62 Mr Wightman on the Advantages of the 
about 120,000 inhabitants, it was estimated that 80 miles of 
levellings would be required to establish a sufficient number 
of permanent bench marks in the town, and it was further 
estimated that these 80 miles of levelling would cost L.200, 
and the addition of the contour lines L.50; and, reckoning 
the number of houses at 26,666, the expense would be 2}. 
per house. 
Remarks on the Advantages and Economy of the Moveable- 
Derrick Crane, improved and introduced into general use by 
WILLIAM WIGHTMAN, Contractor, in the year 1837; but 
more particularly as applicable in the Construction of Bridges, 
Piers, Breakwaters, and Naval Architecture. Communi- 
cated to Ed. New Phil. Journal by the Royal Scottish 
Society of Arts.* With a Plate. 
It is well known, that, previous to the year 1837, no other 
crane was generally known or used in the construction of 
public works, but the common, or, what might be appropri- 
ately called, the gibbet-crane. . These differed sometimes a 
little in form and arrangement of their machinery, but never 
so much as to alter their principle. 
The small figure at A, fig. 3, Plate IT., will give a good idea of 
their general structure, and that which was most in use. They 
were, however, even in their most approved form, always un- 
wieldy, top-heavy, and difficult and dangerous in fixing up or 
taking down; so much so, that more accidents occurred from 
these frequent operations, than from performing their proper 
work. It will also be easily observed, that their capabilities 
for lifting or depositing stone, or other material, in the con- 
struction of works, soon become exhausted, from the point of 
suspension being immoveable to or from the fulcrum, and 
performing only one circle of a radius equal to the length of 
the cross-beam. 
This great defect could, in some measure, be remedied a 
* Read before the Society on 12th April 1844. 
