speculations con- 

 cerning the qualities of the vacuum, and the like; on the con- 

 trary, we must again examine the phenomenon 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 requires an amount of exertion 

 which rapidly increases as the column of liquid becomes higher. 

 Is there, perchance, an ascertainable limit to the action of suc- 

 tion ? As soon as we once begin to experiment in this direc- 

 tion, it can no longer escape us that there is a barometric height, 

 and that it attains to about 30 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 pro- 

 portional to its height and to the specific gravity of the fluid.; 

 we have thus determined the specific gravity of the atmosphere, 

 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 mercury-column. Such expe- 

 riments suggest the question, Whether the laws of falling bodies, 

 with which we have become acquainted at the surface of the earth, 

 do not likewise undergo modification at greater distances from 

 the ground. And if, as apriori 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 accom- 

 plished, is to make observations and carry out measurements 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 

 objects and spaces which forbid all thought of experiment, 



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