432 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1908. 



shall have a datum point from which we may judge. To this, science 

 has in fact been able to supply an answer within certain limits. 



The starting point of the demonstration is the law of universal 

 gravitation. You will remember that the English scientist Newton 

 was the first to recognize the influence of that force in the making of 

 the world. Gravitation holds the earth together, holds us fast to its 

 surface, determines what is " up " and " down." As the child's 

 strength increases it must learn to stand erect in opposition to gravi- 

 tation; with advancing years the old man feels more and more its 

 down-pulling force. It compels the moon to describe its orbit around 

 the earth ; it determines the movement of the planets around the sun. 

 Summing up his observations, Newton was led to the conclusion that 

 all matter without exception is mutually attractive, no matter what 

 may be the size of its particles or the distance between them. Accord- 

 ing to his law, the smaller the mass, and the greater the distance, the 

 less is the attraction ; but it never becomes zero. We are thus driven 

 to the conclusion that the various objects which we encounter on the 

 surface of the earth also attract each other. Is this true ? In daily 

 life, indeed, the effect is not perceptible. Your thoughts may at this 

 moment be turning to the electrical and magnetic phenomena in whicli 

 attraction is distinctly observed ; but these do not belong here ; they 

 are the effects of forces quite different in their nature from gravita- 

 tion. In point of fact, by means of delicate instruments it has been 

 possible to demonstrate and measure the mutual attraction of the 

 objects that surround us. If a body, say a metal globe or any other 

 object, be so suspended as to be protected against disturbance, while 

 the thread that holds it offers a minimum of resistance, and if there- 

 upon another body, say a lead weight, be made to approach it, the 

 suspended body will be seen to move toward the approaching body, 

 and thus fall toward it, even though slowly, just as the apple, de- 

 tached from the tree, falls to the earth. As the approaching body is 

 small in comparison with the earth, it is to be expected that some 

 minutes will elapse before the suspended body shows a movement of 

 even a few millimeters, while the apple falling to the earth passes 

 through a distance of 5 meters in the very first second. The observa- 

 tion of the mutual attraction of surrounding bodies is a task to which 

 physicists have devoted much attention and on which they are still 

 constantly engaged. Numerous observers have spent on this problem 

 all their labor and ingenuity for years, and have used all the available 

 resources of micro-mechanics. In this way we have at last arrived at 

 a very accurate estimate of universal gravitation and are able to state 

 with precision the force of attraction exerted on each other by two 

 bodies measured in grams or kilogi'ams. Now these are the observa- 

 tions which may be employed in estimating the mass of the earth. 

 We know directly the force with which the earth attracts bodies on 



