48 THE POPULAR SCIENCE MONTHLY. 



line is uncertain in about the same degree. Of course, on those por- 

 tions of either continent which have been directly connected with each 

 other by geodetic triangulations, no corresponding uncertainty ob- 

 tains ; and as time goes on, and these surveys are extended, the form 

 and dimensions of each continuous land-surface will become more and 

 more perfectly determined. But, at present, we have no satisfactory 

 means of obtaining the desired accuracy in the relative position of 

 places separated by oceans, so that they can not be connected by 

 chains of triangulation. Astronomical determinations of latitude and 

 longitude do not meet the case ; since, in the last analysis, they only 

 give at any selected station the direction of gravity relative to the 

 axis of the earth, and some fixed meridian plane, and do not furnish 

 any linear measurement or dimension. 



Of course, if the surface of the earth were an exact spheroid, and 

 if there w^ere no irregular attractions due to mountains and valleys, 

 and the varying density of strata, the difficulty could be easily evaded ; 

 but, as the matter stands, it looks as if nothing short of a complete 

 geodetic triangulation of the whole earth would ever answer the pur- 

 pose — a triangulation covering Asia and Africa, as well aa Europe, 

 and brought into America by way of Siberia and Behring's Straits. 



It is, indeed, theoretically possible, and just conceivable, that the 

 problem may some day be reversed, and that the geodesist may come 

 to owe some of his most important data to the observers of the lunar 

 motions. When the relative position of two or more remote observato- 

 ries shall have been precisely determined by triangulation (for instance, 

 Greenwich, Madras, and the Cape of Good Hope), and when, by im- 

 proved methods and observations made at these fundamental stations, 

 the moon's position and motion relative to them shall have been de- 

 termined with an accuracy much exceeding anything now attainable, 

 then by similar observations, made simultaneously at any station in 

 this hemisphere, it will be theoretically possible to determine the 

 position of this station, and so, by way of the moon, to bridge the 

 ocean, and ascertain how other stations are related to those which 

 were taken as primary. I do not, of course, mean to imply that, in 

 the present state of observational astronomy, any such procedure would 

 lead to results of much value ; but, before the Asiatic triangulation 

 meets the American at Behring's Straits, it is not unlikely that the ac- 

 curacy of lunar observations will be greatly increased. The present 

 uncertainty as to the earth's dimensions is, however, a sensible em- 

 barrassment to astronomers, only in dealing with the moon, especially 

 in attempting to employ observations made at remote and ocean- 

 separated stations for the determination of her parallax. 



As to the form of the earth, it seems pretty evident that before 

 long it will be wise to give up further attempts to determine exactly 

 what spheroid or ellipsoid most nearly corresponds to the actual figure 

 of the earth ; since every new continental survey will require a modi- 



