October 1, 1897.] 



KNOWLEDGE. 



235 



Generally we perceive nothing ; a dazzling whiteness, 

 but no spots at all — the very opposite of what happens in 

 the case of the moon, Mars, or Jupiter. Even where, 

 by some extraordinary chance, the observer does perceive 

 some marking, it is altogether undefined, doubtful, faint, 

 and in many cases evanescent. If we are convinced of 

 the reality of some marking or other, and make a sketch 

 of the disc, it often happens that in a few hours, or one or 

 two days, a week, a fortnight or so, we may again observe 

 practically the same configuration. We take the drawings, 

 which seem to be in agreement more or less, and we fancy 

 we can trace in them the rudiments of a Cytherean chart. 

 (Thus a new map of Venus is published on an average 

 every ten years.) In the same way, another astronomer 

 makes other drawings which are in as good agreement with 

 each other, but which very often do not at all resemble 

 the first series. 



Fia. 2. ^Further Sketches of Venus bv Cassini, as preserved by Ms son, 

 April 2l8t,, '1667. 



Similarly, if we compare drawings made at different 

 epochs — say, the drawings of last century — with those 

 made at the present time, they bear no resemblance. In 

 1726, Bianchini, working under the bright Italian sky, 

 made a number of drawings, and traced a map with 

 continents and seas ; and these configurations the author 

 considered sufficiently certain and stable for nomen- 

 clature, notably the regions Galileo, Columbus, Vespucci, 

 and King Emmanuel. If we look for these configurations 

 in modern illustrations of Venus, we shall look 

 in vain ; whereas the general aspects of the 

 moon. Mars, or Jupiter may be readily identified 

 even in the oldest drawings. 



Thus, in the first place, there is this essential 

 difference between Venus and the other planets, 

 that nothing sure, stable, or perpetual is discover- 

 able upon its disc ; while, on the other hand, the 

 conditions under which Venus can be observed 

 from the earth, the absorption by its atmosphere, 

 and the clouds which must overspread it con- 

 tinually, effectually prevent us from distin- 

 guishing, except on rare occasions, any of its 

 surface markings. If we carefully examine the 

 observations which have been made for more 

 than two centuries, to determine the rotation of 

 Venus, we find them insufficient for any definite 

 result. We will briefly recall them here. 



(1.) The first interesting observations con- 

 cerning the configui'ations and rotation of the 

 planet Venus were made by Domenico Cassini, at 

 Bologna, in 1666. On the 14th of October of 

 that year, at 5.45 p.m., he perceived a bright 

 spot near the terminator and two elongated dark 

 spots near its western limb. That was all he 

 noted in this year, for afterwards he was un- 

 successful in his attempts to view the planet. It was not 

 until the 20th of April following, that, a quarter of an 

 hour before sunrise, he once more beheld, upon the other 

 half of the Venus hemisphere — viz., the eastern half, 

 then visible — a dark patch, elongated and pointed in 

 shape, and also a brilliant spot near the terminator. 



lower down or south. A quarter of an hour later, 

 just at sunrise, the bright spot had become sensibly 

 displaced from the sout/i to the north. On the morrow 

 (21st of April), at sunrise exactly, he again saw the 

 spot, and again displaced from south to north as on the day 

 before. In rising it reached the centre of the disc, about 

 thirty-six minutes after sunrise. Cassini saw it again on 

 the 9th, 10th, and 13th of May and on the 5th and 6th of 

 June ; but no more observations are recorded in his life- 

 time : in fact, no observations at all until the year 1726. 

 From these observations Cassini concluded that Venus 

 possesses either a motion of liliration, or of rotation from 

 soutJi to north. 



We give Cassini's diagrams and the record of his 

 observations in his own words. ■■ 



The sketches (1, 2, and 3) are, for the first, dated 

 the 14th of October, 1666, and for the others, the 28th 

 of April, 1667, a quarter of an hour before, 

 and at, sunrise respectively. " I will not ven- 

 ture," says he, "to express my opinion on 

 these phenomena as boldly as I did on the 

 sjots on Jupiter and Mars. Yet I may say 

 (supposing this shining spot of Venus has 

 always been the same), that the planet com- 

 pletes its movement of either revolution or 

 libration in less than one daij, so that in 

 twenty-three days, approximately, it shows 

 the same aspect." 

 Thus, according to Cassini, if there be any rotation, it 

 should be in the direction from south to north, and be 

 completed in less than twenty-four hours. 



Seventy-three years later, f his son, Jacques Cassini, by 

 way of explanation of his father's observations, added to 

 these three drawings those of the 21st of April, which do, 

 indeed, very clearly show the displacement of the white 

 spot. These we also reproduce here. 



Now, nothing is less sure than that dark streak and that 



Fig. ;!.-IIhistrations of Venus by Bianchini, February ISth and 50th, 172P. 



white spot. The former, most likely, is merely an optical 

 effect ; indeed, Cassini himself attached no importance to 



* From the Journal den Saimnts (or 1607, p. 2B5. I have repro- 

 duced the original sketch, though much defaced. No exact copy of it 

 has ever been published, eitlier by liis son, or in the works of 

 Bianchini or Schroeter. 



t Element d'Asfronomie, 1740, p. .511. 



