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XIV. A Letter from Mr. Peter Dollond, to 

 Nevil Mafkelyne, F. R. S. and AJlrono- 

 mer Royal\ defer ibing fome Additions and, 

 Alterations made to Hadley's ^uadrant^ 

 to render it more ferviceabk at Sea. 



Reverend Sir, 



Read March 29, JTT^ HE particular attention, you have 

 I always mown 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 Tea is fo well 

 known, that no mention need be made of the im- 

 portance of any improvements in the conftruction, 

 that may render the obfervation more exact, and 

 occafion more frequent opportunities of making 

 them. 



The glafTes of the Hadley's quadrant mould have 

 their two furfaces perfect planes, and perfectly par- 

 allel to each other. From feveral years practice in 

 grinding thefe glafTes, I have found out methods of 

 making them to great exact nefs ; but the advantage, 

 that mould arife from the good nefs of the glafTes, 



has 



