4^0 An Account of the Measurement 
with safety, suppose the length of the standard chain B to be 
exactly the same then, as at the period when Mr. Ramsden 
compared it with the points inserted into the cast iron bar, 
mentioned in the first account of the trigonometrical operations. 
The measurement of this, the fourth base, commenced on the 
6th of June; and was continued, without much interruption 
from bad weather or other causes, till the 28th of July, when it 
concluded with the 263d chain, the overplus, 38,321 feet, being 
carefully determined, by means of a silver wire and pointed 
plummet let fall over the point marking the north-west extre- 
mity of the base. The two chains were then carefully compared 
with each other ; when it was found, that the wear of the chain 
A was exactly one division on the micrometer-head, or 
part of an inch. As the length of this base is nearly the same 
as that on Sedgemoor, it was reasonable to suppose that the 
elongation of the chain, by the working of the joints in each 
measurement, would be found the same, provided no injury had 
taken place from accidental circumstances, or rusting of the 
pivots and holes, during the time the chains were laid up in the 
Tower. After the reduction of the base, I shall have occasion 
to show that my ideas were correct in this point, as Mr. Berge 
has lately remeasured both chains. 
Angles of the great Triangles observed in the Tears 1800, 1801. 
At Beacon Hill . 
Between 0 , „ Mean. 
The north and south end of base - 20 47 19 1 „ 
20 [ 19,75 
20,25] 
