Another fact needs to be considered. Since the earth’s orbit is in the form of an ellipse, with the sun at one of the foci, the earth must pass nearer the sun in some parts of its orbit than in others. By the laws of gravity, when nearer, the attraction between the earth and sun is greater, and if this were not balanced by increased velocity along its orbit the earth would fall into the sun; and, on the other hand, when farther off this attraction is less, and if this were not balanced by a diminution of velocity along its orbit the earth would fly off into space. This varying velocity, together with other complications too technical for a magazine article, gives varying lengths of orbit to the several solar days of the year. If the earth’s orbit were laid out upon paper and, by astronomical calculations, an exact proportionate section were marked off for each solar day of the year, the variable lengths of orbit for the different days of the year would plainly appear to the eye. But, as before explained, the time of a solar day is the time of one revolution of the earth upon its axis, together with the fractional part of another revolution occasioned by one day’s progress of the earth along its orbit. Then it must follow that as the daily sections of the orbit vary in length, the time of the solar day must vary in length. No clock could be made to keep the variable time of true solar days, and if this were possible, the hour, minute, etc., would be variable of length, and hence no standard for time measurements. But by working a simple arithmetical problem of addition and division an average length of day for the year may easily be found. This average day is the mean solar day adopted. Its time is arbitrary and exact, forming a perfect standard for all time measurements. From this the term _mean time_ gains its significance. By referring to the foregoing earth’s orbit laid out on paper, with the true solar days marked off in sections of mathematical exactness, it will be seen that by dividing each section into two equal parts and marking the division point with red ink, the true noon point of each solar day in the year will be conspicuous upon the drawing, and in its proportionate relations in every way. If now we set a pair of dividers or compasses so that the opening shall reach over the exact space on the orbit of one half of the mean solar day, and beginning at the red noon point of one of the four days in the year when the true noon falls at just twelve o’clock--say December 24th--and step the dividers around on the orbit, making a blue point mark at each second step, then as the blue points vary from the red so will the mean time which our clocks keep vary from the true noon of each day of the year. Variation in length of forenoon and afternoon, therefore, may be viewed by common intelligence not only as a fact but as a necessity. PRESIDENT JORDAN’S “NEMINISM.”