354 SURVEY OF THE COAST 



At about two-thirds of their length from below, they are near 

 two inches thick, tapering gradually and equally out on both 

 sides ; so that the lowest end becomes the smallest, and is 

 reduced to about one inch. This form of the legs adds con- 

 siderably to their strength, and prevents them from bend- 

 ing. ' -' 



Of these albidades and the motion works for the plane ta- 

 bles, I had only two constructed in England, tliough in the 

 farther progress of the work a greater number of them would 

 be required. These, however, might be made in this coun- 

 try, using those constructed in England as models. 



Description of the Magnetic JVeedles. 



In the survey of a sea coast to which ships come under the 

 guidance of the magnet, it was of course of interest to ob- 

 serve the variation of the needle, to obtain data for this inte- 

 resting element. 



For this purpose, and not with any view to its use in the 

 actual survey, two needles were constructed ; and 1 intended 

 to join to the observations of the variation, those of the os- 

 cillation, and for which I had a needle of my own. They 

 are constructed exactly on the same principle as the one I 

 had constructed in 1801, by M. Esser, Mathematical Instru- 

 ment Maker at Aran in Switzerland. 



Plate IX. fig. 5, is a vertical section of this instrument, 

 which may suffice to explain its construction. The needle 

 Z, z one foot in length, is in a circular box about an inch high, 

 having an horizontal circle x, x silvered, and divided to eve- 

 ry twenty minutes, A small silver vernier on the needle 

 assists in reading the subdivisions (wliich might however 

 have been carried farther on the circle itself) The circle is 

 divided as usual into 360°, beginning from a radius parallel 



