EXPLANATION OF SOURCE AND USE OF TABLES. 



cin 



lo', 20', 30'. The curved line joining the extremities of these perpendiculars is 

 the parallel required. It may be drawn by means of a flexible ruler. The other 

 parallels are constructed in the same manner. They are all concave towards the 

 north or south according as the map shows a portion of the northern or southern 

 hemisphere. The meridians are drawn in a similar manner through the points 

 {e.g., F, Q, M, J?, S, T, U'\n Fig. 4) having the same longitude relative to the 

 middle meridian. All meridians are concave towards the middle meridian. 



A test of the graphical work which should always be applied is the approxima- 

 tion to equality of corresponding diagonals in the various quadrilaterals formed. 

 Thus in Fig. 4, /X should be equal to IVV, CN to CN', EVio EW, etc.* 



35" 



50' 



dOf 



30( 



20' 



Id 



34." 



Fig.4. 



Tables 25-29 give areas of quadrilaterals, bounded by meridians and parallels, 

 of the earth's surface. They are taken from " Bulletin 50, U. S. Geological Sur- 

 vey." The unit of length used is the English mile, and the areas are thus given 

 in square miles. The method of using these tables is obvious. 



Table 30 gives data for the computation of heights, from barometric meas- 

 ures, in accordance with the formula of Babinet.f This table is taken from the 

 " Smithsonian Meteorological Tables " (the first volume of this series). The 

 manner of using it is explained in connection with the table. 



* It should be noted that C^is not equal to ^ r', A^ and F referring here to points on the 

 developed parallels. 



t Comptes Reiidiis, Paris, 1S50, vol. xxv. p. 309. 



