- 
Chemistry and Physics. 465 
fortnightly and monthly tides consist in an alternate increase and 
iminution of the ellipticity of the elliptic spheroid of which the 
sea level (after elimination of the tidal oscillations of short 
period) forms a part. There are two parallels of latitude, re- 
spectively north and south of the equator, which are nodal lines, 
along which the water neither rises nor falls. When, in the 
northern hemisphere, the water is highest to the north of the 
nodal line of evanescent tide, it is lowest to the south of it, and 
vice versa; and the like is ae of the southern hemisphere. If 
the ocean covered the whole earth the nodal lines would be in 
latitudes 35° 16’ N. and. (at chick latitudes 4—sin’ Zat. vanishes); 
but when the existence of land is taken into consideration, the 
nodal latitudes are shifted. Now according to Sir William 
Thomson’s amended equilibrium theory of pei tides, the hifi 
of the nodal latitudes depends on a certa n definite in tegral, 
whose limits are determined by the acl meres of land on the 
earth’s surface. 
For the purpose of examining the tidal records, it was there- 
fore first necessary to evaluate this integral. Approximation is of 
course unavoidable, and for that end the irregular contours of the 
8 of the q 
existence ofa “apy antarctic continent, the latitude of evanescent 
tide is 34° 40’ if there is no such continent it is 34° 57’. 
eart 
If — is s yielding of the earth, either with perfect or imper- 
fect elasticity, and with frictional resistance to the motion of the 
Water, the height of tide and the time of high-water must depart 
from the laws assigned by the equilibrium theory. Bien conclu: 
sg but differi ing in the time of high- wate & quarter- 
period from éhe theoretical time, viz: about t psn days 
eek for the eacebls ¢ Bee: bate co 
pounded into a single tide, with time 
within a half-period of the theoretical mes and this is the way 
Am. Jour. Scr.—Tuirp Series, Vou. XXV, No. 30,_-JuNE, 1883, 
31 
