VOL. XXXVIII.] PHILOSOPHICAL TRANSACTIONS. 649 



Dr. Muschenbroek conjectured at first, that this sand is an imperfect magnet, 

 or subtile powder of it, which, when it is increased to a larger lump, forms the 

 common loadstones; but when he found by experience that common loadstones 

 exposed to the tire, according to some of the methods abovementioned, rather 

 lost of their force than gained, he altered his opinion, and owns that he has 

 not hitherto penetrated into the knowledge of this matter. 



Whatever it be, it is certain that there are several kinds of this sand, brought 

 from different countries; for it is brought from Persia: and some from Virginia; 

 there is another sort in Italy, common enough at Leghorn: and this last is 

 naturally very attractive; there are two sorts found in the Eber, a river of 

 Hassia; of which one resembles the Italian, and the other consists of large 

 grains, almost as large as hemp-seed, but scarcely having any virtue. Muschen- 

 broek had, besides, a very vigorous sort, which he is told was got near old 

 Ragusa in Dalmatia. The several kinds of this sand are unknown, which time 

 and the diligent observ:!tions of piiilosophers must hereafter show. 



Some Observations made in London, by Mr. George Graham; andal Black-river 

 in Jamaica, by Mr. Colin Camjjbel/, about the going of a Clock ; in order to 

 determine the Differtmce between the Lengths of Isochronal Pendulums in those 

 Places; and communicated by Mr. Bradley. N° 432, p. 302. 



Though it be upwards of 6o years since M. Richer first discovered, that 

 pendulums of the same length do not perform their vibrations in e()ual times in 

 different latitudes; and though several experiments, since made in different 

 parts of the earth, concur to prove, that pendulums swinging seconds are in 

 general shorter, as we approach the equator; yet what the real difference is be- 

 tween their lengths in different latitudes, does not seem to have been determined 

 with sufficient exactness, by the observations that have hitherto been commu- 

 nicated to the public; as may be gathered from the 20th proposition of the 3d 

 book of Sir Isaac Newton's Principia, where they are compared as well with 

 each other, as with the theory of that illustrious author. It is therefore to be 

 wished that more of this kind of experiments could be made with greater accu 

 racy in proper places, by such persons as have sufficient skill and opportunities 

 to do it; that we might be enabled to judge with more certainty concerning the 

 true figure of the earth, and the nature of its constituent parts. 



As an inducement to such as may have it in their power to put the like again 

 into practice, here follows an account of a very curious experiment of this sort, 

 made in Jamaica by Mr. Campbell: his clock, which was made by the inge 

 nious Mr. Graham, was so carefully contrived, that its pendulum might at plea- 



VOL. VII. 4 O 



