The Ordnance Survey of Great Britain. 253 
On the latter scale the’city of London with its environs could 
not be well shown on a sheet of paper less than 300 feet long by 
200 wide. 
When the facts are taken into consideration, that the Ordnance 
Survey is a cadastral one, in other words, that one of its many 
objects is the measurement and definition of all existing boun- 
daries, political, municipal, parochial or private, and a survey and 
valuation of property for assessments, that its maps are accepted 
in courts of law as authoritative on such questions, then the 
problem of the scales of publication is the most important one to 
be considered. 
As an illustration of the relation of the scale of a map to the 
amount of detail, which can well be represented on it without 
confusion, assume for a moment that an observer is stationed 
in a balloon, which can be raised or lowered or placed at any 
desired height above the ground, and in addition that he is pro- 
vided with a horizontal screen on which he is able to trace the 
details of the landscape below. The eye of the observer well 
represents the lens of a camera, and the screen the focussing 
plate. Therefore to produce a perfect image or map of the ground 
below it will be necessary to assume that all parts are stationary, 
balloon, plate and eye. For convenience assume that the eye 
remains over the centre of the screen at a distance of two feet. 
At a height of four miles above the ground the scale of the 
image on the screen would be exactly six inches to one mile, or a 
reproduction of the popular county map, on which every detail 
of importance such as houses, roads, paths, and fences is shown, 
and the smallest scale on which any attempt is made to preserve 
the relative proportions of such details. 
On such a scale the 1/100th part of an inch represents a dis- 
tance of very nearly nine feet on the ground and consequently 
however accurate the map might be in its projection, as an image 
showing the relative positions of all objects of importance on the 
ground, the scale is clearly too small for the measurement of 
areas for valuation purposes, and it is but a reproduction of the 
larger cadastral map. 
Again assume that the balloon is stationed at a height of 
twenty-four miles above the ground, and that the observer 
places his eye at the same distance of two feet above the screen 
and attempts to construct a map from the image on the screen, 
which is now reproduced at a scale of one mile to one inch, or 
