January 13, 1905.] 



SCIENCE. 



49 



experiments as well as the actual test at 

 sea, the suggestion that the two instruments 

 might be used for the determination of 

 differences of gravity was first published 

 in 1894 by Dr. Guillaume, of the Interna- 

 tional Bureau of Weights and ]\Ieasures at 

 Paris. Dr. ]\Iohn, of Christiania, success- 

 fully applied the method by actual tests in 

 various places in Norway for the purpose 

 of determining the gravity reduction of 

 the barometer for meteorological purposes. 

 Doctor Heeker installed his apparatus on 

 a steamship and sailed from Hamburg to 

 Rio Janeiro via Lisbon, Portugal, and 

 Bahia, Brazil, and returned on another 

 steamer to Lisbon, making observations 

 both ways. The results of his observations 

 have been published and show : 



1. That the intensity of gravity on the 

 Atlantic Ocean between Lisbon and Bahia 

 is nearly normal, and agrees with the theo- 

 retical values computed by means of the 

 general formula published by Helmert in 

 1901. 



2. That the difference of gravity at sea 

 in shallow water and in deep water corre- 

 sponds approximately to the difference of 

 gravity between coast stations and inland 

 stations. 



These results were submitted to the Geo- 

 detic Association at the last meeting. 

 Means were provided for another expedi- 

 tion and last March Dr. Hecker began his 

 journey, crossing the Indian Ocean and the 

 Pacific by way of Melbourne and Sydney 

 to San Francisco. Thence he recrossed to 

 Japan and China, and we may look for- 

 ward to an early statement of the results, 

 which are being aAvaited with deep interest. 



As in the case of the pendulum already 

 referred to, there has been in the last dec- 

 ade a decided improvement and simplifica- 

 tion in instrumental means and methods of 

 work. It is only necessary to cite the in- 

 troduction of tapes and wires for primary 

 ba^e measurement, the introduction of the 



transit micrometer for the elimination of 

 personal equation in time determinations, 

 and of the leveling instrument, devised in 

 the coast survey, which is making its way 

 into more general use. With the use of 

 the latter there has just been satisfactorily 

 completed the first precise line connecting 

 the Atlantic, Gulf and Pacific mean sea 

 levels in the coasts of the United States. 



In all countries the determination of the 

 mean sea level and the establishment of so- 

 called bench marks in the interior are being 

 actively prosecuted as they furnish part of 

 the required geodetic data. 



In beginning I referred to the measure- 

 ment of continental areas. Let us see what 

 has been accomplished as to the extent of 

 areal measurement since Snellius intro- 

 duced triangulation into geodesy 289 years 

 ago. In our own hemisphere, so far as I 

 am able to learn, about the three-hundredth 

 part of one per cent, of the area of South 

 America has been covered ; of Mexico about 

 one per cent. ; of the United States about 

 five per cent. Geodetically the British pos- 

 sessions in the western hemisphere are bar- 

 ren. We may say that less than three per 

 cent, of the western hemisphere has been 

 triangulated. 



In the eastern hemisphere we find that 

 about forty per cent, of Europe has been 

 covered, but if we leave out Russia the per- 

 centage rises to eighty per cent, for the 

 rest of Europe. 



The triangulation of Asia is furnished 

 by India and Japan, Java and Sumatra 

 and amounts to about four per cent. 



Australia shows about two per cent., 

 Africa about two and six tenths per cent., 

 making a total for the eastern hemisphere 

 of about seven per cent. 



If we exclude the north and soiith polar 

 regions a little over six per cent, of the 

 available land area has been triangulated, 

 or about one and one half per cent, of the 

 total surface of the globe. These figures 



