LATITUDE AND LONGITUDE. 7 



axis from west to east ; therefore, since the sun en- 

 lightens only one half of the earth at once, he rises 

 to some places at the same absolute moment of time 

 at which he sets to others, and when it is mid-day 

 in one meridian, it is midnight in the opposite one. 



In the space of 24 hours then, the sun has appa- 

 rently described a circle round the earth, it has con- 

 consequently passed over 360 degrees, being at the 

 rate of 15 degrees an hour ; that is to say, from hour 

 to hour answers to meridians, such as pi, p2, ?3, p4, 

 &c. (Plate 2.) which make between them angles of 

 15 degrees; therefore, reciprocally if two meridians 

 are 15 degrees, or 30 degrees, or 45 degrees apart, 

 the difference of time to persons on those meridians 

 will be one hour, two hours, three hours, &c., cor- 

 responding to the distances of their meridians ; and 

 those who were on a meridian 180 degrees distant, 

 that is on the opposite meridian, would therefore 

 have midnight whilst these had mid-day. 



By this we perceive that if we sailed round the 

 world from east to west, that is to say, in a direction 

 contrary to the earth's motion on its axis, we should 

 lose a day on returning again to the same meridian, 

 and vice versa, if we passed round the earth in a 

 direction corresponding to the earth's rotation, that 

 is from west to east, we should gain a day at our 

 return again to the meridian of departure. 



If then, we knew that some phenomenon which 

 could be seen from different places at the same ab- 

 solute moment of time, would occur at a certain 

 hour on some known meridian ; we could, by drawing 

 the same phenomenon under another meridian, de- 

 termine how far those 3 meridians were asunder. If 

 for example, we knew that an eclipse of the moon is 

 seen at Paris on a given day, at 17 minutes past 6 

 in the evening; and observing this eclipse on some 

 other meridian, we perceived it at 39 minutes, 37 se- 

 conds past 5, we should conclude that we were sit- 

 uated east of Paris ; and the difference of time being 

 27 minutes, 23 seconds would give it at the rate of 



