370 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1959 
intensive study in many countries with a view to predicting their oc- 
currence in time for all possible precautions to be taken, and to esti- 
mating the maximum height likely to be reached by the tidal waves 
so that appropriate defenses can be planned. 
The study of all the effects meterological conditions may have on 
water level is likely to be of considerable importance to navigation as 
well as to coastal engineering, because winds and variations in atmos- 
pheric pressure often significantly change the actual depth of water in 
a channel or over a bar. Even now it would pay most large harbor 
authorities to set up a small team to predict these meteorological dis- 
turbances of the tides. 
MEAN SEA LEVEL 
Industrial development of land near sea level—some of it reclaimed 
land—is now so great that changes in mean sea level, which may be 
quite significant over a period of 20 years or so, could have serious 
implications. The question is also important in geodetic leveling, 
where the results depend on information about changes in the volume 
of water in the oceans and about movements of water from one region 
to another, as well as on information about changes in the shape of 
the earth’s crust. Mean sea level is obtained by reading the water level 
every hour from a tide-gage record and averaging the readings over a 
month or a year, with some allowance at the beginning and end of the 
readings to avoid falsifying the average by including part of an un- 
completed cycle of the main semidiurnal or diurnal tide. 
It has long been known that there are appreciable changes in mean 
sea level from month to month and from year to year, and that these 
changes are not merely local, but extend to separate and distant places. 
In most regions the monthly averages show an annual cycle, with the 
lowest mean level in spring and the highest in autumn. This is 
roughly what might be expected from seasonal changes in the density 
of the water and in the balance between precipitation and evaporation. 
The annual range of monthly mean level varies from a few centi- 
meters at oceanic islands in the Tropics to as much as 165 cm. in the 
Bay of Bengal. More complete knowledge of these variations will 
help scientists to study changes of climate, exchanges of water between 
sea, air, and land, and the worldwide circulation of water. It also has 
some bearing on the problems of coastal engineering and geodetic 
leveling. 
Study of the yearly averages shows that there are fluctuations from 
year to year as well as from season to season. On the Atlantic coast 
of the United States these fluctuations are generally less than 2 cm., 
but occasionally they may be as much as 5 cm. Between 1930 and 
1949 there was an overall rise of 10 cm. of the water level in relation 
to the land. At Newlyn, near the southwest extremity of England, 
