The Astronomical Outlook 



sand feet ; the earth's mass in tons is still in doubt 

 by fully one or two per cent. The limits of 

 error have been much diminished by the geodetic 

 operations and gravitational experiments of the 

 last twenty-five years, but there remains abun- 

 dant room for improvement. 



As regards the moon, the theory of her motions 

 has not yet by any means reached finality, and 

 numerous able mathematicians are still at work 

 upon it. It is hardly likely, however, that any 

 great discovery is to be made in this line of re- 

 search. Observation and theory will doubtless 

 draw into closer accordance, until at last their 

 discrepancies will be only such as can fairly be 

 attributed to the inaccuracies of our standard 

 time-keeper the slight irregular changes in the 

 earth's rotation due to occasional geological 

 paroxysms, such as earthquakes and its con- 

 sequent acceleration or retardation by a few 

 thousandths of a second of time. 



The application of photography has already 

 added much to our knowledge of the lunar sur- 

 face, and is certain in a few years to give us 

 charts of the hemisphere which is visible to us 

 far exceeding in accuracy our maps of any but 

 selected regions of the earth. Two large lunar 

 atlases are now being published one by the 

 Paris Observatory and the other by the Lick; a 

 third, on a much larger scale than either of the 

 other two, but based on the same photographic 

 material, is proposed to be issued at Prague. 

 Comparison of these authentic records of the 

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