HOW MAPS ARE MADE. 429 



otherwise, until at last, on Lis return journey along the eastern flank of 

 his route, the Lama with whom he had taken service insisted on his 

 riding, if only to promote flight from robbers, especially the mounted 

 bands of Chiamo-Goloks, of whom travellers are in constant dread. 

 Thus compelled, A. K. mounted a horse, but here also he proved e(|ual 

 to the occasion, for he at once set to work counting tlie beast's paces as 

 indicated by his stepping with the riglit foreleg. In this way he reck- 

 oned his distances for nearly 2'o0 nnlcs, between BarongChaidani (lati- 

 tude oG-^ 5', longitude 07^ ;V), and Thudcu Gomba (h'ltitnde 3;p 17'^ 

 longitude OC^ 43'), and the results do credit alike to the explorer's 

 ingenuity and to the horse's eipiability of pace." 



An account of his Journey will be found in the Scotti.sh Geographical 

 Magazine for 1885, p. 352. 



After the explorer comes the surveyor. His business is to produce a 

 detailed survey or map of the country. The operations of a cadastral* 

 survey on a grand scale, generally made by the Government, are divided 

 into two parts: (1) the great triangular survey, and (2) the top(>grai)h- 

 ical part, or the filling in of the details required for civil information. 



Before we go furtlier we should gain a thorough idea of tlie]»rinciples 

 of triangulation, because on it are founded all the conditions of an accu- 

 rate map. The great i)roperty of a triangle is this, that of all plane 

 geometrical figures it is the only one of which the form can not be 

 altered if the sides remain constant, and that the three angles of a tri- 

 angle are together equal to two right angles, so that if we know two of 

 the angles of any triangle we can at once calculate the third angle by 

 subtracting the number of degrees in the two known angles from 180 

 degrees, which is the sum of two right angles. If also we know the 

 length of one of the sides of the triangle as well as the number of 

 degrees in the angles, a very simple mathematical formula enables us 

 to calculate the length of the other sides. 



Now this is exactly what is done in the great trigonometrical survey 

 made in this country hy the Ordnance Survey: The surveyor measures 

 what is called a hase line. He puri)osely selects an absolutely horizon- 

 tal plane otherwise conveiuently situated for the purpose of measure- 

 ment. The base line is seldom more than 5 or G miles long, but it is 

 measured with "every refinement which ingenuity can devise or ex[>ense 

 command." In the Ordnance Survey of the British Isles — to give an 

 idea of the care with which such base lines are measured-^the original 

 base line, which wason Hounslow Heath, was measured in 1791, first with 

 a steel chain, then v>'ith deal rods, next glass tubes, and lastly, again 

 with the chain; and was over 5 miles long. Another line was subse- 

 quently measured 7 miles long, on Salisbury Plain, in 1794, which is 

 the base of the existing triangulation. The verification line at Lough 



*A cadastral survey is properly aud etymologically a survey by a .s^overnnient for 

 fiscal purposes, the word being derived from the low Latin capitastrum, a register 

 for a poll tax. As such a survey was naturally carried out with the utmost com- 

 pleteness, the term "Cadastral Survey" came to be used equally with the term 

 " Ordnauce Survey" for the great Government survey of Great Britain aud Ireland, 



