Graduation of Jfironomlcal Inftruments . 13 
Mr, Graham lived till the year 1751 ; and during his time 
there were few inftruments of confequence conftruCted with- 
out his advice and opinion. They were for many years done 
by Mr, Sisson, to whom doubtlefs Mr. Graham would fully 
communicate his method of divifion ; and from this fchool 
arofe that very eminent and accurate artift Mr. Bird, whofe 
delicate hand, joined with great care and affiduity, enabled 
him ftill further to promote this branch of divifion ; and which 
being carried by him to a great pitch of perfection, the Com- 
miffioners of Longitude did themfelves the credit, by an hand- 
fome reward, to induce him to publifh to the world his parti- 
cular method of dividing agronomical inftruments ; which 
being drawn up by himfelf, in the year 1767, this matter is 
fully fet forth to the public : I ftiall therefore only take this 
opportunity of obferving, that there feems to be one article in 
which Mr. Bird’s method maybe ftill improved. 
I muft here obferve, that I apprehend no quadrant, that has 
ever undergone a fevere examination, has been found to form a 
perfect arch of 90° ; nor is it at all neceflary it (hould : the per- 
fect equality of the divifions throughout the whole is the firft 
and primary confideration ; as the proportion of error, when 
afcertained by proper obfervations, can be as eafily and 
readily applied, when the whole error of the reCtangle is fifteen 
feconds, as when it is but five. 
In this view, from the radius taken, I would compute the 
chord of fixteen degrees only. If I had an excellent plain fcale, 
I would ufe it ; becaufe I ftiould expeCt the deviation from the 
right angle to be lefs than if taken from a fcale of more moderate 
accuracy ; but if not, the equality of the divifions would not be 
affeCted, though taken from any common diagonal fcale. This 
chord, fo prepared, I would lay off five times in fucceffion, from 
i 3 the 
