. The transit of Venus. when the black drop breaksoff and a flood of light sweeps in between the planetand the sun. This occurs very suddenly, and has beensupposed to indicate the true time of actual contact. By referring to the Philosophical Transactions of1769-70, a large number of descriptions of the phe-nomenon may be read. Some of the appearances areshown in Fig. 15, they are copied from the originalsby Bevis, Hirst, Bayley, and Mayer, respectively—Prof. Grant states that the last one bears a resemblanceto the appearance of Mercury as seen during its transitin. 1868 from the Glasgow Obser
. The transit of Venus. when the black drop breaksoff and a flood of light sweeps in between the planetand the sun. This occurs very suddenly, and has beensupposed to indicate the true time of actual contact. By referring to the Philosophical Transactions of1769-70, a large number of descriptions of the phe-nomenon may be read. Some of the appearances areshown in Fig. 15, they are copied from the originalsby Bevis, Hirst, Bayley, and Mayer, respectively—Prof. Grant states that the last one bears a resemblanceto the appearance of Mercury as seen during its transitin. 1868 from the Glasgow Observatory, the sun being-near the horizon. 1 Monthly Koikes, xxix. p. 17, &c. IV.] THE TRANSIT OF VENUS. 49 In the case of that transit of Mercury, studied bysix experienced observers at Greenwich Observatory,two curious facts appear. Firstly, the times of contactas determined by different observeis vary to the ex-tent of 13^ seconds. And secondly, the shape of theplanet varied considerably with different Fig. 15.—Tne black drop, as observed in 1769. Mr. Stone having noticed a confusion in the lan-guage of the astronomers of the last century as towhich of the two phases was observed, carefullyre-studied their words ; and by supposing the twophases to be separated by a constant interval of time,he utilised both kinds of observation. This constant E 50 THE TRANSIT OF VENUS. [chap. interval of time was deduced from all the observations,and found to be about \J seconds. In this mannerhe arrived at the more accurate value of the sunsparallax. It has been asserted that astronomers claim unduecredit for the accuracy of their measurements, sinceEncke made an error of three or four millions of milesin the calculation of the suns distance. This is notso. A chemist may be able to weigh many substanceswith an error of j-Jq- per cent, or less ; but if the sub-stance to be weighed be only T^ of a milligramme,he might have a larger percentage error. When weconsider how extremely
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