An introduction to astronomy: designed as a text-book for the use of students in college. By Denison Olmsted ...

'i)lS. I1 69
thc10 lunar wave is lowcst,,and hence is to be added to the
height of low water at the time of Neap tfides. I elnce t1he
difference between highl and low water is only about lalf as
great at INeap tide as at Sprincg tide.
28 4. The powecr of the moon or of thel sun to raise te tide
is found by the doctrine of universal gravitation to be Yinversety
(s i/te cube qf /te distace."'llihe variations of distance in the
sun ar re not great enough to influence the tides very mlatcrially,
but tlhe variations in th.]e Imoon's distances have a striking
eflect. T'lle tide; which happen when the moon is in perigee,
are conlsiderably greater than wchen slhe is in apogee; and if
ishe happens to be in perigee at thte time of the syzygics, the
Spring tide is unusually high.
28.5. Thle declinations of the suni and moon cause the im
mediate tide, at a given latitude, to be greater than the opposite
one, or the reverse, according as the declination and latitude
are alike or unlike. When the (leclination is not.hing, both
tides are alike at every place. FIor if the moon is in the plalno
Fig. 67. Fig. 68.
N N
t S e..............-... \..~
of' the equator (i, 57) then tlt higiest points of bot] tidewaves are also on the equator; and, at a given latitude, a place
b)y the earth's rotation is carried round through'P.1.2,'t3 at................. 1.........................................
o a Place, Syst. dut Monde, 1. iv., c. x.
- Diagramss like these arc apt to mislead the learner, by exhibiting the protuber<ance occatsioned by the titdes as mutltch greater than the reality. We must
recollect, that it amounts, at ltie highest, to 0only a very few feet ill eight thousand miles. Were the diagram, therefolre, 1drawIn in just proportions, tle alterlations of figure produced by tlce tidtles would be wholly insensible.