166 Original Letters of Dr. Franklin. 



so apt to gather on the warm walls. A tankard of cold 

 water, will, in a hot and dry summer's day, collect a dew 

 on its outside. A tankard of hot water will collect none in 

 the moistest weather. 



6. 'Tis, 1 think, a mistake, that the trade winds blow on- 

 ly in the afternoon. They blow all day, and all night, and 

 all the year round, except in some particular places. The 

 southerly sea breezes on your coast indeed blow chiefly in 

 the afternoon. In the very long run from the west side of 

 America, to Guam among the Philippine islands, ships sel- 

 dom have occasion to hand their sails, and yet they make 

 it in about sixty days, which could not be if the wind blew 

 only in the afternoon. 



7. That really is, which the gentleman justly supposes 

 ought to be on my hypothesis. In sailing southward, when 

 you first enter the trade wind, you find it N. E. or therea- 

 bouts, and it gradually grows more east as you approach the 

 line. The same observation is made of its changing from S. 

 E. to E. gradually, as you come from the south latitudes to 

 the equator. 



I have not yet had time to transcribe my paper on the in- 

 crease of mankind, but hope to do it shortly, and shall be 

 glad of your and Mr. Todd's sentiments on it. My re- 

 spects to that gentleman ; and be assured that I am, very 

 affectionately, dear Sir, 



Your most humble servant, 



B. FRANKLIN. 



12 Philadelphia, Nov. 8, 1753. 



Dear Sir, 



The first intimation I find of the new air-pump, is in a 

 piece of Mr. Watson's, read to the Royal Society, Feb. 20, 

 1752, where describing some experiments he made in va- 

 cuo, he says — " The more complete the vacuum, ceteris 

 paribus, the more considerable were the effects ; and here, 

 I should not do justice to real merit, were I silent in regard 

 to Mr. Sweaton. This gentleman, with a genius truly me- 

 chanical, which enables him to give to such philosophical 

 instruments as he executes, a degree of perfection scarce 

 to be found elsewhere ; this gentleman, I say, has construe- 



