INTRODUCTION. 81 



of the water proceeds from a change of depth, 

 from the colour of the bottom, from that of the 

 sky and clouds, from the light of the sun, or from 

 foreign substances swimming on the surface of tlie 

 water. The transparency of the sea water would 

 be easiest measured by letting down a flat surface, 

 fastened to the plumb line, painted white, with 

 stripes, or letters of black, or other colour, on it. 

 For want of tliis, a white earthen plate, or a board 

 covered witli white stuff, might be used. The 

 depth at which the board became invisible, or 

 the marks upon it undistinguishable in different 

 waters, would show their relative transparency. 



The opinions respecting the shining of the sea 

 are so very various, that it is well worth the while 

 to acquire more accurate knowledge of the subject 

 by continued researches ; for some persons attri- 

 bute it to the effects of electricity ; others to phos- 

 phoric matter, developed from substances in a state 

 of putrefaction ; others to living, and, for the 

 most part, microscopic animals ; and others, to 

 till these causes together. The fainter shining of 

 the sea most deserves to be enquired into, it having 

 been considered as an electrical or phosphoric de- 

 velopment of light from the water itself ; and it is 

 especially to be examined whether living lumi- 

 nous animalculas, only perhaps of a smaller species, 

 or in less numbers, may not be there and cause 

 the light. Perhaps the best way of doing this, 

 would be to filter a sufficient quantity of sucli 



VOL. I. c 



