NATURAL PHILOSOPHY. 127 



and if one term of it (the velocity of light) must be diminished three 

 per cent, to'suit Foucault's experiment, then we must at the same time 

 diminish the other term (the velocity of the earth) proportionally ; 

 and the old ratio will be preserved, and the value of aberration will be 

 left unchanged. Is it possible, therefore, that there can be an uncer- 

 tainty to the extent of three per cent, in the velocity of the earth ? If 

 so, the tables are turned ; and, instead of employing the ratio which 

 aberration supplies to calculate the velocity of light from the velocity 

 of the earth, as the best known of the two, we henceforth must calcu- 

 late the velocity of the earth from the velocity of light. For Foucault 

 has found the latter by experiment more accurately than astronomy 

 gives the former. If there is an error of three per cent, in the velocity 

 of the earth, it is an error in space and not in time. To diminish the 

 velocity of the earth sufficiently by a change of time would demand an 

 increase in the length of the year amounting to eleven days nearly. 



The only other way of reaching the velocity of the earth is by dimin- 

 ishing the circumference of the earth's orbit, and this, if diminished, 

 changes proportionally the mean radius of the orbit ; that is, the sun's 

 mean distance. The question, therefore, resolves itself into this : Can 

 the distance of the sun from the earth be considered uncertain to the 

 extent of three per cent, of the whole distance 1 



The answer to this question leads to a discussion of the processes by 

 which the sun's distance from the earth has been determined, and the 

 limits of accuracy which belong to the received value. To know the 

 sun's distance, the astronomer studies the solar parallax, which is the 

 angle between the directions in which two astronomers, located at op- 

 posite extremities of the earth's diameter, point their telescopes when 

 they are looking at the sun at the same moment. 



As Kepler's third law establishes a relation between the distances of 

 the different planets from the sun, and their periods of revolution, if th^ 

 astronomer finds either distance by observation, the others can be com- 

 puted from this law. As the solar parallax is only about eight seconds, 

 and an error of one-tenth of a second includes an error of more than a 

 million of miles in the sun's distance, he takes advantage of the law of 

 Kepler, and selects a planet which comes occasionally nearer to the 

 earth than the sun. The choice lies between Venus at inferior con- 

 junction and Mars at opposition. 



What are the results which have been obtained ? 



1. Only two transits of Venus have occurred since the time when 

 the sagacious Dr. Halley invoked the attention of posterity to these 

 rare, astronomical events, as pregnant with the grandest results to 

 science, namely those of 1761 and 17G9. The astronomers of the last 

 century did not neglect the charge which Halley consigned to them. 

 The transit of 1769 was eminently favorable, offering a chance which 

 comes only once in a millennium ; and whatever verdict posterity shall 

 pronounce on the deductions from the observations then made, they 

 will never, says Encke, reproach astronomers or governments with neg- 

 ligence or want of appreciation toward this golden opportunity. The 

 solar parallax which Encke deduced from an elaborate discussion of all 

 the observations, fifty years after they were made, is S".57116. This 

 corresponds to a solar distance of 95,360,000 statute miles. 



Although transits of Venus will take place in 1874 and 1882, and 



