Now suppose the observer wishes to view Mercury on some day, whereon Mercury is an evening star. Take, for instance, June 9th, 1868. We find from 'Dietrichsen' that on this day (at noon) Mercury's R.A. is 6h. 53m. 23s.: and the sun's 5h. 11m. 31s. We need not trouble ourselves about the odd hours after noon, and thus we have Mercury's R.A. greater than the sun's by 1h. 41m. 52s. Now we will suppose that the observer has so fixed his uprights and the two rods, that the sun, seen from the fixed point of view, appears to pass the point of crossing of the rods at half-past seven, then Mercury will pass the cross-rod at 11m. 52s. past nine. But where? To learn this we must take out Mercury's declination, which is 24 deg. 43' 18" N., and the sun's, which is 22 deg. 59' 10" N. The difference, 1 deg. 44' 8" N. gives us Mercury's place, which it appears is rather less than 1-3/4 degree north of the sun. Thus, about 1h. 42m. after the sun has passed the cross-rod, Mercury will pass it between the first and second divisions above the point of fastening. The sun will have set about an hour, and Mercury will be easily found when the telescope is directed towards the place indicated. It will be noticed that this method does not require the time to be exactly known. All we have to do is to note the moment at which the sun passes the point of fastening of the two rods, and to take our 1h. 42m. from that moment. This method, it may be noticed in passing, may be applied to give naked-eye observations of Mercury at proper seasons (given in the almanac). By a little ingenuity it may be applied as well to morning as to evening observations, the sun's passage of the cross-rod being taken on one morning and Mercury's on the next, so many minutes _before_ the hour of the first observation. In this way several views of Mercury may be obtained during the year. Such methods may appear very insignificant to the systematic observer with the equatorial, but that they are effective I can assert from my own experience. Similar methods may be applied to determine from the position of a known object, that of any neighbouring unknown object even at night. The cross-rod must be shifted (or else two cross-rods used) when the unknown _precedes_ the known object. If two cross-rods are used, account must be taken of the gradual diminution in the length of a degree of right ascension as we leave the equator.