32 



SCIENCE. 



[N. S. Vol. I. No. 



A description of Gascoigne's micromctei- was 

 published in the Philosophical Transactions 

 in 1667, and a little before that a similar 

 instrument had been invented by Auzout iu 

 France, but observatories were fewer then 

 than now, and so far as I know J. D. Cassini 

 was the onlj^ person beside Flamsteed who 

 attempted to determine the solar parallax 

 fi-om that opposition of Mars. Foreseeing 

 the importance of the opportunity, he had 

 Kicher dispatched to Cayenne some months 

 previously, and when the opposition came 

 he effected two determinations of the paral- 

 lax ; one being by the diurnal method, from 

 his own observations in Paris, and the 

 other by the meridian method fi-om ob- 

 servations in Fi-ance by himself, Romer 

 and Picard, combined with those of Kicher 

 at Cayenne. This was the transition from 

 the ancient instruments with open sights 

 to telescopes armed with micrometers, and 

 the result must have been little short of 

 stunning to the seventeenth century as- 

 tronomers, for it caused the hoarj^ and gi- 

 gantic parallax of about 180 seconds to 

 shrink incontinentlj' to ten seconds, and 

 thus expanded their conception of the solar 

 system to something like its true dimen- 

 sions. More than fifty years previously 

 Kepler had argued from his ideas of the 

 celestial harmonies that the solar parallax 

 could not exceed 60 seconds, and a little 

 later Horrocks had shown on more scientific 

 grounds that it was probablj' as small as 14 

 seconds, but the final death-blow to the 

 ancient values ranging as high as two or 

 three minutes came from these observa- 

 tions of Mars by Flamsteed, Cassini and 

 Richer. 



Of course the results obtained in 1672 

 produced a keen desire on the part of as- 

 tronomers for further evidence respecting 

 the true value of the parallax, and as Mars 

 comes into a favorable position for such in- 

 vestigations only at intervals of about six- 

 teen years, they had recourse to obsei-vations 



of Mercury and Yenus. In 1677 Ilalley 

 ol>served the diurnal parallax of Mercury, 

 and also a transit of that planet across the 

 sun's disk, at St. Helena, and in 1681 J. D. 

 Cassini and Picard observed Venus when 

 she was on the same parallel with the sun? 

 but although the obsei-vations of Yenus 

 gave better results than those of Mercury, 

 neither of them was conclusive, and we now 

 know that such methods are inaccurate 

 even with the powerful instruments of the 

 present day. Nevertheless, Halley's attempt 

 by means of the transit of Mercury ulti- 

 mately bore fi-uit hi the shape of his cele- 

 brated paper of 1716, wherein he showed 

 the peculiar advantages of transits of Yenus 

 for determining the solar parallax. The 

 idea of utilizing such transits for this pur- 

 pose seems to have been vaguely conceived 

 by James Gregory, or perhaps even by 

 Horrocks, but Halley was the first to work 

 it out completely, and long after his death 

 his paper was mainly instrumental in induc- 

 ing the governments of Europe to undertake 

 the observations of the transits of Yenus in 

 1761 and 1769, from, which our first accu- 

 rate knowledge of the sun's distance was 

 obtained. 



Those who are not familiar with practical 

 astronomjr niay wonder why the solar par- 

 allax can be got fi-om Mars and Yenus, but 

 not fi-om Mercury, or the sun itself. The 

 explanation depends on two facts. Firstly, 

 the nearest approach of these bodies to 

 the earth is for Mars 33,870,000 miles, 

 for Yenus 23,651,000 miles, for Mercury 

 47,935,000 miles and for the sun 91,239,000 

 miles. Consequently, for us Mars and 

 Yenus have verj' much larger parallaxes 

 than Mercury or the sun, and of coiirse the 

 larger the parallax the easier it is to meas- 

 ure. Secondly, even the largest of these 

 parallaxes must be determined within far 

 less than one-tenth of a second of the truth, 

 and while that degree of accuracj' is possible 

 iu measuring short arcs, it is quite unat- 



