252 PHILOSOPHICAL TRANSACTIONS. [aNNO 17 8/. 



ments of the very best kinds, there should remain an uncertainty in this res- 

 pect of 2 or 3 seconds of time : for in the latitude of Greenwich 3 seconds 

 correspond to 477, and in that of Oxford to 474-1- fathoms. These however 

 are points which must be left to the respective astronomers to settle in the best 

 way they can ; and it is not to be doubted that the Astronomer Royal will 

 throw a new and very satisfactory light on the matter, in the paper which he 

 proposes about this time to lay before the r. s., along with M. Cassini's Me- 

 moir, which, for that purpose, has now been nearly 2 years in his possession. 



With regard to the trigonometrical operation (which may be considered as 

 infallible, because, by means of the base of verification, it will prove itself, and 

 if small errors unavoidably arise in the course of a long suite of triangles, the 

 maximum of these may be always ascertained,) Gen. R. has no doubt that the 

 distance between Greenwich and the point P in his map may thence be de- 

 termined to a very small number of fathoms, perhaps to 15 or 1 6 on a difference 

 of longitude of about 2° 20' 20'', and therefore to about ^-oth part of a second 

 of time on each degree. This, for any useful purpose, will certainly be ad- 

 mitted to be sufficiently near the truth, and is probably considerably nearer than 

 it will be brought for many years to come, by a mean of the best observations 

 of the heavenly bodies, if these should be found in the present state of the 

 matter to leave it yet doubtful to 2 or 3 seconds. 



The astronomical difference of time may also be obtained by experiments on 

 the instantaneous explosion of light; but these he would propose to be made 

 subsequently to the trigonometrical operations. The station of Tatterlees, to- 

 wards the eastern extremity of our range of chalk hills, or some point near it, 

 would seem to be the most proper for the place of explosion, because it can be 

 seen from Bottle-hill, on the same range, and nearly in the meridian of Green- 

 wich Observatory. It is not to be doubted, that Tatterlees may be seen from 

 Fienne Windmill, or even perhaps from that of the Brunemberg; since they 

 are both situations on the continuation of the same range in France, the 

 distance being shorter too, and little land, but chiefly sea, intervening. Let us 

 then suppose, that the two astronomers with their clocks and transit-instruments 

 are posted, one at Bottle-hill, and the other at the Brunemberg, while gun- 

 powder is repeatedly exploded at Tatterlees, or while the Indian lights are al- 

 ternately exhibited, and again covered by an extinguisher prepared for the 

 purpose, which operation may be repeated several times the same evening; it is 

 certain, that a just mean being taken between the instants so marked by the re- 

 spective clocks, well regulated before-hand, the difference of time between the 

 two extreme stations will thus be obtained to a very considerable degree of ac- 

 curacy, and probably more to be relied on than that resulting from the com- 

 parison of the observations of the heavenly bodies. 



