THE PLANET MARS IN FEBRUARY 1869. 
43 
by Sir W. Herschel in the years 1775-1783, one or two by 
Maraldi early in the eighteenth century, and two by Hooke in 
the year 1666, which seemed sufficiently distinct to be available 
for the purpose I had in view. I found that Kaiser’s value did 
not bring these several views into accordance. For example, in 
a period of seventy-nine years, Kaiser’s value brought out a 
discrepancy corresponding to an hour’s rotation of the planet ; 
and when the full period of 198 years which separated the 
earliest of Hooke’s from the latest of Dawes’ drawings was 
examined, a discrepancy resulted which there was no mistaking. 
In other words, when the aspect of Mars was calculated back- 
wards from the date of one of Mr. Dawes’ latest drawings, with 
Kaiser’s value of the rotation-period, a result was deduced which 
differed wholly from the view given by Hooke in 1666. This 
result was confirmed later, when I was able to make use of a 
drawing of Mars taken by Mr. Browning with one of his eight- 
inch reflectors in February 1867. Here a period of 201 years was 
made use of, and it need hardly be said that, when once one has 
obtained a value so near to the true rotation-'period as to be 
certain of the exact number of rotations which have taken place 
in so long a period, the error affecting the value of a single 
rotation is very small. In the following table I present the 
results of calculations applied to three long intervals, viz. from 
March 12, 1666, 12 h. 20 m. (astronomical time and new style), 
in each case, to 
(i) April 24, 1856, lOh. 50m. 
(ii) November 26, 1864, llh. 46m. 
and (iii) February 23, 1867, 6h. 45m. ,* 
the drawings corresponding to (i) and (ii) having been made by 
Mr. Dawes, the one corresponding to (iii) by Mr. Browning. 
Int. 
Interval in 
Seconds 
Cor. for 
Geo. Long. 
Cor. for 
Phase 
Corrected Interval 
in Seconds 
Number of 
Rotations 
Resulting 
Rotation Period 
(ii) 
(iii) 
5999524200 
6270650760 
6341394300 
0° 
-248 
-273 
-12° 
0 
+ 3 
5999521246 
6270589()96 
6341326590 
67682 
70740 
71538 
88642'737 
88642-734 
88642-734 
It results that Mars’ rotation period lies between 24 h. 37 m. 
22*73 s. and 24 h. 37m. 22*74 s. Within these limits, this result 
may, I think, be depended upon ; because an error of one- 
hundredth part of a second in a single rotation would mount up 
to 715 seconds, or nearly a quarter of an hour in the long 
interval numbered (iii) ; and the appearance of Mars changes 
very perceptibly in a quarter of an hour. 
But I would call attention here to the absurdity of assigning 
