406 SURVEY OP THE COAST 



to be laid off from them within the limits of usual dividers; 

 and in this manner every point was laid off by rectangular 

 ordinates, from the nearest sides of the trapezium of the pro- 

 jection. 



The accuracy of the projection is therefore the basis upon 

 which the accuracy of the wliole work depends, and to this 

 great attention is to be pa"'d. It will, on every sheet, be;gin 

 by a right anyrle in the middle, extending over the whole pa- 

 per both in latitude and longitude ; and to ol)tain this wi^h 

 accuracy, as well as in all other parts of the construction, it 

 is necessary to assist the eye with a maa;nifying glass, to aug- 

 ment the sharp vision of the small points, which it is only al- 

 lowed to make, if the sinking of the beam compass shall not 

 introduce errors in the work. 



It is not allowed to use any compasses but tliose with rec- 

 tangular points. For the smaller parts, the proportional 

 beam compasses, No. 46 of the Catalogue of Instruments, 

 "Were intended. They are very convenient in the hand, and 

 easy to support against making too deep points. To prevent 

 the beam compasses from making deep impressions, and to 

 ease their guidance, I used to suspend them over the table 

 by a roller with a counterpoise. 



It is evident that in no case whatever a method of laying 

 off a point, by means of an angl*" any how constructed, can 

 be ajiplied in these works. The only place where these are 

 admissible is in the plane table operations, and in laying off 

 points of soundings with the station pointer, observed by the 

 prottlem of three points. 



The projection which I intended to use was the develop- 

 ment of a part of the earth's surface upon a cone, either a 

 tangent to a certain latitude, or cutting two given parallels 

 and two Meridians, equidistant from the middle meridian, and 

 extended on both sides of the meridian, and in latitude, ordy 

 so far, as to admit no deviation from the real magnitudes, 

 sensible in the detail surveys. I had just commenced some 

 calculations relative to the question, — which radius of the 

 earth was most advantageous to admit the greatest extent to 



