■454 PHILOSOPHICAL TRANSACTIONS. [aNNO 1767. 



ratio of 100 to 118, and yet will be more than about tVtt of the greatest effect 

 above calculated for a single vertical float-board; so that the found dimensions 

 must be a small matter altered, and the quantity baa increased by ■^. 



For 6 = 0.3y6 &c. For 6 flo 0.277 &c. 



9 = 0.336 &c. 7 0.281 &c. 



12 = 0.323 &c. 9 0.285 &c. 



18 = 0.295 8rc. 12 ....,.,,,,.. 0.284 &c. 



an infinite number = 0.214 &c. 18 . .,^. .^ 0.276 &c. 



2° if/ = \a. an infinite number 0.238 &c. 



In this second case, Q float-boards are to be taken, instead of 6 shown by the 

 table, though the difference will be but very small ; and we shall have an effect 

 which will be -[Vo- of that of a vertical float-board, and in that ratio that the 

 quantity baa, found by the above formulae, must be increased. 

 3°If/=0.866a, 



for 12 fl = 0.099 &c- 



18 = 0.099 &c. 



36 = 0.104 &c. 



an infinite number . . = 0.103 &c. 



In this third case, the difference is still very small, and the effect resulting 

 from 36 float boards, will be tVo- of the effect of a single vertical float-board. 



XXXVI. A New Method of Constructing Sun Dials, for any given Latitude, 

 without the Assistance of Dialing Scales or Logarithmic Calculations. By 

 James Ferguson, F. R. S. p. 889. 

 This new method is given, improved, at p. 95 of the author's Select Exercises, 



published in 1773. 



XXXVIL On the Formation of Islands. By Alex. Dalrymple, Esq. p. 394. 



There is not a part of natural history more curious, or perhaps to a navigator 

 more useful, than an inquiry into the formation of islands. The origin of islands 

 in general, is not the point to be discussed; but of low, flat, islands in the wide 

 ocean: such as are most of those hitherto discovered in the vast South-sea. 

 These islands are generally long and narrow, they are formed by a narrow bar of 

 land, inclosing the sea within it; generally, perhaps always, with some channel 

 of ingress at least to the tide, commonly with an opening capable of receiving a 

 canoe, and frequently sufficient to admit even larger vessels. 



The origin of these islands will explain their nature. What led Mr. D. first 

 to this deduction, was an observation of Abdul Roobin, a Sooloo pilot; that all 

 the islands, lying off the n. e. coast of Borneo, had shoals to the eastward of 

 them. These islands being covered to the westward by Borneo, the winds from 

 that quarter do not attack them with violence. But the n. e. winds, tumbling in 



