Dr. Gregory's Strictures on Don Rodriguez!. IQil 



one second either in the observations at Arbury Hill, or at 

 Dun nose ; and those at Clifton are, by the Don's own con- 

 cessions, out of the question. 



First, the mauner o^ Jix'wg the zenith-sector could not 

 lead to error. For, " to procure for the external stand (says 

 Col. Mudse, Phil. Trans. J 803), and thence for the whole 

 apparatus, a firm foundation, I caused four long stakes to 

 be driven into the ground, (one for each foot of the stand,) 

 to which its feet were firmly screwed down. The surfaces 

 of the stakes were cut off smooth, and brought into the 

 same horizontal plane, by which means the interior frame 

 and sector were placed much within the limits of their se- 

 veral adjustments." The whole was inclosed in a suitable 

 observatory. 



Don Rodriguez may perhaps think the French method 

 of fixing their instruments, on some occasions, preferable 

 to this. The reader shall judce. Their instruments, both 

 tor taking horizontal and vertical angles, were sometimes 

 placed on tottering stages, so as to give anomalies in the an- 

 gles of from 5" to S"; furnishing, as Delambre terms ihem, 

 " le tourment des observateurs." Thus, at p. 46, Discours 

 Preliminaire, we are told that at Chatillon there was a 

 high wooden stage erected for an observatory, in which the 

 carpenter had so badly done his work that *' le moindre 

 vent agitait toute la machine, de mani^re, non sculement k 

 rendre les observations moins siires, mais a iuquicter les 

 oOservatenrs." And on turnine; to p. 174, tome i. it wil! 

 be seen that the observers had not to contend with a gentle 

 eale; for they there tell us of the " grand vent qui agitoit 

 le signal el ['instrument." The whole was blown down 

 shortly after. Will Don Rodriguez place reliance on ob- 

 servations made from such a platform in such a wind, and 

 notwithstanding doubt the observations made with a stable 

 instrument by the English ? And let him not forget, that 

 whatever error was thus occasioned in the distance between 

 Boiscommun and Cli~Miiilon, is more than doubled in ail 

 the remaining trianyles of the series, by reason of the bad 

 shape of the triangle, Chatillon, Eoiscomniun, Chateau- 

 Jieuf. 



If no error in the English observations can be fairly im- 

 puted to the manner oi' Jixing the zenith sector, neither 

 can any be ascribed to the " construction" of the inslru- 

 njtnt itself. This was most positively declared by two very 

 excellent judges, the late Astronomer Royul, and tht Hon. 

 Henry Cavendish, on their close examination of the instru- 

 ment. It will also be inferred, without hesitation, by all 



competent 



