10 ELEMENTARY PHYSICS. 



east to west and is equidistant from each of the poles. The merid- 

 ians are great circles passing around the globe from north to south, 

 crossing the equator at right angles and meeting at the poles. 

 The parallels (of latitude) are smaller circles parallel to the 

 equator. The tropics, marking the highest latitude receiving 

 the vertical rays of the sun, are situated twenty-three and one-half 

 degrees both north and south of the equator. The polar circles 

 are situated (to correspond with the tropics) twenty-three and 

 one-half degrees, the one north of the south pole, and the other south 

 of the north pole, and are the limits of hght when the sun is verti- 

 cal at the tropics. The ecHptic marks the apparent path of the 

 sun from tropic to tropic, and as it continues around the earth re- 

 quiring three hundred and sixty-five days for the sun to pass to its 

 end it divides the seasons and days, hence the weeks and months 

 of the year. Latitude is the distance from the equator to the pole, 

 measured in either direction : thus there are ninety degrees of north 

 latitude and a corresponding number of degrees of south latitude ; 

 the length of a degree of latitude is sixty-nine and one-half miles. 

 Longitude is the distance east or west from any given meridian 

 measured on the equator ; the length of a degree of longitude is 

 also sixty-nine and one-half miles. The relative difference in lon- 

 gitude between two places marks, also, the difference in time 

 between those places ; for since there are 360° degrees around 

 the earth traversed in twenty-four hours, one hour of time must 

 correspond to fifteen degrees, or four minutes to one degree. 

 When, therefore, a place is so many degrees from any given locaHty, 

 it is easy to see what will be the difference in time between these 

 places ; if east the time is slower, if west faster than the given time 

 at the given place. 



Applying other simple laws we find that the nautical mile is equal 

 to one and one-sixth English miles : or there are sixty of them to 

 a degree of latitude ; it is therefore natural to compute distances 

 in nautical miles by applying the latitude as a measure of distance. 



Though these remarks may not now seem in place, their sugges- 

 tion will become evident at once in measuring distances without the 



