584 - LECTURE XLVII. 



suddenly with a head of several feet in height. These deviations probably 

 depend on the magnitude of the actual displacement of the water, which in 

 such cases bears a considerable proportion to the velocity of the tide, while 

 in the open ocean a very minute progressive motion is sufficient to produce 

 the whole elevation. The actual progress of the tides may be most con- 

 veniently observed, by means of a pipe descending to some distance below the 

 surface, so as to be beyond the reach of supe'rficial agitations, and having 

 Avithin it afloat, carrying a wire, and indicating the height of the Avater on a 

 scale properly divided. 



We have hitherto considered the tides so far only as they are occasioned) 

 by the moon; but in fact the tides, as thej' actually exist, depend also ort 

 the action of the sun, which produces a serves of effects precisely similar ta 

 those of the moon, although much less conspicuous, on account of the 

 greater distance of the sun, the solar tide being only about two fifths of the 

 lunar. These tides take place independently of each other, nearly in the 

 same degree as if both were single; and the combination resulting from 

 them is alternately increased and diminished, accordingly as they agree, or 

 disagree, with respect to the time of high water at a given place; in the 

 same manner as if two series of waves, equal among themselves, of whiclt 

 the breadths are as 29 to 30, be supposed to pass in the same direction over 

 the surface of a fluid, or if two sounds similarly related be heard at the same 

 time, a periodical increase and diminution of the joint effect will in either 

 case be produced. Hence are derived the. spring and neap tides, the effects 

 of the sun and moon being united at the times of conjunction and ojjposition, 

 or of the new and full moon, and opposed at the quadratures, or first and 

 last quarters. The high tides at the times of the equinoxes are produced by 

 the joint operation of the sun and moon, when both of them are so situated 

 as to act more powerfully than elsewhere. 



The lunar tide being much larger than the solar tide, it must always de- 

 termine the time of high and law water, which, in the spring and neap tides, 

 remains unaltered by the efTcct of the sun; so that in the neap tides, the 

 actual time of low water is that of the solar high water; but at the inter- 

 inctliate times, the lunar high water is more or less accelerated or retarded. 

 The progress of this alteration may easily be traced by means of a simple 



