across the sun's disc.
The moon did not commence to cross until 6 hours and 7 minutes later, by
which time the earth had passed over three-quarters of the sun's
diameter. The earth was 8 hours and 42 minutes in transit, and the moon,
which crossed a little lower down, was 8 hours and 31 minutes in
crossing."

"That must have been an interesting sight," said John, "and I should
like to have the opportunity of watching a similar transit."

"I am afraid you never will," said Merna, "for the transits only occur
at long intervals. The previous transits occurred in November 1879,
November 1800, May 1700, and May 1621. There will not be another until
May 1984, and the next after that will not occur until November 2084."

"I am sorry to hear that," remarked John, "for even if I stayed here, I
should not be likely to live long enough to see the next transit.
Possibly you may do so, Merna; you are so much younger than I am."

"Yes," Merna replied, "it is not unlikely that I may see another such
transit, for the average length of our lives on Mars is about equal to
one hundred and thirty of your years, so that leaves me an ample margin
of time."

I then went on to remark that as another result of the thinness of the
Martian atmosphere twilight is much shorter than on the earth, the
light being less diffused when the sun is below the horizon, and
refraction also considerably less than we experience.

In this connection, I mentioned to M'Allister that we can often see the
sun and the moon apparently above the earth's horizon when they are, in
fact, below it. This is caused by the refractive power of our dense
atmosphere, which has the effect of making both the sun and the moon
appear a little higher up than they really are.

"That is something new to me, Professor," exclaimed M'Allister; "and I
cannot say I quite understand how refraction, as you term it, has the
effect you mention."

"It may help you, then," I answered, "if I tell you that water acts very
much in the same way; and there is a simple and fairly well-known
experiment you might try for yourself, which would make the matter
perfectly clear to you. It is as follows:--

"Take a teacup and place a shilling at the bottom of it, then move back
until you quite lose sight of the coin. Ask some one to pour some clean
cold water gently into the cup, and, as it fills, the refraction of the
water will apparently reduce the depth of the cup, and thus bring the
coin full