248

THE PLANET MAES.

course, consistent with the diminished attraction of gravity,to which we have previously referred, which would cause aman weighing ten stone here to weigh less than four stoneupon Mars , and enable one fifteen feet high to be as light andagile as a man six feet high would be upon the Earth .

In connection with the above reference to the diminution ofthe attraction of the force of gravity upon the surface of Mars ,—a result which depends upon its size as well as upon itsdensity,—it may be well to mention, that the planet is reallysmaller than the occasional brilliancy of its light might leadmany persons to suppose ; its volume being only about }th ofthat of the Earth , and its weight between Jth and ^th.

It is, indeed, a somewhat curious fact that, as time has goneon, successive observations have given smaller and smallervalues for the diameter of the planet. It may be rememberedthat we stated that this was also the case with Venus. In bothinstances, this result is probably related to the irradiationwhich surrounds the bright images seen in a telescope, fromwhich the measures of their diameters have been made. Thebetter the telescope employed, the more accurately can theboundary of their discs be determined. Consequently thecontinued improvement in modern instruments has diminishedour estimates of their size. The diameter of Mars is nowbelieved to measure about 4,200 miles, or between that and4,300 miles ; instead of 4,800, or 4,900 miles, which was untilrecently the accepted estimate.

It should also be remembered, that the diminution in thevalue assigned by recent investigations to the distance of theSun from the Earth diminishes our estimate of the distances ofall the planets, and consequently of all their diameters whichhave been deduced from those distances combined with theirapparent sizes. If, for instance, we alter the Sun’s distancefrom 95,000,000 miles to 93,000,000 miles, we must take ¥ 3 T thsoff our previous value for every planet’s diameter. We havementioned these facts lest any of our readers should find itdifficult to reconcile the values assigned to the diameter ofMars in text-books of a few years since, with some of thestatements made in this lecture.