30 PHILOSOPHICAL TRANSACTIONS. [aNNO 1735. 



of gravity in islands is very probably occasioned by the vicinity of a great quan- 

 tity of water, which being specifically lighter than land, attracts less in propor- 

 tion to its bulk. And we find by computation, that the odds in the pendulums, 

 between theory and practice, is not greater than what may be accounted for on 

 that supposition. We may also observe, that though the matter of the earth 

 were entirely uniform, yet the hypothesis of its being a true spheroid is not 

 near enough the truth to give the number of vibrations which a pendulum 

 makes in 24 hours. And suppose the true figure were known, the inequalities 

 of mountains and vallies, land and water, heat and cold, would never allow 

 theory and experiments to agree. But after the French gentlemen, who are 

 now about measuring a degree, and making experiments with pendulums in the 

 north and south, shall have finished their design, we may expect new light in 

 this matter. 



Of the Mexican Filtering Stone. By Dr. Abraham Voter, F, R, S. &c. N° 438, 

 p. 106. Abridged from the Latin. 



This stone has the name filtre from its porosity, by which it suffers liquors 

 to pass through it : and for this reason pots and mortars are made from larger 

 pieces of it, to strain liquors, particularly water to drink: for it is thought, 

 that the water filtered through this stone is freed from all its impurities, and 

 becomes clearer and purer, and more wholesome. Hence these stones are 

 highly valued in Japan, and sold at the price of gold; because the Japanese, 

 who know nothing of the stone or any other disorder in the kidneys, and who 

 prefer health far before all other blessings, are of opinion, that these petrified 

 fungi have the power of prolonging life. This species of fungus, it is said, 

 grows on the rocks in some places of the gulph of Mexico, about 100 elns 

 under water, and spontaneously hardens and petrifies in the air. Dr. Vater 

 does not attempt to determine the origin of the filtre stone, nor its produc- 

 tions, though both appear to be very suspicious, and invented only to prevent 

 its being thought a common stone. For Lentilius writes that there are 

 vessels made of two sorts of it ; one of a dark grey colour, like the lapis scis- 

 silis from Canada, and sold at a dearer rate; and others of a tophaceous colour, 

 of the growth of Italy. And, according to Le Clerc, in his physics, it is like- 

 wise dug up in the bishopric of Liege, and much used in Holland. Dr. 

 Ehrhart of Memmingen, presented Dr. Vater with a choice collection of fos- 

 sils, among which was a tophus very porous, found about Memmingen, and 

 which he was assured would strongly imbibe water. For, no sooner does it 

 touch the surface of the water, but the water ascends, and is carried quite 



