PIGUKE OF THE EARTH. 321 



deciding wliether the curvature of tlie two terrestrial hemisplieres should be 

 regarded as identical or distinct. At the end of the last century, as has been 

 before intimated, La Caille had transported himself from France to the Cape of 

 Crood Hope, in the design of co-operating in the solution of various astronomical 

 problems which in that remote country seemed to call for an intelligent ob- 

 server, and having there executed the measurement of a small arc of the mei idian, 

 he obtained for the irregularity of the globe a much smaller value than any of 

 the analogous ones deduced in Europe and America. How was this anomalous 

 result to be explained? In oue of two manners: cither by attributing it to a 

 real defect of symmetry in the form of the earth, or to an error, not easily to 

 be avoided, in the operations of La Caille; but as the first was contradictory 

 of the received theory and opposed to many facts well ascertained by other 

 observers, and as the second was scarcely admissible in view of the recognized 

 talent, industry and conscientiousness of the French savant, no one knew which 

 alternative to adopt. Everest, on his return from India, inspected the locality 

 where La Caille had operated, and at sight of the mountains which surrouncl 

 it concluded that the distinguished astronomer might easily have deceived him- 

 self, or neglected certain precautions without which no geodesic labor can really 

 afibrd a sufticieut guarantee of certainty. Maclear, with due regard to the 

 indications of Everest, undertook in 1837 an operation analogous to that pre- 

 viously executed by La Caille, though on a larger scale and with better material 

 resources ; and the result now confirmed the previsions of the theory, or the 

 identity of form of both terrestrial hemispheres. 



Nor has it been only in a direction from north to south that astronomers and 

 geometers have essayed to estimate the dimensions of the earth. When a com- 

 parison of the first results obtained in the proceedings directed to that object 

 had revealed, not only the defective sphericity of our globe, but the irregulari- 

 ties or accidents which interrupt its presumed ellipticity, whether from one pole 

 to the opposite, or even in passing from one meridian to another not far distant, 

 the attempt was also made to measure oue or more arcs of parallel. That by 

 this means, as by the former, and still better by a combination of both, a know- 

 ledge of the form and volume of the earth might be obtained, is readily con- 

 ceived ; and when it is considered that this new operation is even more delicate 

 and troublesome than the other, the reader will scarcely wonder that till a quite 

 recent epoch the number of arcs of pai-allel measured bore no proportion to the 

 arcs of meridian. The Franco-Spanish commission, charged with measuring an 

 arc of the latter sort in Peru, proposed also to determ'uu the value oi a degree 

 of parallel, which in those regions would have been a degree of the equator, 

 but a difierence of views as to the execution, added to the dii.iculties of the 

 enterprise, led to a relinquishment of the project before it had begun to be 

 carried into efixict. At the same epoch, 1734 to 1740, the Cassinis, Maraldi 

 and La Caille measured in France two arcs of parallel, one in the latitude of 

 • Paris, the other across Provence; and still later Lambton undertook in India a 

 work of the same kind; but these first essays led to no definite result, and 

 s.'rved only to show at once the utility of the undertaking and the difiiculties 

 which its adequate accomplishment would present. The measuri;ment of a 

 great arc of parallel, stretching from the neighborhood of Bordeaux to Padua, 

 or from the ocean to the Adriatic, over an extent of 13° and at a latitude of 

 45^ 40', was commenced u» 1811 under the direction of Colonel Crossean, and 

 continued in 1820 across upper Italy by Carlini, Plana and other astronomers 

 and geometers of Italy, Fi-ance and Switzerland. Besides this operation, which 

 forms an epoch in the annals of geodesy, there must also be mentioned the 

 measurement of another arc upon the parallel of Paris, from Brest to Strasburg, 

 executed between 1818 and 1823, by the French functionaries Bonne and 

 Ilenry ; another completed from Greenwich to Valcntia in the west of Ireland, 

 by Professor Airy ; and a third commenced in 1857 by W. Struve, in 52^ of 

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