496 Transactions. — Miscellaneous. 



An interesting article on geodetic measurement appears in the 

 October number of Nature, and shows that at the present 

 time great activity is displayed and much interest shown in 

 endeavouring to arrive at a more accurate solution of the 

 great problem involved in determining the figure of the earth. 



The Swedes and Eussians (jointly) are measuring an arc 

 of meridian in Spitzbergen, the field-work of which is now all 

 but completed ; and a well-equipped army of French experts 

 leave shortly for the equator to measure and to lengthen the 

 Peruvian arc. 



Lacaille's arc at the Cape, first measured some hun- 

 dred and fifty years ago, since corrected (in 1840), and still 

 later largely extended, is as yet the only measurement 

 effected in the Southern Hemisphere, the exact form of 

 which, from want of more complete 'data, is largely arrived 

 at by inference. Sir David Gill, however, proposes to carry a 

 chain of triangulation from the Cape to Cairo, and thence by 

 the coast of the Levant and through the isles of Greece to 

 connect with the existing European systems. This grand 

 conception, if given effect to, would mean an unbroken chain 

 of geodetic measurement from the Cape to the Shetland 

 Islands. At present the Cape triangulation extends from 

 about 28° S. to 35°, the southern limit of Cape Colony, which 

 is nearly on the same parallel as the northernmost point of 

 this Island, Cape Agulhas being in 34° 50' S. and North Cape 

 in 34° 22' S. 



This colony covers 13° of arc, including the 45th parallel, 

 to which (in the Northern Hemisphere) so much importance 

 was attached when the Formentara arc was measured ; and it 

 is also exceptionally situated in this respect : that the curva- 

 ture of the earth in this hemisphere cannot elsewhere be as- 

 certained in such high latitudes, excepting in South America. 



The cost of a geodetic survey is not now prohibitive, 

 though it necessarily increases with the degree of accuracy 

 demanded, but up to j^in. error per mile of base and Jin. 

 error per mile of the triangulation the cost is moderate. 



The elaborate apparatus formerly used in the measurement 

 of primary bases by means of short rods or bars is now dis- 

 carded in favour of steel bands ranging from 80 ft. to 500 ft. 

 in length (the most favoured length being 300ft.), and, by 

 means of these, results varying from a probable error of 1 in 

 1,000,000 to 1 in 1,500,000 (or, say, 1 in. error in twenty 

 miles) can be obtained. I have averaged the results of prob- 

 able error in five separate bases measured by the United 

 States Coast and Geodetical Survey in 1885 with a steel band 

 of 300 ft., and it amounts to 1 in 840,000, or about ^g-in. per 

 mile. Such exactitude is only possible with a measure of 

 absolute length, the modulus of elasticity and coefficient of 



