140 



BOOK V. 



Both these surveyors, as well as the others, in the first place make use 

 of the haulage rope. These they measure by means of others made of linden 

 bark, because the latter do not stretch at all, while the former become very 

 slack. These cords they stretch on the surveyor's field, the first one to 

 represent the parts of mountain slopes which descend obliquely. Then the 

 second cord, which represents the length of the tunnel to be driven to reach 

 the shaft, they place straight, in such a direction that one end of it can touch 

 the lower end of the first cord ; then they similarly lay the third cord straight, 

 and in such a direction that its upper end may touch the upper end of 

 the first cord, and its lower end the other extremity of the second cord, and 

 thus a triangle is formed. This third cord is measured by the instrument 

 with the index, to determine its relation to the perpendicular ; and the length 

 of this cord shows the depth of the shaft. 



Some surveyors, to make their system of measuring the depth of a shaft 

 more certain, use five stretched cords : the first one descending obliquely ; 

 two, that is to say the second and third, for ascertaining the length of the 

 tunnel ; two for the depth of the shaft ; in which way they form a quadrangle 

 divided into two equal triangles, and this tends to greater accuracy. 



STRETCHED CORDS : A FIRST. B SECOND. B THIRD. C FOURTH. C FIFTH. 



D QUADRANGLE. 



These systems of measuring the depth of a shaft and the length of a 

 tunnel, are accurate when the vein and also the shaft or shafts go down to the 



