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XIV. A Letter from Mr- Peter Dollond, to 
Nevil Mafkelyne, F. R . S. and Aftrono- 
mer Royal ; defcribing fame Additions a?id 
Alterations made to Hadley’s Quadrant ^ 
to render it ?nore ferviceable at Sea . 
Reverend Sir, 
Read March 29, t | > H E particular attention, you have 
I always fhown to any improve- 
ment tending to the advantage of aftronomy or na- 
vigation, makes me take the liberty to trouble you 
with an account of fome additions and alterations I 
have lately made to the Hadley’s quadrant. 
The general ufe of this inftrument at fea is fo well 
known, that no mention need be made of the im- 
portance of any improvements in the conftru&ion, 
that may render the obfervation more exadt, and 
occafion more frequent opportunities of making 
them. 
The glades of the Hadley^s quadrant fhould have 
their two furfaces perfect planes, and perfectly par- 
allel to each other. From feveral years pra&ice in 
grinding thefe glades, I have found out methods of 
making them to great exadtnefs ; but the advantage, 
that fhould arife from the goodnefs of the glades, 
