248 LIGHT SCIENCE FOR LEISURE HOURS. 



Ireland, a base-line nearly eight miles longwas measured 

 near Lough Foyle, in Ireland, and another nearly seven 

 miles long on Salisbury Plain. Trigonometrical opera- 

 tions were then extended from Lough Foyle to Salisbury 

 Plain, a distance of about 340 miles ; and the Salisbury 

 base-line was calculated from the observations made 

 over this long arc. The difference between the measured 

 and calculated values of the base-line was less than five 

 inches ! As we have stated, the trigonometrical survey 

 of India will bear comparison with the best work of our 

 surveyors in England. 



A revenue survey is prosecuted for the definition of 

 the boundaries of estates and properties. The opera- 

 tions of such a survey are therefore carried on conform- 

 ably to those boundaries. 



The topographical survey of a country is defined by 

 Sir A. Scott Waugh to imply the measurement and 

 delineation of the natural features of a country, and the 

 works of man thereon, with the object of producing a 

 complete and sufficiently accurate map. Being free 

 from the trammels of boundaries of properties, the 

 principal lines of operations must conform to the fea- 

 tures of the country, and objects to be surveyed.' 



The only safe basis for the topographical survey of 

 a country is a system of accurate triangulation. And 

 where the extent of country to be surveyed is large, 

 there will always be a great risk of the accumulation of 

 error in the triangulation itself ; which must, therefore, 

 be made to depend on the accurate results obtained by 

 the great trigonometrical operations. In order to secure 



