36 



the perpendicularity of the plane capable of 

 being determined by means of a plummet and 

 a large air-bubble level ; 

 A graphometer by Ramsden, placed on a cane, 

 with a magnetic needle, and a wire meridian 

 to take magnetical azimuths ; 



A dipping needle oi twelve inches, constructed 

 on the principles of Borda and Le-Noir. This 

 instrument, of the most perfect execution, was 

 ceded to me, at the time of my departure, by 

 the French board of longitude. The figure of 

 this instrument will be found in the narrative 

 of the voyage of d'Entrecasteaux % the astro- 

 nomical part of which was composed by a 

 learned navigator, Mr. de Rossel. An azimuth 

 circle serves to find the plane of the magnetic 

 meridian, either by correspondent dips, or by 

 seeking the position in which the needle is 

 vertical, or observing the minimum of the 

 dippings. The instrument is verified by ob- 

 serving on the east and west side, and chang- 

 ing the poles ; 



A variation compass by he Noir, constructed on 

 the principles of Lambert, and furnished with 

 a wire meridian. The nonius was divided at 

 every two minutes ; 



A needle twelve inches long, furnished with 

 sight-vanes, and suspended to an untwisted 

 thread according to the method of Coulomb. 



* Vol, ii.p. 14. 



