The Ordnance Survey of Great Britain. 245 
The one instrument that has received the greatest develop- 
ment in the modern type is the quadrant, a simple graduated are 
from whose center was suspended a plumb-line, or which carried 
a movable arm with raised sights for measuring horizontal or in- 
clined angles. This arm has retained the name alhidada derived 
from the Arabic. 
- Such was the trigonometrical instrument used by the earliest 
navigators and astronomers for determining latitudes, and by 
surveyors and artillerists for finding ranges. 
In the latter part of the 16th century Thomas Digges, surveyor 
and author, conceived the idea of combining two such graduated 
ares in one instrument, the one placed horizontally and the other 
in a vertical plane, the whole supported on a rigid stand or tripod, 
and he called the same his 7’heodolitus, which is said by DeMor- 
gan to have been the origin of the name of the modern instru- 
ment. 
In the earliest books in the practice of artillery and of survey- 
ing, the crescent of the dreaded Moor appears in the woodcuts 
illustrating range finding or trigonometrical surveying generally 
floating over the tower of some captured castle or town, which 
it 1s desired to bombard. This clearly demonstrated that the 
chief use of trigonometrical instruments was for military pur- 
poses. 
Among the instruments of surveying of this period which 
became practically obsolete in England in the present century, 
but which is most widely used elsewhere, is the plane-table, 
unquestionably one of the earliest instruments invented for 
measuring or recording angles. 
At the period 1570, when the Germans claim that it was 
invented by Pretorius, a professor of the University of Nurem- 
burg, it was unquestionably in use in England, and it is men- 
tioned by Thomas Digges, in his Pantometria, published in 1590, 
as a platting instrument for such as are ignorant of arithmet- 
ical calculations. On the relative merits of the theodolite and 
plane table authorities still differ. 
Throughout Europe great activity in the development of the 
practical applications of geometry soon followed the exchange of 
ideas brought about by the introduction of printing. 
Side by side with the important geographical discoveries of 
the age came the minor improvements in scientific instruments 
: VOL. II. 17 
