Mathematical Inflruments. 7 
iontal, by parallel plates and fcrews. On 
the index, and over the compafs-box is fixed 
a double fextant, which moves exadtly in a 
vertical circle, with a fpirit level, and over 
that a telefcope, fo contrived, that when the 
bubble refts in the middle of the fpirit tube, 
the interfedtion of the hairs in the telefcope 
will cut an exadt level. The double fextant is 
divided in fuch a manner as to fhew on one 
fide thereof the degrees and minutes of any 
altitude or depreffion within the extent of its 
divifions. On the other fide are divifions for 
taking the height of timber Handing in feet ; 
and on the limb, there are alfo divifions for 
meafuring its breadth. It mud be alfo ob~ 
ferved here, that both horizontal and vertical 
angles are obferved at the fame time, which 
is extremely ufeful in laying down plots, 
when the hypothenufa! are to be reduced to 
horizontal lines ; when the telefcope is direct¬ 
ed to any objedt, the whole inftrument is 
fixed in fo firm a manner, that on directing 
the telefcope to the next, the limb remains 
entirely ftedfafl, which in other inflruments 
of this fort, is very difficult to be effected. 
36 Circumferentors, the principal furveying in¬ 
ftrument ufed in the Weft Indies. It is verv 
* » • * 
fimple, yet expeditious in the pradtice, and 
conffts only of a bra'fs circle, with a compafs 
divided into 360 degrees, on the center of 
which is fufpended a magnetic needle, and 
an index, on whofe extremities are two fights; 
the whole is mounted on a flaff, and fome- 
times for conveniency, on a ball and focket. 
37 Gunters, or four pole chains. 
38 Air levels which fhew the line of level, by 
means of a bubble of air and fpirits of wine 
.hermetically 
