Mr. Wollaston’s Def crip f ion of See.- 
fyflem of wires invented by Dr. Bradley, and called by the 
French Reticule Rhsmboide , whence it has commonly obtained 
in Engliffi the name of the Rhomboid. 
In the former I was difappoin ted by the weather ; which from 
the time I went into the country, in the middle of May, till 
the end of June, when that conftellation came to the meridian 
in the day- light, afforded me very few evenings fit for obfer- 
vation. 
In the latter I failed, through the imperfection of myinfitru- 
ment, or my own want of fkill in the ufe of it ; for though 
a fingle fet of ohfervations in any one evening would appear 
very good, yet when reduced by calculation, and confronted 
with other repeated trials, they never gave me the fatisfa&ion 
I wilhed. 
The rhombus (for a rhombus, and not a rhomboid, it ought 
mold properly to be called) is very good in theory ; but very 
difficult to get executed with precifion, and liable to fome inac- 
curacy in the obfervation. The truth of it depends upon the 
longer diagonal being exadly twice the length of the ffiorter 
one ; which requires an awkward angle (53 0 f 48") at the ver- 
tex, not eafily to be hit by the workmen, and therefore feldom 
fufficiently tiue. Befide this, as the fides of the rhombus, on 
which depends the calculation for differences of declination, are 
but 26° 33 x 54" declining from the perpendicular or horary 
wire, a very fmall error in obferving the paffage of a ftar 
makes a very material difference in the refult. 
This determined me upon making trial of a fquare placed' 
angularly (an addition to M. Cassini’s wires at 45 0 , asanay be 
feen in tab. XII, fig. 1.) which feems to anfwer better. I 
muff confefs I have not yet had opportunity for trying it fo 
completely as I could wifii : but I was unwilling to let this 
year 
