378 SURVEY OF THE COAST 



which the longer diameter is in the direction of the lines, 

 and the shorter perpendicular to the same. The intersec- 

 tion of these two lines forms below a short and sharp edge 

 which cuts with such ease and keenness, that on silver the 

 tool cannot even be left to press with its full weight, and on 

 brass its weight alone is sufficient to make a deep and sharp- 

 ly defined line in one stroke. 



The verniers may be easily divided, by placing them on 

 the instrument itself at such a proportional distance within 

 the circumference of the circle as the part shall be into which 

 they are to divide a subdivision of the instrument, and then 

 dividing as usual. This produces evidently, when they are 

 removed to the circumference, the loss of one division upon 

 their full length, in their comparison with the divisions of 

 the circle. On the dividing engine, with a tangent screw, 

 this division can be made by the mere adjustment of its re- 

 volutions. 



On some of my instruments, the pin holes which served 

 for this operation, and which are usually filled up again, are 

 left open, that a vernier may be restored again from the di- 

 vision of the instrument itself, if need should be. 



It may not be improper here to mention some of the de- 

 tails which are to be observed in the choice of instruments, 

 or in the direction of their construction ; as their influence 

 on the practical use of the Instruments is greater than might 

 be supposed, and as they are not always attended to by every 

 artist. 



The quality of the metal of which the framing part of tlie 

 instrument is made is not indifferent, as the stability of the 

 instrument depends in some measure upon it. H brass is 

 used, this is subject to very great variation, without being ob- 

 served by a person not well acquainted witli the subject. 

 The mixtures of copper and tin, properly cannon metal, 

 though usually called bell metal, are much more easily dis- 

 finguished in quality, and therefore preferable in many in- 

 stances. The use of this metal for tlie axes, when the sock- 

 ets are brass, or for the sockets, when the axes are steel, 



