^4-3 ■ Mr. Wor.LASTON*s Defcription of a 



year (lip by, vvithout making it known ; fmce, I think, from' 

 wh:it I have done with it, I may be confident of its utihty *. ' 

 The properties and advaniuges of Inch a fyftem of wires ' 

 fcarcely need to be pointed out to afironomers. The whole ex« ' 

 terit of the field is employed as it is in the rhombus (the want 

 of which was faid to be Dr. Bradley's objedion to M. Cas- ■ 

 siNi's wires) ; but being formed of right angles or half- right 

 angles, to which workmen are moft accuflomed, they will 

 always be apt to execute their part better; and the obliques; 

 from the dltferences being juif double to what they are in the' 

 rhombus, give the comparative declinations with twice the^ 

 certainty. To this the number of correfponding obfervations 

 in the paflage of every flar add confiderably ; fuice you may 

 calculate its dllfance from the center C, from the angle D or E • 

 or from one of the intermediate angles K, as you fhall fee occa- 

 fion. The fame indeed you may do in the rhombus from D or 

 from E; or, if the rhombus be formed of wires, from the' 

 angle at L, fig. 2. ; but only with half the precifion. The' 

 refult of a (ingle pafTage of any one ftar (excepting towards the*' 

 extremities of the field) gives the extent of the field equally ia^ 

 each, provided the declination of the flar be known, by de-i 

 ducing its diftance from thofe feveral angles ; and fuch deduc- 

 tions ferve as a ilill farther check upon every obfervation ; be-' I 



1 



* What is here offered is by no means to be underftood as recommending any " 

 fyftem of wires in preference to aftual meafurement with a micrometer, but to 

 render the ufe of them as convenient as may be to fuch gentlemen as are not 1 

 provided with better inftruments. The equatorial micrometer with a large field 

 (fuch as I have feen at Mr. Aubert's, of Mr. Smeaton's conftruftion) I take to 

 be the beft inftrument for taking differences of right afcenfion and declination 

 out of the meridian ; and far fuperior to any fyflem of fixed wires, or indeed to 

 any equatorial feitor whatever. 



caufe, 



