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XX. An Account of the Trigonometrical Survey carried on in 
the Tears 1791, 1792, 1793, and 1794,, by Order of his Grace 
the Duke of Richmond, late Master General of the Ordnance. 
By Lieut. Col. Edward Williams, and Capt. William Mudge, 
of the Royal Artillery ; and Mr. Isaac Dal by. Communicated 
by the Duke of Richmond, F. R. S. 
Read June 25, 1795. 
INTRODUCTION. 
A general survey of the island of Great Britain, at the 
public expence, was (as we learn from the LXXVth Vol. of 
the Philosophical Transactions) under the contemplation of 
Government as early as the year 1763, the execution of which 
was to have been committed to the late Major General Roy, 
whose public situation and talents well qualified him for such 
an undertaking. Various causes procrastinated this event till 
the year 1783, when the late M. Cassini de Thury trans- 
mitted a memoir to the French ambassador at London, which 
paved the way to a beginning of this important work. Calcu- 
lated for the advancement of science, this memoir was pre- 
sented to the King, and readily met with the approbation of 
a monarch, so eminently distinguished, from the aera of his 
reign, for his liberal patronage of the arts and sciences. By 
