^SS, dURVEY OF THE COAST 



on the box, one directed forwards in tlie manner described, 

 the other backwards on the other side of the I)ox. These 

 will serve for the purpose of verification, and may be taken 

 from the collection of instruments, of which all the parts are 

 double. 



It is evident that half the breadth of the box, or distance 

 of the directing telescope from its middle, must be consi- 

 deied in viewing the signals used in the alignments of the 

 base. This condition will be satisfied by making the breadth 

 of the signals equal to that of the box, and by directing the 

 telescope to the proper side of each signal in the fore and 

 back observations. 



In relation to the fourth requisite of the apparatus for 

 measuring base lines, viz. the ascertaining of the position ol 

 each part towards the horizon, the simplest and easiest mode 

 might seem to be to measure the line actually horizontal, 

 and in case a difference of level should occur, in which it 

 would be too inconvenient to follow the same level, to fall 

 or rise to a different level by means of a plumb line. But 

 this mode should not be adopted when great accuracy is re- 

 quired, as experience would soon prove to an attentive ope- 

 rator. The deviations from the level to be obtained will 

 increase so as to be unsatisfactory, and the placing of the 

 apparatus will be found too tedious, and at the same time 

 liable to inaccuracy. 



The measurements were generally taken as near as pos- 

 sible to the soil. This supposes of course that the lines mea- 

 sured in the intervals were all straight lines, which probably 

 was not the case. 



I considered that the shortest as well as most accurate 

 mode of proceeding was to ascertain the true position of 

 each bar when it was actually employed in the operation. 

 Horizontal measurements, however, should be taken when- 

 ever the ground is favourable to them ; and when this is not 

 the case, the tripods should be fixed to the proper inclina- 

 tion by means of the level above mentioned. 



