26 
POPULAR SCIENCE REVIEW. 
and extent of presentation of the northern hemisphere), in about 
the most favourable part of Mars’ orbit. 
As respects the inclination of Mars’ axis to the plane of his 
orbit, and the other elements on which his seasons and the 
appearance he presents to us depend, we have the determi- 
nations of Sir W. Herschel. He estimated that the Martial 
spring equinoctial point was situated (in 1787) in longitude 
79° 28' (the longitude of Mj in the figure is 78°), the obliquity 
of the Martial ecliptic 28° 42', and the inclination of Mars’ 
equator to the earth’s orbit 30° 18". I fear it will be considered 
somewhat rash to impugn results obtained by Sir W. Herschel. 
Standing, as he does, in the very foremost rank among ob- 
servers, and facile princeps as an interpreter of observations, 
astronomers justly look on his opinions almost as laws. Yet I 
think, if we consider the nature of the observation, and the 
character of the instruments used by Herschel, we must recog- 
nise the fact that he attributed to his results an exactness they 
were not capable of attaining to. The pictures of Mars given 
by Herschel are sufficient to show that the instrument he used 
was far inferior in defining power to those with which Delarue, 
Dawes, Lockyer, and Phillips have examined the planet. Now, 
let us see on what indications furnished by Mars (thus viewed) 
Herschel founded the determinations above recorded. Deferring 
to the paper in the Philosophical Transactions ,* we find that 
the indications he trusted to were the motions of spots across 
Mars, and the appearance or disappearance of certain bright 
spots near the Martial poles. In fact, from the nature of the 
case, it is obvious that no other sort of evidence was available. 
The necessary observations were repeated at intervals, as the 
weather permitted, and carefully reduced (on just mathematical 
principles) in accordance with the motions of Mars and the 
earth in their respective orbits. Now, if we consider the minute- 
ness of the disc presented by Mars, the variable appearance of 
the spots and points upon his surface, and the extreme difficulty 
of assigning, with any approach to exactness, the period or 
place at which a spot or point becomes visible on the edge of a 
rotating sphere, even when such sphere is distinctly (and per- 
manently) marked, we shall see that, even with the best modern 
instruments, it would be impossible to determine the inclination 
of Mars’ axis within two or three degrees, or the place of his 
vernal equinox within seven or eight degrees. Those who are 
best able to appreciate Herschel’s work as an astronomer will be 
precisely those who will most clearly recognise the difficulty of 
the problem he attacked. It is to be wished that some of our 
modern observers would re-examine the subject. That very 
Phil. Trans . 178 4, p. 241. 
