HOW TO WOEK WITH THE TELESCOPE. 
469 
spaces, each subtending a degree, and therefore two inches in 
length if the eye is distant nine feet and a half, and proportion- 
ately less or greater, according to the distance of the eye. 
Suppose, now, that the observer has erected such rods to aid 
him in observing Mercury, on November 22 next, in the even- 
ing. Referring to Dietrichsen’s Almanack, we find that on that 
day Mercury’s R.A. and dec. exceed the sun’s by lh. 31m., 
and 5° 19', respectively. Therefore, viewed from the fixed 
point, Mercury will pass the cross-rod lh. 31m. after the sun, 
and between the fifth and sixth marks beloiv the fastening. 
If the sun cross the rod at about 3 p.m.. Mercury will pass 
it about half an hour after sunset, and probably be visible to 
the naked eye until 4h. 55m. p.m., when he sets. 
"With a little ingenuity a similar method may be applied 
to determine from the position of a known object that of any 
neighbouring unknown object, even at night. The first rod 
must be divided like the cross-rod, and the cross-rod shifted, 
or else two cross-rods used, when the unknown 'precedes the 
known object. That such methods are efficient may be gathered 
from an instance. On one occasion, by simply noting the 
hour and angle at which the sun passed the gable-end of a 
house at some distance, I estimated the time and place at 
which Mercury should emerge from behind the house, and 
picked him up in a 5 -foot achromatic so soon after emergence 
that the outline of the house (out of focus, of course) was in 
the field of view with him. 
Observation of Mars may be usefully pursued with small 
telescopes. A valuable series of maps of this planet was 
constructed by Beer and Madler with the instrument already 
mentioned, in 1830, when Mars was in opposition near 
perihelion. Owing to the eccentricity of Mars’s orbit, his 
distance from the earth (when he is in opposition) varies from 
61,000,000 when he is in aphelion, to less than 35,000,000 
when he is in perihelion. Oppositions near perihelion recur at 
alternate intervals of about fifteen and seventeen years. Unfor- 
tunately, when favourably situated as to distance. Mars is 
south of the equinoctial, and therefore not favourably 
situated for observation in northern latitudes, having a low 
altitude on the meridian. As respects altitude. Mars will be 
very favourably situated for observation at the opposition 
which will take place next January; but he will not appear 
nearly so large as he did in October, 1862. The vernal 
equinox of Mars’s northern hemisphere occurs when the lon- 
gitude of the planet is about 78J°, corresponding to the position 
of the earth in the beginning of December. Knowing this, 
the student can easily determine what proportion of either 
hemisphere of Mars is turned towards the earth at any time ; 
