PICURE OF THE EARTH. 



333 



fton at Arbiiry Hill, that the observations of the stars are erro- 

 neous nearly 5 seconds, notwithstanding the goodness of the 

 instrmnenis, and the skill and care of the observer. Bat, before 

 I insist farther on (his head, I will answer one objection that 

 may be made to the principles of the method that I have pursued 

 in this Memoir. 



Those astronomers, v^'ho !)ave hitherto undertaken the mea- objtction. The 

 sureraent of der^rees of the meridiem, have deduced their elements He- _ 



"' ,11 peiicieiit on tiie 



measures by simply dividing the linear extent by the number eliiptica'.figurc, 

 of degrees and trjnutes found by observation of the fixed stars are too uncer- 

 taken at the two extrenciities of the arc. This is indeed 'he ^'^ , j j^^ 

 most simple that can be adopted 5 and it has the advantage of calculating th« 

 being independent of the elliptic figure of the earth, especially '"'^*''^' *» ^ 

 in arcs of small extent. The elements dependent on this 

 figure, are too uncertain to be employed in calculating the 

 angular intervals in the short distances between successive 

 stations, even as a means of verification, without risk'of com- 

 mitting greater errors than those to which astronomical observa- 

 tions can be liable. Accordingly one cannot safely make any 

 use of it in cases where great accuracy is required. 



I must admit the justness of this objection, and must there- 

 fore shew the extent to which it really applies to the present 

 subject. 



In the first place, I may suppose, that in consequence of^*^*'""*® .. 

 r ^ ■ \ . . .... Other hand, if 



some fault m the instrument, with respect to vertical position, diir. meast!i«« 



construction, or some accidental derangement, there is an giye diff. ellip- 



r 1 . , , . ^ , /- I soldi — it nui»~ 



error or some seconds in the observations of the fixed stars. ^^^^ ^^ adnm- 

 How is this to be discovered ? This is not to be done by com- ted th^t the 

 paring the value of a degree on the meridian, as deduced from t'i(,'"'i,e ascr'ib- 

 threse observations, with the results of other measurements in ed to the fig of 

 distant parts of the globe. For if we find that these degrees ^^^^^^^^q^^^'^'^ 

 so taken do not agree in giving the same ellipsoid, we are notenurj ot oLii. 

 to attribute all the dilTerences to irregularities of the earth, 

 without supposing any error on the part of the observer, of his 

 instrument, or of other means employed in his survey. 



But this, in fact, is what has generally been done. It must, 

 however, be acknowledged, that the majority of observers 

 have not been in fault, as they could do nothing better j but 

 loo much reliance has been placed on the goodness of their 

 instruments, their means, ami other circunn;;:tances. It is true 



that 



