August 21, 1896.] 



SCIENCE. 



237 



1894 Flagstaff observations of the terminator by 

 Mr. Douglass, Mr. Lowell writes that ' ' Of the 

 736 irregvilarities observed, 694 were not only 

 recorded, but measured. Of these, 403 were 

 depressions. It is singular, in view of their 

 easy visibility, that, with the exception of 

 Schroeter, in the last century, no one should 

 have noticed them before." 



Mr. Lowell rejects 346 out of 403 depressions 

 as not real, since they lay on the dark areas of 

 the planet and were due to the smaller irradia- 

 tion at those places. He holds that the re- 

 maining 57 depressions were due to clouds 

 within the terminator, and 291 projections were 

 clouds outside the terminator ; because if they 

 were mountains the number of depressions 

 should equal the number of projections. To 

 my mind, the argument is not convincing. If 

 we remove 196 of the projections which are 

 described as 'long and low,' and which some 

 experience in observing them leads me to as- 

 cribe to excessive irradiation, we shall have 95 

 projections and 57 depressions of the ' short 

 and sharp variety.' When we consider that 

 these clouds or mountains (or something else) 

 are immersed in an illuminated atmosphere, we 

 cannot expect the projections and depressions 

 to be equal in number. The problem will not 

 be settled until it is determined whether or not 

 the projections occupy fixed and the same posi- 

 tions at many successive oppositions — the phase 

 and atmospheric conditions being equal. 



I confess my inability to unravel Mr, Low^ell's 

 discussions of Mr. Douglass' observations. 

 "When it was a question of detecting a twilight 

 effect it was the illuminated atmosphere which 

 formed the visible and measurable terminator. 

 "When it was a question of proving that Mars 

 was extremely level, and would, therefore, lend 

 itself to general irrigation, it was the land sur- 

 face that formed the visible terminator ; and 

 since this terminator was always "compara- 

 tively smooth, * * * we know that, relatively 

 to his size, he has no elevations or depressions 

 on his surface comparable to the lunar peaks 

 and craters." Lastly, the several hundred ir- 

 regularities observed on the terminator, vary- 

 ing from those extremely high to those very 

 low, were attributed to clouds. The terminator^ 

 then, is formed by the illuminated atmosphere 



and not by the land surface ; secondly, there 

 are no significant elevations and depressions on 

 the surface, because the terminator, formed by 

 the land surface, is comparatively smooth ; and 

 thirdly, the extensive irregularities on the 

 terminator, which 'may be seen every night,' 

 are due to clouds. 



Mr. Lowell writes of the ' long and low ' 

 irregularities that the projections averaged 

 0^^.136 in height ; the depressions 0^^.125 in 

 depth. These are the distances from the ap- 

 proximately elliptic arc that would have formed 

 the apparent terminator if the irregularities 

 had not existed. Thus we have the heights of 

 the irregularities from a curve that did not 

 exist given to three decimals of a second of 

 arc ! And there is nothing to show that the 

 varying distances of the planet were taken 

 into account, either. Every practical astron- 

 omer knows that the first decimal place is 

 uncertain; the systematic errors in such cases 

 can easily and generally do exceed a tenth of 

 a second. To say that the results are accurate 

 because they are the mean of a large number 

 of observations is to say that if a stranger to 

 Colorado's clear atmosphere should waken un- 

 expectedly on Pike's Peak and guess the dis- 

 tances to several hundred neighboring peaks, 

 the mean of all the guesses would be very near 

 their average distance. 



There is not much demand for mathematical 

 analysis in a popular book on Mars, nor is the 

 application of that little always happy. On 

 pages 183-134, after stating that practically all 

 the canals follow the arcs of great circles, and 

 necessarily appear curved when viewed ob- 

 liquely, the author writes, ' ' apparent straight- 

 ness throughout is only possible in compara- 

 tively short lines. For a very long arc [of a 

 great cii'cle] upon the surface of a revolving 

 globe tilted toward the observer to appear 

 straight in its entirety it must lie due north 

 and south. ' ' This is incorrect. If the apparent 

 center of the planet's disc is at 18° south lati- 

 tude, which was the average for Mars in 1894, 

 then every arc of evei-y great circle that can be 

 drawn in any direction through any point that 

 lies on the minus 18° circle of latitude will ap- 

 pear straight twice every day. An infinite 

 number of such circles can be drawn. Mr. 



