152 C. E. Burron.—Physical Observations of Mars. 
the compilation of a drawing embodying the results of the best views obtained 
during the period occupied by a group of sketches, and defined by the times given 
on the drawing. When possible, no one group of sketches was allowed to occupy 
more than half an hour, although when definition continued sufficiently good, two, 
or even three groups of sketches have been obtained in one night. Each of the — 
drawings now laid before the Royal Dublin Society is a compilation made according 
to the method described above. 
No comparisons with the results of other observers were made until each group of 
sketches was finished. Every detail when drawn was repeatedly compared with 
the image in the telescope. On each finished drawing is entered the longitude of 
the central point of the disc, computed for the middle instant between the times of 
beginning and ending by interpolation from the invaluable ‘ Ephemeris for Physical 
Observation of Mars,’ published by Mr. Marth in vol. xxxvii., No. 8 of the Monthly 
Notices, and the drawings are numbered zn the order of increasing longitudes, not 
according to their chronological succession. It was thought preferable to retain 
the same scale for the drawings as that employed for the series of views taken in 
1871 and 1873, and published in the twenty-sixth volume of the Trausactions of the 
Royal Irish Academy, although the planet’s apparent diameter was considerably 
greater at the last than at the two previous oppositions just mentioned ; for all the 
drawings are then directly comparable, while their dimensions appear sufficiently 
large to do justice even to the minutest details seen. 
The atmospheric conditions were usually good, and the greater altitude of the 
planet in 1879 as compared with 1877, probably compensated observers in the 
northern hemisphere for its mcreased distance and diminished diameter. Some 
degree of confidence is therefore felt in the result of comparisons with the splendid 
series of drawings obtained by Mr. Green at Madeira, and by Mr. Dreyer at Parsons- 
town in 1877. These comparisons appear strongly to support the impression which 
seems now to be pretty generally entertained, that the great majority of the 
markings are permanent, and that differences of aspect are almost entirely due to 
altered projection (orthographic, with a variable axis), and where not thus explicable, 
they may usually be traced to temporary and partial obscuration by something in 
the nature of mist, frost, or snow, mainly dependent on the Martian seasons. 
Instances of such obscuration will be noticed when we come to the detailed 
descriptions of the drawings. In more than one case, drawings which ought to be 
duplicates differ noticeably in a way which is not accounted for by ascribing the 
discrepancy to errors of mere drawing, or the slight change of projection between 
the epochs of the sketches (see for example the sketches of Oct. 24, Dec. 1, and 
Jan. 5). The changes in the polar snows, especially in the northern snow spot, 
were very marked,* the very evident and large snow cap seen near the N. Pole 
having become very small and almost indiscernible between the beginning of 
* See the Summary of Results, page 164. 
