188 The Planet Mars : a Fragment. 



found essential to explain and reconcile many of my figures ; 

 and as the mode by which I obtained my diagram may possibly 

 be serviceable to some brother amateur who may have been 

 engaged in a similar study, or may be of use at some future 

 opposition, it may be well to describe it. A circle cut out in 

 cardboard, and divided round the edge into twenty-four hours, 

 numbered (like those of Eight Ascension) from W. by S. to E., 

 is intended to represent the visible portion of the planet, viewed 

 with the S. pole in the centre. This is capable of being turned 

 on its axis within a somewhat larger circle drawn on another 

 piece of cardboard, and divided by radii from the common 

 centre into sections, three of which coincide with two of the 

 hour- divisions on the moveable circle; each, therefore, corre- 

 sponds to forty minutes of time ; and as, fortunately, the rota- 

 tion of Mars exceeds that of the earth by very nearly this 

 quantity,* each of the divisions on this outer circle in succes- 

 sion, reckoned the opposite way to the hours on the inner 

 moveable circle, indicates the position of the observer's eye on 

 each successive night as long as the circle lasts, that is, for 

 thirty-six days ; but as the whole series of observations much 

 exceeds this period, three other circles are drawn just outside of 

 it, one beyond the other, which carry forward the series of 

 days successively in turn from Sept. 16 to Dec. 20. Any day 

 and hour, when a satisfactory sketch was taken, being then 

 fixed upon as a standard, as for instance, Nov. (>, 8h. 45m., the 

 moveable card representing Marsbeing so set as to have that hour 

 coinciding with the given day in one of the outer circles, will 

 stand in a right position with respect to any other day in all 

 the four circles, at the same hour : for any earlier or later time 

 of the evening, the corresponding hour-mark on Mars (the 

 moveable card) being brought opposite to the standard day 

 (Nov. C), the diagrams will be set correctly for that time for 

 every day in tlio series. Sketches taken at any time may then 

 be transferred to the moveable card, in that peculiar conven- 

 tional kind of projoction which is used in similarly constructed 

 maps ; and though the distortion will be very great, idehtifica- 

 1 ion, the chief object, will bo attained. One consideration only 

 interferes with the simplicity and facility of this plan — the 

 orbital movement of the planet during a long series of observa- 

 tions, in oonsequenoe of Avhich the position of the apparent 

 centre of the disc will bo somewhat altered ; but this may be 

 allowed for by constructing a littlo table, by which a correction 

 may be applied to the hour of observation. Nov. <> being fixed 

 upon as zero, this correction fortunately proved always subtrac- 

 tive ; it amounted to 43m. on Sept. 19, and 37m. on Dec. 20. 



* The exact duration of tlio solar day and night upon Mara is fixed by Beer, 

 and Mudlcr at 24h. 3'Jm. 357s. 



