214 Gen, Roy’s Account of a 
With regard to the trigonometrical operation (which may 
he confidered as infallible, becaufe, by means of the bafe of 
verification, it will prove itfelf, and if fmall errors unavoidably 
arife in the courfe of a long fuite of triangles the maximum 
of thefe may be always afcertained), I have no doubt that the 
difhnce between Greenwich and the point P in the map 
may thereby be determined to a very fmall number of fathoms, 
perhaps to fifteen or fixteen on a difference of longitude of 
about 2° 20 x 20", and therefore to about -^th part of a fecond 
of time on each degree. This, for any ufeful purpofe, will 
certainly be admitted to be fufficiently near the truth, and is 
probably confiderably nearer than it will be brought for many 
years to come, by a mean of the bed: obfervations of the hea- 
venly bodies, if thefe fhould be found in the prefent Rate of 
the matter to leave it yet doubtful to two or three feconds. 
The aftronomical difference of time may likewife be ob- 
tained by experiments on the inftantaneous explofion of light ; 
but thefe I would propofe to be made fubfequently to the trigo- 
nometrical operations. The Ration of Tatterlees , towards the 
eaftern extremity of our range of chalk hills, or fome point 
near it, would feem to be the moft proper for the place of ex- 
plofion, becaufe it can be feen from Bottle-hill , on the fame 
range, and nearly in the meridian of Greenwich Obfervatory. 
It is not to be doubted, that Tatterlees may be feen from Fienne 
Windmill , or even perhaps from that of the Br unember g ; fince 
they are both fituations, on the continuation of the fame range 
in France, the diftance being fhorter too, and little land, but 
chiefly fea, intervening. Let us then fuppofe, that the two 
aftronomers with their clocks and tranfit-inftruments are polled, 
one at Bottle-hill , and the other at the Brunemberg , while gun- 
powder is repeatedly exploded at Tatterlees , or while the Indian 
lights 
