168 C. E. Burtoy.—Physical Observations of Mars. 
The prevalence of the minus sign and the large amount of the differences in the 
last column of longitude under the heading G—D is due to the selection by Mr. 
Green of a first meridian 11° to the east of the point which seems to have been 
adopted by Mr. Marth in his Ephemeris before mentioned. In strictness, therefore, 
11° ought to be added to Mr. Green’s longitudes to eliminate this constant differ- 
ence. 
Similarly, the constant differences of longitude between the results obtained by 
Professor Schiaparelli from Professor Kaiser's and from his own observations, and 
those which I have deduced, amount respectively to—1° and to—3°. 
GENERAL SurvEY OF RESULTS. 
The accompanying chart,* the fundamental points of which have been laid down 
in accordance with the Table of Areographic co-ordinates given on page 166, includes 
the whole of the apparently permanent markings seen at Dunsink, and at Lough- 
linstown. 
The details not included in the list of positions have been inserted by eye-esti- 
mation, and it was my aim throughout this part of the work, to distinguish what 
was accidental and temporary from that which was permanent. The principal aid 
available for making this discrimination has been the well-known and long 
established fact that the dark markings, as a rule, have always occupied sensibly 
the same positions on the globe of Mars whenever they were distinctly seen, and 
that in many instances of apparent change it has been found, by persevering 
scrutiny, that the alteration was due to the formation or removal of luminous veils, 
akin to terrestrial clouds, which concealed or disclosed to view the subjacent 
features of the planet’s actual surface. 
Consequently, I have been led to insert on the chart every dark shading seen, 
even if detected on one occasion only ; and to ignore those light markings which 
do not find a place in all the drawings which include their positions. As the 
position of Mars’ axis was very unfavourable in comparison to those taken up at the 
oppositions of 1871, 1873,'and 1877, for scrutiny of either of the polar regions, and 
as the projection adopted for the chart is free from distortion as regards latitude, it 
has seemed unadyisable to construct special polar charts from the observations of 
1879-80. 
The Polar Snows.—On the whole the northern snow spot was far brighter and 
larger than the southern, notwithstanding that the northern pole was some distance 
from the limb within the averted hemisphere, and the southern the same distance 
within the visible hemisphere. Generally a few bright specks were all that could 
be seen of the southern snow, while the northern appeared to extend on Dec. 10 as 
far as the northern boundary of Nasmyth Inlet in N. latitude 45°. If concentric 
with the N. pole this frozen region must consequently have had a diameter of about 
* Fig. 1, Plate VILL 
