14 BROWN : 



internal contact of Mercury, which is a similar phenomenon." 

 So far Dr. Halley in 17 16. 



Professor Airy, likewise Astronomer Royal, sums up his 

 lecture on the same subject before the working men of Ipswich, 

 in March, 1848, in words which we quote as coming from one 

 having authority and which dispose of our only remaining 

 difficulty, the inexact knowledge of the sun's angular semi- 

 diameter. 



"I may now mention that though the principles of the 

 method are given with perfect correctness in the explanation 

 given above, and though the process must thus be contem- 

 plated in order to enable him to select stations in the most 

 advantageous positions, yet an astronomer's calculation is not 

 made in that form. He proceeds entirely by the method of 

 parallax. The process, strictly speaking, is algebraical, but 

 it may be correctly described in the following manner. He 

 assumes a certain value in seconds tor the sun's horizontal 

 equatoreal parallax and then from the known proportion of 

 the distances of Venus and the sun he computes the horizontal 

 equatoreal parallax of Venus, the parallax being greater as 

 the distance is less. Thus, if the sun's horizontal parallax be 

 assumed at ten seconds and if it be known that at that time 

 the distances of Venus and the sun from the Earth are in the 

 proportion of 28 : 100 (7 ; 25, as above), then he must take 

 the horizontal equatoreal parallax of \^enus at thirty-five and 

 five-sevenths. 



"Then from knowing the Earth's form he computes the 

 horizontal parallax of each at the place of observation ; that 

 is, how much each of them is apparently depressed by paral- 

 lax. Venus being nearer than the sun is apparently more 

 depressed than the sun and is therefore moved by parallax off 

 the sun's limb if she is lower, or upon the sun's limb if she is 

 higher. From this he calculates by a troublesome mathemat- 

 ical process how much the time of \^enus' entering or leaving 

 the limb has been accelerated or retarded by parallax. And 



