the Trigonometrical Operation . 269 
that no error of any confequence had accumulated in carrying 
on the operation to thefe diftant points refpedtively, even as far 
as the remote fhore of the Northern Ocean. 
Finally, in order to preferve the primitive fcale of diftances* 
whereon the accuracy of the recent operation, and all future 
ones that may hereafter be connected with it, muft always be 
fuppofed to depend, it is indifpenfably neceffary to eftablifh, 
without lofs of time, fome permanent marks at the extremities 
of the bafe on Hounflow Heath*. Thefe fhould be low cir- 
cular buildings, riling but a few feet above the furface of the 
Heath, compofed of the hardeft materials, fuch as granite, and 
conftru&ed in the moft durable manner by dove-tailing the 
ftones into each other. They would referable thofe bafements 
of ancient croffes we often meet with, formed into regular 
Heps, whereby the afcent is rendered eafy to the top of a 
circular table or platform, of fufficient dimensions for the re- 
ception of the great inftrument on any future occafion. 
In the interior part of thefe little buildings, metal tablets 
would be inferted, containing the name of that much-beloved 
Monarch in whofe reign the operation was begun, and thefe 
buildings executed ; the diftance from one to the other; the 
angle of the bafe with the meridian and alfo the, magnetical 
variation. 
It is not to be doubted, that the refpedtive lords of the ma- 
nors will readily veft in the Royal Society, the property of the 
two fmall fpots of the Heath fufficient for the ere&ion of thefe 
* Soon after the measurement of the bafe, Mr. Mylne, F. R. S. at my 
dehre, was fo obliging as to give a defign for a building of this kind, which, 
being conftru£fed nearly in the way of the Eddyltone Light-houfe, executed by 
the ingenious Mr. Smeaton, would anfwer very well. 
termini. 
