364 OF THE phænomena and affections

pected by former astronomers. Mayer’s lunartables, in particular, are considered as extremelyaccurate; for being compared with above twohundred observations, as well ancient as modern,they were seldom found to differ above a minutefrom the truth.

But as the theory of the moon’s motion istoo intricate to admit of a popular illustration,I shall leave this subject for the present, and pro-ceed to give you some account of the rest of theplanets. Mercury, as I have already mentioned,is too near the fun to be often seen ; but Venus,being higher in the system, and more easy tobe observed, is found to be diversified with spots,and to have all the various phases and ap-pearances of the moon. Mountains and vallieshave also been discovered in this planet, bymeans of good instruments; and from the mo-tion of her spots it is determined, that flie re-volves round her axis from west to east in thespace of about twenty-three hours.

The face of Mars, on the contrary, is alwaysfound to be round and full, as his superior situ-ation requires; excepting at thy time of thequadratures, when a small part of the unen-lightened hemisphere being turned towards us,his disc appears like the moon about three daysafter the full. This planet is also diversified withspots like the moon, by which his diurnal revo-lution is ascertained in the direction from west

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