208 Mr. Ivory on the Figure of the Earth. 



nometers. It appears, therefore, that the longitudes, so far 

 as they have been observed, confirm my former argument, 

 drawn from the perpendicular measurements, in favour of the 

 spheroid of revolution deduced from the lengths of meridional 

 arcs. It would be very desirable to add to the instances al- 

 ready noticed, the difference of longitude between Dover and 

 Falmouth which Dr. Tiarks determined by chronometers; 

 but I have not made the calculations requisite to find the geo- 

 detical value of the same quantity. 



The measurement of portions of a parallel of latitude lately 

 made in France and the north of Italy, may be viewed pre- 

 cisely in the same light as the operations relative to Dover 

 and Portsmouth considered above. A chain of triangles be- 

 ginning at Marennes near Bourdeaux was carried eastward 

 through France to Geneva, and then continued in the same 

 direction to Padua. By means of the triangles the differences 

 of longitude of the intermediate stations were computed on 

 the supposition that the earth is a spheroid of revolution 

 having its compression equal to ^^^^ or •00324. The same 

 differences of longitude were likewise determined astronomi- 

 cally by fire-signals observed fx'om station to station. From 

 these operations it was found that the differences of longitude 

 in time between Marennes and Geneva, and Marennes and 

 Padua, are as follows * : 



Geodetically 29' 2"-220 — 51' 57"-340 



Astronomically.... 29 1 -078 — 51 56-121 



Diff: 1-142 Diff: 1-219 



If the geodetical and astronomical differences of longitude 

 had come out the same without error, the only inference must 

 have been that the earth coincides in its figure with the as- 

 sumed spheroid. And how much must be abated fi'om the 

 force of this argument, if the errors do not exceed the pro- 

 bable amoimt of the discrepancies unavoidable in such obser- 

 vations ? Every fire-signal is an independent experiment ; and 

 every irregularity of observation has its influence in the total 

 result: and hence it must be evident that the accumulated 

 amount of the erroi's of so many different operations, is not 

 only not improbable, but even that the greatest skill and cai'e 

 could alone have kept it within so small a compass. The 

 measurement of the parallel, therefore, furnishes a strong pre- 

 sumption that the earth is a solid of revolution such as is de- 

 duced from the length of the meridional arcs : and as this con- 

 clusion follows immediately from a comparison of the experi- 



* Com. des Terns, 1829. pp. 28!), 2.00, 291, 293. 



mental 



