184 CELESTIAL MEASURINGS AND WEIGHINGS. 



tedious but impracticable, except on a carefully-levelled 

 plain free from all obstructions. Nevertheless, when the 

 object is to measure any large tract of country, or to con- 

 struct a chart of a territory by what is called a " Trigono- 

 metrical Siirvey^^ it is indispensable to lay down and 

 measure, no matter at what cost of time and labour, 

 some one such very long line, as a ^'- base line ;'" and to 

 mark its two extremities in some very distinct and per- 

 manent manner : so that their linear distance (a large 

 multiple of the original standard unit) shall not only be 

 exactly known, but shall be capable of being appealed 

 to for all future time, or at least till the whole work is 

 completed, as a new^ and larger unit, ^'- the length of the 

 base,'' to which all other distances in the survey are tem- 

 porarily referred. These, being subsequently reduced 

 by calculation to multiples and fractions of the original 

 unit, all the dimensions of the territory become finally 

 known in yards, feet, and inches. For the purpose of 

 measuring sucli a base, the ground must be cleared and 

 reduced to perfect horizontality (or any slight inclina- 

 tion exactly taken account of), and the intended base 

 line allifieated hy placing a telescope a little beyond one 

 of its proposed extremities, so as to command them 

 both, and as it were to fore-shorten its whole length into 

 one point, the intersection of two wires in its focus. 

 Anything seen in the telescope to the right or left of 

 this point, or above or below^ it, is out of the line. 



(10.) Whenever lengths are to be added by the re- 

 petition of one and the same unit, there is always a pos- 

 sibiUty of error arising from imperfect juxta-positions 



