GEOGRAPHICAL INSTRUMENTS. z$$ 



triumphs of the geodesist have been secured ; and that a degree 

 of precision in measurement has been reached which to the 

 uninitiated really sounds almost incredible. 



Surveys are the basis of statistics and of administration, and 

 consequently rigorous accuracy is necessary. Hence their ope- 

 rations are most valuable aids to pure science, and are espe- 

 cially identified with investigations connected with the shape of 

 the earth, and with measurements of arcs of the meridian. Surveys 

 on a trigonometrical basis have now been commenced in all, and 

 completed in most of the countries of Europe (except Turkey) ; 

 and the United States Coast Survey has also been converted into 

 a regular trigonometrical survey on rigorous principles. 



In Great Britain the Ordnance Survey was begun in April, 

 1784, when General Roy measured a base line on Hounslow 

 Heath. The triangulation of the British Isles was commenced in 

 1784, and completed in 1852, and about 250 points have thus 

 been determined with the most extreme accuracy. The two base 

 lines, on Salisbury Plain and on the borders of Lough Foyle, 

 were measured by the lo-feet compensation bars invented by 

 General Colby. The Lough Foyle base was measured in 1827, 

 that on Salisbury Plain in 1849. An idea of the minute accuracy 

 of these measurements may be formed from the fact that, when a 

 portion of the Lough Foyle base was re-measured, the difference 

 between the old and new measurements was only one-third of the 

 finest dot that could be made with the point of a needle. The 

 accuracy of the base line measurements was put to a still more 

 rigid test. The bases on Lough Foyle and Salisbury Plain are 

 360 miles apart, and taking the measured length of the former to 

 start with, the length of the Salisbury Plain base was computed 

 through the intervening network of triangles, and compared with 

 its measured length. The difference was found to be about 

 5 inches. The instruments used in the triangulation were two 

 3-feet theodolites made by Ramsden, an 1 8-inch theodolite by 

 Ramsden, and a 2-feet theodolite by Troughton and Simms. 



