234 SCIENTIFIC APPARATUS. 



The sides of the triangles average 35 miles in length, but some 

 are over 100, and one in miles long. The two distant stations 

 of the side of a triangle were connected by the use of a hehostat, 

 or revolving plane mirror for reflecting the sunbeams from the 

 point to be observed towards the far-off station. But, in this 

 climate, sometimes weeks elapsed before the gleam of light was 

 seen. Similar accuracy was obtained in measuring vertical angles, 

 the difference between the height of one of the Scotch mountains 

 so measured, and as ascertained by spirit level, being one inch and 

 a half. 



The exact measurement of an arc of the meridian of 10, 

 from the Shetlands to the Isle of Wight, was included in the 

 operations of the survey. By combining this measurement with 

 those of arcs of the meridian measured near Quito on the equator, 

 in India, France, Hanover, Denmark, Prussia, Russia, and 

 Sweden, Colonel Clarke found the figure of the earth to be a 

 spheroid with an equatorial semi-axis of 20,926,350 feet, a polar 

 semi-axis of 20,853,429 feet, ellipticity -rsisT. By 1860 the 

 trigonometrical surveys on the Continent were also approaching 

 completion, and it was determined, by connecting them, to 

 measure an arc of the 52nd parallel of latitude, from Valentia on 

 the west coast of Ireland, to Orsk on the river Ural, a distance 

 of more than one-fifth the circuit of the globe. The English 

 triangulation was successfully connected with those of France and 

 Belgium in January, 1862. 



As a preliminary step to the measurement of this great arc of 

 parallel, the standards of the world were sent to Southampton, 

 for comparison with those of England, in 1865. Those of India, 

 Russia, Belgium, Prussia, Austria, Spain, Italy, and the United 

 States were compared by Colonel Clarke at Southampton. These 

 two great works, namely, the comparison of the standards of 

 length, and the connection of the various national triangulations 

 for measuring an arc of parallel, are the crowning achievements of 

 geodesy. The latter operation is still in progress. 



