Mr. Wollaston's Befcnption of, &c. 347 



iyflem of wires invented by Dr. Bradley, and called by the 

 French Reticule Rhomboide, whence it has comnaonly obtained 

 in Englifh the name of the Rhomboid. 



In the former I was difappointed 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- 

 vatlon. 



In the latter I failed, through the imperfecllon of my Inftru- 

 ment, or my own want of Ikill in the ufe of it ; for though 

 a fingle fet of obfervatlons in any one evening would appear 

 very good, yet when reduced by calculation, and confronted 

 with other repeated trials, they never gave me the fatisfadioa 

 I wiflied. 



The jrhombus (for a rhombus, and not a rhomboid, it ought 

 moft properly to be called) is very good in theory ; but very 

 difficult to get executed with preclfion, and liable to fome inac- 

 curacy in the obfervatlon. The truth of it depends upon the 

 longer diagonal being exactly twice the length of the fhorter 

 one ; which requires an aukward angle (^^'^° Y 48^^) at the ver- 

 tex, not eafily to be hit by the workmen, and therefore feldom 

 fufficiently true. Befide this, as the fides of the rhombus, on 

 which depends the calculation for differences of declination, are 

 but 26° 2>3^ 54^'' declining from the perpendicular or horary 

 wire, a very fmall error in obferving the pafl'age of a flar 

 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°, as may be 

 feen in tab. XII. fig. i.) which feems to anfwcr better. I 

 rnufl: confefs I have not yet had opportunity for trying it fo 

 completely as I could wifli : but I was. unwilling to let this 



year 



