GEOGRAPHICAL LATITUDE. 777 



between the points of observation will be to the entire circumference as 

 the difiereuce in latitude is to 300 degrees. The earth being a spheroid 

 makes the calculation much more complicated, though the same princi- 

 ple of proportion between distance and degrees of arc remains as the 

 basis thereof. But inasmuch as it is known that the earth has consider- 

 able irregularities, it is not possible to determine its exact form from 

 one or two measured lines; but for entire accuracy the whole surface of 

 the land should be surveyed. To carry out as f;ir as possible this object, 

 there was formed iu 1861, the Middle European Commission for meas- 

 uring degrees. 



This commission working steadily from year to year will gradually 

 cover the entire surface of Europe with a net-work of surveyed tri- 

 angles, and thus be able to add greatly to our knowledge of the true 

 form of the earth. 



The improvement in measuring distances on the surface of the earth 

 has kept equal pace with the advance in astronomical accuracy. The 

 ancients accepted the distances as reckoned by travellers or at best the 

 land measurement of government emploj'es on a line not straight. 

 The Arabs measured one or more lines of a degree's length, by what 

 means we do not know, and with so little accuracy that there was a 

 difference in the results of three-quarters of a mile. The first measure- 

 ment of modern times was made in the sixteenth century by counting 

 the revolutions of a carriage wheel on an ordinary road. Finally in 

 1615, in The Netherlands, was made the first application of trigonome- 

 try to land measurement, and though the result was not nearly so 

 accurate as that of the wheel measurement, a new principle had been 

 introduced, which was destined to revolutionize investigations of this 

 nature, and by gradual improvement iu its application, to furnish re- 

 sults so accurate as to leave practically nothing to be desired. This 

 principle is that which forms the foundation of trigonometrical sci- 

 ence, viz, that the value of a line and two angles of a triangle being- 

 known the other quantities can be determined. From that time this 

 has been the method employed, with the exception of the attempt bj' 

 Mason and Dixon to measure an entire degree in Pennsylvania, which 

 failed to produce an accurate result, and of Norwood's survey from 

 London to York in 1634-'35. The best method having been disccfvered, 

 there remained still the possibility of enormous progress iu accuracy, 

 especially iu two directions: (1) iu measuring the angles of the tri- 

 angles; (2) in measuring the base-line. These have now been brought 

 to such perfection that a leading authority is of the opinion that 

 further improvement in this direction can not increase the accuracy of 

 the result so much as the outstanding uncertainty due to Irregularities 

 in the form of the earth.' 



Before passing to a chronological consideration of the most impor- 

 tant surveys made to determine the length of a degree, it may be well 



' Bessel, Gradmessung, 428. 



