Zach on Repeating Circles. 407 



^°^' isioJ - ^^'^^ ™^^^' ^^^ apparent velocity of the 

 waves, to which the rate of the ship, 6^ knots per hour, being 

 added, the sum 29,49 is the true velocity of the waves in sea 

 miles per hour. 



The velocity of the waves no doubt depends greatly on 

 the force and continuance of the wind that puts them in mo- 

 tion. This may account for the great difference in the results 

 of the few experiments that have been made for ascertaining 

 the velocity of the waves of the sea. Capt. Horsburgh men- 

 tions that Dr.Wollaston, by some experiments, found the ve- 

 locity of the waves near the coast of Yorkshire to be nearly 

 60 miles per hour; and that Captain Tate found the waves in 

 the China Sea to have a velocity of only 1 6 miles. It would 

 therefore be desu-able that more experiments should be made 

 on this subject. 



Trusting that this will be allowed a place in your valuable 

 Journal, I am, gentlemen. 



Your most obedient servant, 

 56, Great Hermitage-street, David Thomson. 



May 8, 1823. 



LXXXIV. On Repeating Circles. By the Baron de Zach. 

 [Concluded from p. .363.] 

 A FTER all these experiments and reflections upon repeating 

 -^-*- circles, we thought we might make this further experiment, 

 whether such a circle could show the effect which would be 

 produced on the plumb-line and on the level by the attraction 

 of mountains. This idea was more especially suggested by 

 the favourable nature of the place for such an investigation. 

 The city of Marseilles, which stands at the distance of 8000 

 toises from the sea, is surrounded on the north by a chain of 

 calcareous mountains, which rise to the height of two or three 

 thousand feet above the level of the sea. To the south-west, 

 at the same distance from the town, is a small rocky island, 

 level with the water, called Vile de Planier, upon which a 

 lighthouse has been erected. This favourable situation sug- 

 gested to us the idea of going to make the observation o^ laii- 

 tude at the foot of these mountains, and in the midst of the 

 sea ; and afterwards to combine these two points ofoli'^ervation 

 by a geodesical o})eration, in order to see whether we should 

 find, in the differences of the astronomical and geodesical lati- 

 tude, some anomaly similar to that found, at infinitely smaller 

 distances and elevati<jns, between Barcelona and Montjouy. 

 This anomaly the astronomers and geometricians of France 



did 



