250 LIGHT SCIENCE FOR LEISURE HOURS. 



Ireland, a base-line nearly eight miles long was mea- 

 sured near Loch Foyle in Ireland, and another nearly 

 seven miles long on Salisbury Plain. Trigonometrical 

 operations were then extended from Loch Foyle to 

 Salisbury Plain, a distance of about 340 miles; and 

 the Salisbury base-line was calculated from the obser- 

 vations made over this long arc. The difference between 

 the measured and calculated values of the base-line was 

 less than Jive inches ! As we have stated, the trigono- 

 metrical 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 con- 

 formably 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 

 features 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 there- 

 fore be made to depend on the accurate results obtained 

 by the great trigonometrical operations. In order to 



