292 THE POPULAR SCIENCE MONTHLY. 



rotation. It was found, however, that Clark's calculations were con- 

 siderably affected by certain anomalies probably existing in some of 

 the geodetic calculations employed, and it seems that a majority of 

 those competent to judge in these matters endorse the theory of a 

 revolving ellipsoid. 



By the term " figure of the earth " is understood the geometrical 

 form of an ideal surface coinciding with the mean level of the sea, and 

 prolonged in thought beneath the continents. In fact, geodetic calcu- 

 lations are always reduced to the sea-level, the altitudes of the stations 

 being first determined from levels based on the nearest coast-line. The 

 great difficulty is to accurately determine this level for a given station. 

 For a long time it was supposed that the surface of the open sea was 

 a horizontal ; in other words, that it was parallel to the surface of 

 liquids in repose, and perpendicular to the direction of the plummet- 

 line. But this definition is insufficient, as may easily be shown. The 

 apparent vertical indicated by the plummet-line or determined at the 

 level of the sea, is simply the direction of weight, which may be mate- 

 rially affected by local attractions due to an irregular distribution of 

 the masses composing the soil. The vicinity of a mountain will deflect 

 the plummet to a considerable degree, and a subterranean cavity may 

 cause a deflection in the opposite way. 



Let us now imagine the continents divided by a network of canals 

 that connect all the seas, thus making of them one continuous sheet of 

 water, as it were. Setting aside, for the purpose of the illustration, the 

 oscillations caused by the tides, this sheet of water, assumed to be im- 

 movable, which represents the mean level of the sea, will exhibit eleva- 

 tions and dej)ressions attributable to the local influences that deflect 

 the plummet-line. The attraction of the continents causes a notable 

 elevation of the sea-level along the coast, and a proportionate lowering 

 of the mid-ocean. This influence of continents was described by M. 

 Saigey in 1842, who gave as the probable height of the sea on the 

 coasts of Europe thirty-six metres. Seven years later Mr. Stokes, the 

 celebrated English physicist, attacked the question, bringing to bear 

 upon it all the resources of mathematical analysis ; and Philipp Fischer, 

 in 1868, estimated that the disturbance of level due to the attraction 

 of continents might amount to nine hundred metres. The mean level 

 of the sea is, therefore, in all probability, an irregularly undulated sur- 

 face, and the ideal or geometrical surface of the earth a regular s]Dhe- 

 roid, deviating but little from this average level, the accidental irreg- 

 ularity of which is in some way equalized. 



The triangulations by which the terrestrial arcs are measured define 

 the dimensions and configuration of this spheroid by the comparison 

 of distances measured on the earth with the corresponding angular 

 amplitude ascertained from the astronomical latitudes and longitudes 

 of the stations. The most delicate part of the operations consists in 

 ascertaining the local attractions that cause the deviations of the plum- 



