NEW OBSEKVATIOXS OF THE PLANKT MERCUnT. 447 
position of the markings has something to snj. AnJ in (liis connection it is dlaiinclJjr 
to be noted that, though in a general way syininetric, the two sidu^ nrc bj nn iiuan» 
the same. The markings are suggestively bilaf oral without Ijcin'^ like. Now wlun 
we take three facts into account: (1) the syinnietry noitli jind «ou(h ; (2) the sym- 
metry east and west; (3) the relation of Testudo regio to the line of iucmh lihmtion 
we have pretty clearly hinted that the planet has turned the ,^nine fui o to Uic $un 
from before such time as the markings were made; and that the markings are due 
to some cause which has since operated upon this face to the exclusion of the other. 
This cause can be none other than the heat received from the Sun. The continuous 
hiding of one hemisphere from the Sun's rays would cause a greater contraction in it 
than in the other in the days when the cooling was going on, and this unequrd cool- 
ing of one side would result in cracks, — cracks which we now see s<ereo<y])ed in 
the markings. This explanation, which has been suggested to me, f^ccms ilic most 
probable one to adopt. Besides accounting for the general fymmetry of tlic mark- 
ings, it explains in addition why they should be lines rather thaji p;itoh(>«. 
2G. Genesis of Isochronism. — We perceive, also, that this theory of their g( m >is 
agrees with what we should expect to find. i?or the present practically air 
condition of Mercury must have resulted very early in its career as a body scpnrnic 
from the primal nebula. In consequence there can since then have been neither 
air nor water upon the planet, — none of the wherewith to cause tides to break its 
rotation. The slowinsr up must therefore have been brought about by Hubslnntinl 
CS8 
o 
tides, — tides, that is, of the whole body, — and therefore have taken plac before 
the planet had solidified. Consequently when the planet was solidifying it \\as 
k 
eady presenting the same face in perpetuity to the Sun, from which crn 
uld inevitably result. We conclude, then, that Mercury is not only a dead 
rid now, but that it has never been anything else. 
27. Surface Visible. — From the rotation period and the libration it nipears that 
we see at one 
five ei"-hths of the planet's surface, and 
three eighths remain forever hidden from view because forever unilluminatcd. 
28. Measures of the Planet's Diameters. - Between the 2d of Septen»ber, 18%, 
and the end of March, 1897, I made 109 sets of meas.ires of the planets diameters. 
These were taken through the phase axis and the diameter perpendicular to it. Each 
three double diameters. The sets were nearly all complete. The 
mean of each set was then expressed in seconds of arc, corrected as m the tables. 
especially for irradiation, reduced to distance unity, ami tabulated. The rcsnlfs fen- 
the polar measures are given in Table IV.; those for the equatorial ni Table \. 
Vol. IL No. 4.-3. 
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