26 INTRODUCTION", 



being less attracted by her than the other parts which are nearer to 

 her, gravitate less towards the earth's centre, and consequently must 

 be higher than the rest. Those parts of the earth, on the contrary, 

 where the moon appears on the horizon, or ninety degrees distant 

 from the zenith and nadir, will have low water; for, as the waters 

 In the zenith and nadir rise at the same time, the waters in their 

 neighbourhood will press towards those places, to maintain the 

 equilibrium ; and to supply the place with these, others will move 

 the same way, and so on to the places ninety degrees distant from 

 the zenith and nadir, where the water will be lowest. By combining 

 this doctrine with the diurnal motion of the earth, above explained,, 

 we shall be sensible of the reason why the tides ebb and flow twice 

 in a lunar day, or about twenty-four hours fifty minutes. 



The tides are higher than ordinary twice every month, that is, 

 about the times of new and full moon, and are called Spring Tides ; 

 for at these times the actions of both the sun and moon are united, 

 and draw in the same straight line ; and consequently the sea must 

 be more elevated. At the conjunction, or when the sun and moon 

 are on the same side of the earth, they both conspire to raise the 

 waters in the zenith, and consequently in the nadir ; and at the op- 

 position, or when the earth is between the sun and moon, while one 

 occasions high water in the zenith and nadir, the other does the 

 same. The tides are less than ordinary twice every month, about 

 the first and latt quarters of the moon, and are called Neap Tides: 

 for in those quarters, the sun raises the waters where the moon 

 depresses them, and depresses where the moon raises them ; so 

 that the tides are only occasioned by the difference by which the 

 action of the moon, which is nearest us, prevails over that of the 

 sun. These effects would be produced uniformly, were the whole 

 surface of the earth covered with water; but since there are a 

 multitude of islands and continents which interrupt the natural 

 course of the water, a variety of appearances are to be met with in 

 different places, which cannot be explained without considering the 

 situation of the shores, straits, and other objects that have a share 

 in causing them. 



Currents... .There are frequently streams or currents in the 

 ocean, which set ships a great way beyond their intended course. 

 There is a current between Florida and the Bahama^islands, which 

 always runs from south to north. A current runs constantly from 

 the Atlantic, through the Strait of Gibraltar, into the Mediterra- 

 nean. A current sets out of the Baltic Sea, through the Sound or 

 strait between Sweden and Denmark, into the British channel, so 

 that there are no tides in the Baltic. About small islands and 

 head-lands in the middle of the ocean, the tides rise very little ; 

 but in some bays, and about the mouths of rivers, they rise from 12 

 to 50 feet. 



CONSTRUCTION AND USE OF MAPS. 



Maps. ...A map is a representation of the earth, or a part of it. A 

 map of the world is a delineation in perspective of the globe, as it 

 would appear to an eye placed in a particular point. The circles 

 bounding such a map represent the brass meridian, and the curve 

 lines running across at every ten degrees show the latitude north 

 Or south of the equator. The top and bottom are the north and 



