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the Threads of the Screw, and of the preclfe Num- 
ber of Seconds correfponding to them. 
But altho’ thefe Points could be fettled with great 
Certainty, I was neverthelefs obliged to make one 
Suppofition ; which perhaps to fome Perfons may 
feem of too great Moment in the prefent Inquiry, 
to be admitted without an evident Proof from Fads 
and Experiments. For I fuppofie , that the Line of 
Collimation of my Telefcope has invariably preferved 
the fame Dircdion, with refped to the Diviiions up- 
on the Arc, during the whole Courfe of my Ob- 
fervations. And indeed it was on account of the 
Objcdions, which might have been raifed againft 
fuch a Tojluiate , that I thought it neceffary, to con- 
tinue my Series of Obfervations for fo many Years, 
before I publifhed the Conclufions, which I fhall at 
prefent endeavour to draw from them. 
Whoever compares the Refult of the feveral 
Trials, that have been made by the Gentlemen of 
the Academy of Sciences , for determining the Ze- 
nith Point of their Sedor, fmee their Return from 
the North ; will, I prefume, allow that mine is not 
an unreafonable or precarious Suppofition : fince it 
is evident, from their Obfervations, that the Line of 
Collimation of that Inftrument underwent no fenfi- 
ble Change in its Diredion, during the Space of 
more than a whole Year 5 altho' it was feveral times 
taken down, and fet up again in different and re- 
mote Places; whereas mine hath always remained 
fufpended in the fame Place. 
But befides fuch a flrong Argument for the Pro- 
bability of the Truth of my Suppofition , I have the 
Satisfadion of finding it affually verified by the 
Obfervations 
