DRIFTING LIGHT- WA VES. 95 



adjustments of the instrument a cause of error against 

 which precautions are certainly very necessary. He satisfied 

 himself that when sufficient precautions are taken no dis- 

 placements take place such as Lockyer, Young, and others 

 claimed to have seen. But he submitted the matter to a 

 further test As the sun is spinning swiftly on his axis, 

 his mighty equator, more than two and a half millions of 

 miles in girth, circling once round in about twenty-four 

 days, it is clear that on one side the sun's surface is swiftly 

 moving towards, and on the other side as swiftly moving 

 from, the observer. By some amazing miscalculation, Secchi 

 made the rate of this motion 20 miles per second, so that 

 the sum of the two motions in opposite directions would 

 equal 40 miles per second. He considered that he ought 

 to be able by the new method, if the new method is 

 trustworthy at all, to recognize this marked difference 

 between the state of the sun's eastern and western edges ; 

 he found on trial that he could not do so ; and accordingly 

 he expressed his opinion that the new method is not trust- 

 worthy, and that the arguments urged in its favour are 

 invalid. 



The weak point in his reasoning resided in the cir- 

 cumstance that the solar equator is only moving at the 

 rate of about i\ miles per second, so that instead of a 

 difference of 40 miles per second between the two edges, 

 which should be appreciable, the actual difference (that is, 

 the sum of the two equal motions in opposite directions) 



lounts only to -2,\ miles per second, which certainly 

 secchi could not hope to recognize with the spectroscopic 



jwer at his disposal. Nevertheless, when the error in his 

 jning was pointed out, though he admitted that error, 

 ie maintained the justice of his conclusion ; just as Cassini, 



iving mistakenly reasoned that the degrees of latitude 

 should diminish towards the pole instead of increasing, and 



iving next mistakenly found, as he supposed, that they do 

 linish, acknowledged the error of his reasoning, but 



isisted on the validity of his observations, maintaining 



