APPENDIX. 



Observations on the Ufe of the Megameter in Marine 



Surveying. 



^ I ^HE greateft difEculty in marine furveying is that 

 of obtaining an accurate bafe, from the extremities 

 of which the angles may be taken with precifion, for 

 afcertaining the bearings and dlftance of headlands and 

 fhoals, when either want of time or other circumftances 

 make it impradlicable to land and meafure a bafe. The 

 ufual way is, to eftimate the diftance by the log, and to 

 take the angles by the compafs. This method is liable to 

 many errors, and affords no means of correcting or dis- 

 covering them. The Megameter, conftrudled upon the 

 principles of the obje6l-glafs micrometer, defcribed by 

 M. de Charniere and applied by him to find the longi- 

 tude at fea, I thought might be ufefully applied to 

 marine furveying. That which I ufed was made by 

 Ramfden, with fome improvements. The advantages I 

 imagined might be derived from this inftrument were, 

 a more corre£t and expeditious manner of determining the 

 pofition of coafts, and the diftance of {hoals or the fhip 

 from headlands. This inftrument being divided to ten 

 feconds, an angle may be taken by it with great ac- 

 curacy to five feconds. The height of a fhip's maft-head 

 above the water being known, it is eafy to find with this 

 inftrument, by a fingle pbfervation, the diftance between 



R 2 two 



99 



