264 EXPERIMENTS and OBSERVATIONS 



fea fhore at Oftend was nearly as good as that at the 

 mouth of that river. Although we may, to a great de- 

 gree, adopt his fentiments, ncverthelefs 1 think it pro- 

 bable that this increafed purity does not entirely depend 

 lipon the ocean ; for I have found the air over the Bays 

 of Chefapeak and Delaware of the fame degree of purity 

 with the atmofphere of the ocean. And hence I am in- 

 clined to think, that the air over a large body of water 

 is always purer, cseteris paribus, than that of the adjoin- 

 ing land, owing perhaps to a decompofition, which the 

 water may fuffer from the adtion of the Sun's rays ; and 

 this may iikewife be affifted by its alfo abforbing many 

 foreign matters Avhich on land are more or lefs intimately 

 mixed with the air in a mechanical way. This opinion 

 is conlirmed by Dr. White's experiments, who fays: " the 

 air over the river Ouze was conftantly purer than that of 

 the garden by 2 or 3 degrees." Fhilolophical Tranfac- 

 tions, vol. 68. — And in the fame paper he obferves, that 

 the fame happened with the air of the fofs when the 

 marfiies were overflowed. 



When I firfl engaged in thefe experiments it was my 

 intention to perform them only on fea air ; but I foon 

 found it neceflary to repeat them on land air for the fake 

 of comparifon. The fubjedl increafed ©n my hands. 

 The atmofphere of marlTies prefented itfelf as worthy 

 of ferious inveftigation. 1 therefore performed fome ex- 

 periments upon it; but proper length of time is necef- 

 fary to their repetition ; and for this realon I mufl omit 

 them for the prefent, and merely relate thofe I perform- 

 ed on the air of this city, its environs, and on the ocean. 



I fhall firft proceed to the enquiry whether the at- 

 .mofphere differs in purity in different iituations on land ? 



The opinion that the air is purer in the country and 

 on the tops of mountains than it is in towns, is adopted 

 by many ; therefore in afferting the contrary we muft 



prepare 



