240 PHILOSOPHICAL TRANSACTIONS. [aNNO 1787, 



regulated his clock by correspondent altitudes; but M. Messier corrected his by 

 a transit instrument, which however has no meridian mark. For the present I 

 infer, that we may take the difference of meridians 9"™ 20*, as being within a 

 very few seconds of the truth, till some more occultations of fixed stars by the 

 moon, already observed, or hereafter to be observed, in favourable circumstances, 

 and carefully calculated, shall enable us to establish it with the last exactness. 

 To collect and calculate such observations I have not leisure at present; but the 

 field of calculation is equally open to the celebrated astronomers of Paris, the 

 observations made at this place being now published annually. 



The extensive geometrical operations recommended by the late M. Cassini de 

 Thury, and commenced under the direction of Major-general Roy, p. r. s., by 

 his exact measure of a base on Hounslow-heath, may also, when completed, 

 determine the difference of meridians of Greenwich and Paris to great exactness. 

 But they do not seem to me likely to throw any new light on the difference of 

 latitude of the two Observatories, because the uncertainty we are still under about 

 the true figure and dimensions of the earth, and the irregular attractions arising 

 from the irregular external figure, and unequal density of the internal parts of 

 the earth, would prevent us from drawing any accurate conclusions, or such as 

 we could confide in, from those geometrical measures, with respect to so large a 

 quantity as 2° 38' 26" the difference of latitude; and, at all events, it must be 

 less exact, as it is less direct, to determine the difference of latitude of two places 

 from the measured distance of the two parallels compared with the length of a 

 degree in the intermediate latitude, inferred from former measures of degrees, 

 which were themselves determined with the help of astronomical observations, 

 than to infer it from the immediate astronomical observations made at the two 

 Observatories, in the manner I have already deduced it. 

 Greenwich, Feb. 21, 1787. Nevil Maskel-yne, Astron. Royal. 



XIX, An Account of the Mode proposed to he followed in Determining the 



Relative Situation of the Royal Observatories of Greenwich and Paris. By 



Major-general Wm. Roy^ F. U.S., andA.S, p. 188. 



Two years have nearly elapsed since an account of the measurement of a base 

 on Hounslow-heath was laid before the r. s., being the first part of an operation 

 ordered by his Majesty to be executed for the immediate purpose of ascertaining 

 the relative situations of the Royal Observatories of Greenwich and Paris; but 

 whose chief and ultimate object has always been considered of a still more impor- 

 tant nature, namely, the laying the foundation of a general survey of the British 

 islands. 



When the operation commenced in 1784, it was not doubted, that in 1786, 

 at latest, we should have been able to have proceeded with the series of triangles 



