2o Mr . mountaine’s Letter to Mr . maskelyne. 
taken at different periods, and thofe over the whole face 
of the terraqueous globe ; but until that law is certainly 
known, charts can be conftrudted only from time to time 
from the lateft obfervations. I am, See. 
W. MOUNT AINE. 
P. S. From the differences, times confidered, between 
thefe identical obfervations of douglass, and Dr. Hal- 
ley’s chart in 1700, Mr. Charles leadbetter at- 
tempted, with great labour, to delineate a fet of curves for 
the year x 740, but without luccefs. He feems, by his un- 
finifhed work, to have plunged himfelf into an inextrica- 
ble labyrinth. I have the manufeript chart and his table 
of differences now in my cuftody. His attempt was frui- 
trated by the opinion he poffeffed of an uniform and re- 
gular mutation ; and this failure further convinced me of 
the neceflity of proceeding upon other principles. 
On 
