to face with that dragon of the century—the unending struggle for food and raiment. GEOGRAPHY OF THE HEAVENS. BY CHANCELLOR M. B. GOFF, Western University of Pennsylvania. THE SUN, While affording such accurate methods of determining time, does not directly furnish that by which we are accustomed to be guided, so that astronomers are wont to speak of _solar time_, as _mean solar time_, and _apparent solar time_, the former being that kept by our clocks and the latter that from which the former is estimated. If the earth moved around the sun at an unvarying rate, then each time the sun threw the shadow of a vertical rod directly north (in the northern hemisphere) would indicate noon at the point where the rod was located; and the interval between two such successive shadows would be exactly one day, and we could divide the time into twenty-four equal spaces and call each space one hour. Now, this is what we imagine the earth to do; or rather, for the sake of convenience, and because the results are the same in either case, we conceive the sun to move around the earth, and make a _mean_ or _mock sun_, move around the earth in a day of twenty-four hours of equal unvarying length, and call this a _mean solar day_. But the _true_ or _apparent solar day_ is considerably different from this, being sometimes less and sometimes more than twenty-four hours in length, the _true sun_ reaching the meridian as much as 16¼ minutes before or after the _mean sun_, both reaching it together only four times each year, viz.: On April 1, June 15, September 1, and December 24. Of course it would be impossible to construct a time-piece that would keep pace with the _true sun_. Indeed, it is difficult enough to construct one that will keep with the mean sun. But all difficulty is obviated by making clocks whose _rate of error_ can be determined. This rate being known it is easy to estimate the correction, and thus obtain exact time. For example, suppose a clock to _gain_ 0.24 of a second per day, then in two days it will gain two times 0.24 of a second, and in three days three times 0.24 of a second, etc. These amounts subtracted from the noon-time of the clock would give the correct noon. For any other hour, a part of the 0.24 depending on the number of hours after noon must be subtracted. If the clock _loses_, then in a similar manner the proportioned loss must be added. In actual practice, we may say that even the best chronometers do not keep exact time; and every one has to be “corrected” in the manner indicated.