EELATIONS OF TIME AND SPACE. 145 



being placed above the horizontal line. The mean solar day is the 

 average of all the solar days iu the year, and the deviation irom it, 

 arising from both the causes already specified, being proportionably 

 represented at any date in the year on the two curves, A and B, 

 already described, the effect arising from the combination of the two 

 will be represented by curve C, which indicates the equation or cor- 

 rection of time. This will be found to be nothing at irregi^lar inter- 

 vals, to wit : about the 15th of April, 15th June, 1st September. 

 and 24th December. 



It may be well to notice to some extent the difference of local time; 

 as it will enable us readily to answer a curious question that has often 

 been discussed at much length : If from a first meridian, at which it is 

 noon — say of Tuesday — we count around eastward, we shall find it to 

 be later and later in the afternoon of Tuesday, until arriving at the 

 opposite side of the earth, in longitude 180, we shall find it to be the 

 midnight with which Tuesday is about to end and Wednesday begin ; 

 but if from the same first meridian we should count around westward, 

 we would find it to be earlier in the morning of Tuesday, until arriving 

 again at the longitude of 180 degrees, we should find the time to be 

 the midnight with which Tuesday is about to begin. By comparing 

 these two results it will be perceived that the difference of time between 

 two places situated beyond 180 degrees — one on the one side and the 

 other on the other — must be all of 24 hours. This difference, which 

 exists about midnight, must continue to exist as time moves on, at any 

 other hour of the day. In passing this limit, therefore, the name of 

 the day of the week must be changed in the reckoning. Now, as 

 Christianity, and Avith it civilization, are overspreading the earth, it 

 would seem most suitable that the first meridian of reference should 

 be the meridian of Jerusalem. 



The question to which allusion has already been made, is as follows: 

 Suppose a traveller to be transferred westward as fast as the earth 

 revolves from sun to sun eastward, so that if at the outset it were 

 noon with him it would continue so, it is required to know when and 

 where it would be necessary for him to change the name of the day. 

 Now, it has already been seen that the place loliere this ought to 

 happen, is in the longitude 180 degrees from the first meridian fixed 

 upon, and the time when this must occur is noon by the very condi- 

 tions of the problem. 



I will now notice the calendar, though I may do it but briefly. It 

 has, of course, for its object the distribution of time, a primary- 

 division of which is the day. Anciently the twenty-four hours of the 

 day were named after the sun, the moon, and the five planets, as 

 follows: Sun, Venus, Mercury, Moon, Saturn, Jupiter, and Mars. 

 The first hour of the first didcy was named after the sun. This also 

 gave name to the day, and it was called Sunday. The other hours of 

 the day were named after the other planets, <fec., in the series; and 

 then the series was begun anew^, and having been gone through in 

 this way three successive times, twenty-one hours received their 

 names, and the remaining three hours were named by commencing 

 the series a fourth time and proceeding as before. The first hour of 

 10 



