Our magnetic receiver had been placed in position, the Morse register was attached; the whole apparatus was in one of the upper rooms of the observatory, in proximity with the telescope through whose glass for days we had watched the approach of our sister planet.  As the night settled down upon us we had taken our seats for a few instants at a table in a lower room engaged in one of those innumerable desultory talks upon our project and their, even to us, somewhat problematic character.  Everything connected with that evening, apart from its having been carefully recorded in my diary and notebooks, is very distinctly remembered by me.  I recall my father reading from a letter to Nature, May 15, 1884, by Mr. W.F.  Denning, discussing “The Rotation Period of Mars.”  From my note-book I find the passage literally transcribed: 

It read—­“Notwithstanding his comparatively small diameter and its slow axial motion, the planet Mars affords especial facilities for the exact determination of the rotation period.  Indeed, no other planet appears to be so favorably circumstanced in this respect, for the chief markings on Mars have been perceptible with the same definiteness of outline and characteristics of form through many succeeding generations, whereas the features, such as we discern on the other planets, are either temporary, atmospheric phenomena, or rendered so indistinct by unfavorable conditions as to defy measurement and observation.  Moreover, it may be taken for granted that the features of Mars are permanent objects on the actual surface of the planet, whereas the markings displayed by our telescopes on some of the other planetary members of our system are mere effects of atmospheric changes, which, though visible for several years and showing well defined periods of rotation cannot be accepted as affording the true periods.  The behavior of the red spot on Jupiter may closely intimate the actual motion of the sphere of that planet, but markings of such variable, unstable character can hardly exhibit an exact conformity of motion with the surface upon which they are seen to be projected.  With respect to Mars’ case, it is entirely different.  No substantial changes in the most conspicuous features have been detected since they were first confronted with telescopic power and we do not anticipate that there will be any material difference in their general configurations.

“The same markings which were indistinctly revealed to the eyes of Fontana and Huyghens in 1636 and 1659 will continue to be displayed to the astronomers of succeeding generations, though with greater fullness and perspicuity owing to improved means.  True, there may possibly be variations in progress as regards some of the minor features, for it has been suggested that the visibility of certain spots has varied in a manner which cannot be satisfactorily accounted for on ordinary grounds.  These may possibly be due to atmospheric effects on the planet itself, but