86 LECTURES 



The number of primary planets known to astronomers "has been 

 quadrupled within the last few years. Instead of the five planets 

 known to Ptolemy and Copernicus, and even to Newton, we now have 

 upwards of sixty. Scarcely a month passes that does not add some- 

 thing to the aggregate of our knowledge of the heavens — that does 

 not reveal some new wonder to the telescope, or disclose some new 

 treasure to the far-reaching analysis of the mathematician. Never 

 could it be said with a profounder meaning than now that " the hea- 

 vens declare the glory of God." 



The mechanism of the heavens, in proportion as we comprehend 

 more and more of its vastness and seeming complexity, bears witness 

 to the enduring order and harmony of the universe, and points with 

 unerring certainty to the superintending agency of an intelligent and 

 infinite -creator. 



In the course of our discussions we shall have occasion not only to 

 state the results of investigation, but to witness the struggles and 

 triumphs of the human mind in working its way to the solution of 

 some of the great problems of science. 



We spontaneously pay the tribute of our homage to all great 

 achievements. But in no case is homage more just or more enduring 

 than that which all cultivated minds pay to him who stands as the 

 minister and interpreter of Nature, and makes known to us her laws 

 and her mysteries. Many such adorn the annals of astronomy. 



But I must not, in these preliminary remarks, encroach upon the 

 time which belongs to the special subject of the evening, which is — 



Tlie figure and magnitude of the earth. 



This may be called the great problem of astronomy. Its import- 

 ance is apparent from this, that the radius of the earth is the unit of 

 measure — I may say the measuring rod — which we are compelled to 

 use in determining the dimensions of the solar system. Whatever 

 error obtains in our knowledge of this will be repeated and multiplied 

 in all our estimates of jjlanetary distances. Hence the importance 

 which attaches to this problem, and the great pains which has been 

 taken to solve it with the utmost accuracy. 



That the general figure of the earth is spherical was an opinion held 

 by many of the ancient astronomers. This was obvious from the 

 manner in which vesfecls disappear at sea. It was also indicated by the 

 circular shadow of the earth upon the moon in the case of eclipses. 

 In contemplating the extent of countries, and the distance of one 

 country from another, the question of the earth's magnitude would nat- 

 urally — almost necessarily, be suggested. It would be useless to 

 attempt to trace the successive steps in the progress of this inquiry 

 among the ancients. Aristotle relates that the mathematicians prior 

 to his time had found the circumference of the earth to be 400,000 

 stadia. But the length of the Grreek stadium is unknown, so also is 

 the method by which this result was obtained. The fact mentioned 

 by Aristotle is only of interest as showing that the question had en- 



