far
advanced that it became necessary to decide whether this was really
a suitable location. I had grave doubts on the subject. A mountain
side is liable to be heated by the rays of the sun during the day,
and a current of warm air which would be fatal to the delicacy of
astronomical vision is liable to rise up the sides and envelope the
top of the mountain. I had even been informed that, on a summer
evening, a piece of paper let loose on the mountain top would be
carried up into the air by the current. But, after all, the proof of
the pudding is in the eating, and Holden united with me in advising
that an experienced astronomer with a telescope should be stationed
for a few weeks on the mountain in order to determine, by actual
trial, what the conditions of seeing were. The one best man for
this duty was S. W. Burnham of Chicago, who had already attained
a high position in the astronomical world by the remarkable skill
shown in his observations of double stars. So, in August, 1879,
huts were built on the mountain, and Burnham was transported thither
with his telescope. I followed personally in September.

We passed three nights on the mountain with Captain Floyd, studying
the skies by night and prospecting around in the daytime to see
whether the mountain top or some point in the neighboring plateau
offered the best location for the observatory. So far as the
atmospheric conditions were concerned, the results were beyond our
most sanguine expectations. What the astronomer wants is not merely
a transparent atmosphere, but one of such steadiness that the image
of a star, as seen in a telescope, may not be disturbed by movements
of the air which are invisible to the naked eye.

Burnham found that there were forty-two first-class nights during his
stay, and only seven which would be classed as low as medium. In the
East the number of nights which he would call first-class are but few
in a year, and even the medium night is by no means to be counted on.
No further doubt could remain that the top of the mountain was one
of the finest locations in the world for an astronomical observatory,
and it was definitely selected without further delay.

Sometime after my return Mr. Floyd sent me a topographical sketch
of the mountain, with a request to prepare preliminary plans for the
observatory. As I had always looked on Professor Holden as probably
the coming director, I took him into consultation, and the plans
wer