244 National Geographic Magazine. 
Little change (in the art of mensuration or surveying) seems to 
have been made until the early part of the 17th century when 
simple boundary line maps accompanied the terriers of the sur- 
veys made in Ireland in 1634, by order of Lord Stafford, then 
viceroy. Great improvements were introduced about that time in 
Sweden by Gustavus Adolphus, which must have become known 
to Cromwell, for in 1654, the “ Down Survey,” as it was called, 
comprised maps of the townlands, and baronies over two-thirds 
of the surface of Ireland, that is, comprehending about 20,000,000 
of English acres. 
It may not be uninteresting or irrelevant to bestow a few re- 
marks upon the development and methods of surveying in the 
seventeenth century, many of which have descended with little 
modification to the present day. 
When man first conceived the idea of owning real property the 
art of geometry or surveying became a necessity. Interest in 
other worlds than our own, and the measurement of time, led to 
the development of the science of astronomy, and of graduated 
instruments for measuring angles. Many of the most refined 
modern instruments are but slight modifications of original Ara- 
bian models, and the practice of linear surveying, or the subdi- 
vision of land into triangles, and geometrical figures, whose area 
could be computed, has been carried on without modification for 
centuries. 
The greatest development took place after the introduction of 
artillery in the methods and instruments used for tr igonometrical 
surveying or range-finding. Every principle which is to-day 
known and applied in the construction and use of modern trigo- 
nometrical surveying instruments can be traced in a modified 
form to the construction and application of the instruments of the 
sixteenth and seventeenth centuries. . 
In the practice of artillery, the first important question is the 
distance or range of the enemy. As in war it was clearly impos- 
sible to obtain the same by direct linear measurement, instruments 
were devised for measuring the range trigonometrically, all based 
on the calculation of a single triangle, the base and two angles of 
which could be measured. These instruments were simply modi- 
fied to the extent of furnishing in the instrument itself a constant 
base or angle so that only one or at most two measurements were 
necessary. 
