Chap. I.

GEOLOGY.

637

effect is shown in the diagram, where the sun, S, and the moon, M, are supposed to he drawingtlie waters of the earth, ABCD. The moon’s action alone is shown by the spheroidc k g f, and the combined effect of both sun and moon by the spheroid E F G H. Whenthe moon is in quadrature with the sun, the action of the one diminishes the effect of the

M

Fig. 582.

other, as shown in the figure e f g h ; the moon would have produced the effect, but as thesun’s attraction depresses the water at e and g , and raises it at f and /<, the combined effectof the two luminaries will be that shown at E F G H, and such an effect is called aveap-tide.

The highest spring-tide does not occur immediately after the new or full moon, butabout the third or fourth tide afterwards, and the lowest neap about the same time afterthe quarters.

The magnitude of a tide is the difference between the highest flood and the lowest ebbduring the same day.

Tlie high and low state of the tide occur twice during a lunar day, or at the rate of12 hours 25 minutes and 14 seconds. The tide is, however, 9 or 10 minutes longer ebbingthan flowing, and the higher it is at the ebb, the lower it generally sinks on the sameday.

The atmosphere is subject to elevations and depressions like those of the sea, andcause variations that affect the winds and weather; but their influence cannot be very great,for the addition of a few feet to the height of the atmosphere would be productive of littlechange. The height of an aerial tide must correspond with the height of the observabletides of the ocean, and the alteration of atmospheric pressure may be measured by the dif-ference between the actual form and the spheroid of equilibrium. Near the equator thereis a periodical variation in the state of the atmosphere far greater than in our climate, butwhich is not caused by the action of the moon, as it happens regularly at the same hourof both day and night. The atmosphere is affected by a current from east to west, likethat of the sea, which is attributed to the attraction of both sun and moon. Since Newtonexplained his theory of the tides, and the effects of the laws of gravitation, the theory hasbeen much improved by the labours of later mathematicians: but no problems aremore difficult to solve than those which belongs to such investigations; and the causes andcircumstances upon which they are dependent are so remote, that it is more than probablethey will remain for a length of time unknown to us; and it is absolutely necessary, beforewe can arrive at a conclusion, that we should have ascertained the depth of the seathroughout the globe.

The great wave which follows the moon, or the tide , is an undulation in which there is notmuch progressive motion, except where it approaches the shore ; and the usual time of highwater is about two or three hours after the moon is on the meridian.

Where the passage of this wave is confined, the tides are not the result immediately ofthe sun and moon; the narrowing the mass of water elevates their level considerably ; thisis very evident where the waters of the Atlantic are opposed by the coast of Ireland , anddivided into three different branches, one passing up the British Channel , another west ofIreland and Scotland , and the third into the Irish Channel. The first of these moves at arate of 50 miles an hour, and passes through the Dover Straits, so as to reach the Nore atmidnight during spring tides.

The second branch is more rapid, reaching the north of Ireland six hours before; threehours afterwards it has arrived at the Orkney Islands , and three hours more, or at twelveo’clock, we find the same wave, extending eastward to the Naze of Norway ; twelve hoursafterwards, it has progressed through the German Ocean, and arrived at the Nore, meetingthe morning tide, that left the mouth of the channel only eight hours previously, so that