556 PHILOSOPHICAL TRANSACTIONS. [aNNO 1754, 



diligent attention, he assures that he has been able to settle a point much contro- 

 verted by naturalists, in regard to the copulation of fishes. The most generally 

 received opinion has been, that they did not copulate ; but that the female did 

 cast her spawn in the water, and that then it was fecundated by the spermatic 

 matter of the male. Mr. Tull, in contradiction to this hypothesis, asserts, that 

 he has frequently seen fishes in actual copulation ; and that this is generally done 

 before the ova arrive at maturity. 



After Mr. Tull has castrated his fish, they are put into the water where they 

 are intended to continue. He makes no particular appropriation, neither with 

 regard to the ponds into which they are put, nor does he give them any particu- 

 lar nourishment ; but they take their chance in common with other fish, as 

 though they were not castrated. And he remarks further, that if tolerable care 

 is taken, very few fish die of the operation, when performed in the manner 

 here described; though heretofore, when, instead of the belly, he made the 

 opening in the sides of the fish, numbers died, from his wounding the intestines, 

 and frequently dividing the ureters. 



evil. An Attempt to point out, in a concise Manner, the Advantages which 

 will accrue from a Periodic Review of the Variation of the Magnetic Needle, 

 throughout the knoivn IVorld; addressed to the Royal Society, by Wm. Moun- 

 taine and James Dodson, Fellows, and requesting their Contribution thereto, 

 by Communicating such Observations concerning it, as they have lately made, 

 or can procure from their Correspondents in Foreign Parts, p. 875. 

 About the year I700, Dr. Halley having collected together many observations 

 on the variation of the needle, in several parts of the world, drew (on a mercator 

 chart) certain lines, showing the quantity of that variation, in those parts of the 

 world, over the representation of which those lines were drawn ; but as the quan- 

 tity of this variation is in a perpetual state of fluctuation, in perhaps every part 

 of the world, it had been so much changed in the space of about 40 years, that 

 when the writers of this paper endeavoured, about the year 1744, to draw on it 

 other lines to answer the purposes above mentioned, they found that those laid 

 down by Dr. Halley were grown entirely useless ; and that a system of such 

 lines, or something analogous, should be performed once in every 10 or lj2 

 years at least, m order to answer the purposes intended by that gentleman. 



In the reconstruction of them, the writers received the assistance of the com- 

 missioners of the navy, and of the directors of the East-India and African com- 

 panies, having leave to peruse the journals of those mariners which were under 

 the direction of each respective body ; from these, and a few private communica- 

 tions, they were enabled to draw the proper lines over the most frequented seas. 

 and to make some attempts toward doing the same in those least so ; a copy 01 



