C, E. Burroy.— Physical Observations of Mars. 163 
points of Dawes Forked Bay were seen as one ill-defined point, which is not 
wonderful, considering that they were some 50° distant from the central 
meridian, Near the eastern limb a trace of Phillips Island was clearly seen, and 
Burchardt Land could be faintly descried close to the opposite point of the disk. 
The amount of detail visible was far too great to be transferred with accuracy to a 
drawing within the time available for the purpose ; especially as regards the junction 
of the Kaiser Sea and Ocean of Dawes, where the unavoidable omissions have been 
most numerous owing to the constant slight flickering of the image, which prevented 
the eye from dwelling on minute points of detail sufficiently long to be perfectly 
assured that their true forms had been made out. 
AREOGRAPHIC PosITIONS OF THE MARKINGS AS DEDUCED FROM THE DRAWINGS. 
A full discussion of the results obtained naturally includes the determination of 
the places of the markings observed, in other words, of their areographic latitudes 
and longitudes. This has in the present instance been accomplished by the method 
first made use of, so far as | am aware, by the late Professor Kaiser of Leyden, 
details of which are given by him in the third volume of the Annals of the Leyden 
Observatory ; pp. 50-51. Briefly, his process was firstly to compute the position 
of the axis of Mars, and of the assumed first meridian, for certain dates included 
within the period embraced by the observations ; then to construct from these data 
diagrams, representing on the orthographic projection the circles of longitude and 
latitude on the same scale as the drawings, but on transparent paper ; finally, 
placing these diagrams successively upon each drawing in such a position that the 
polar diameter of the diagrams passed through the centre of the polar snow 
spot, and the outlines were accurately superposed, it was easy to read off on the 
diagram the angular distance of any spot from the central meridian and from the 
equator. Knowing by calculation the areographic longitude of the centre of Mars’ 
disk at the time of observation, it was a very simple matter to ascertain that of 
any spot on the drawing. This process was repeated with the second diagram, and 
the true place of the spot found by interpolation for the date of observation. 
In the case of the drawings now laid before the Royal Dublin Society hardly 
any calculation was required, thanks to the Ephemeris of Mr. Marth, already 
referred to, which furnished all the numerical data needed. Two diagrams were 
constructed as above described; the first answering to 1879, Oct. 14, the second 
corresponding to 1879, Dec. 8, and 1880, Jan. 2. Owing to the distribution of the 
drawings in time, it was found safe to use these two diagrams for the whole term 
of the observations, as the corrections for change of the planet’s position never 
exceeded 5° in latitude forany drawing. The method of using the diagrams was the 
same as Prof. Kaiser’s, but the longitude of the centre of the disk was interpolated 
from Marth’s Ephemeris. Though the results can scarcely attain to the accuracy 
reached by Professor Kaiser, yet as they furnish a numerical measure of the 
