VOL. XL.] PHILOSOPHICAL TRANSACTIONS. 131 



their knowledge and experience in the like kind of measureinenls : whence we 

 may expect to succeed and execute it better than could have been done else- 

 where, by appl)ing timely remedies against the difficulties that occurred in 

 other places. 



These operations are to be founded on a basis of the greatest length possible; 

 which must be actually measured, and with the greatest exactness ; as it is to 

 serve for a foundation to the measurement of all the triangles. And in this 

 point too there is a very great convenience near Petersburg, seeing on the ice, 

 we may measure out a basis, greater than has been hitherto taken, namely, 

 from the coast of Ingria about Peterhoff, to the coast of Finland toward Syster- 

 beck. There is not less than 20 wersts distance between these two extremities, 

 and this great distance may be measured very exactly, the ice being very even. 

 And as this basis is situated between the Isle of Cronstad and Petersburg, in a 

 direction nearly perpendicular to the distance from Petersburg to Cronstad ; 

 there can be no better method for inferring thence, by exact observation of the 

 angles taken at the extremities of this basis, the distance from the centre of the 

 imperial observatory, to the steeple of the new church at Cronstad; which two 

 objects are seen reciprocally from each other, and are not less than 30 wersts 

 asunder : and this distance, onee known exactly, will serve as a foundation for 

 all the triangles to be taken ; of which each of the sides may have not less than 

 from 30 to 40 wersts, according as objects are found advantageously situated 

 for that purpose. It should begin with the mountain of DouderhofF. which, 

 with the imperial observatory, and the steeple of Cronstad church, forms one 

 of the most convenient triangles imaginable for the subject proposed. 



In taking observations at these three places, it must then be tried to discover 

 others of the same advantageous situation ; but when no remarkable objects are 

 found of the desired situation and distance, they must be erected on purpose, in 

 the same manner as in other countries. The most necessary instruments for 

 executing this undertaking, are, besides the ordinary astronomical instruments, 

 a common quadrant of between 2 and 3 feet radius, for observing the angles of 

 the triangles ; and a portion of a circle of the greatest radius that can be con- 

 veniently had, for observing the arches of the heavens corresponding with the 

 distances measured on the earth. As to the instrument for observing the arches 

 of the heavens, its radius ought not to be less than from 12 to 15 feet : but it 

 is not necessary that it should contain a large portion of a circle. It is only re- 

 quisite to have this portion somewhat larger than the arch of the heavens in- 

 tended to be measured. Thus, as the meridians, which may be traced in Russia, 

 can be extended only between 22 and 23 degrees, it will suffice that the instru- 

 ment employed, be a portion of a circle of 30 degrees 



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