1 8 PIONEERS OF SCIENCE IN AMERICA. 



the passage of perspirable matter through them. Animals that 

 have open pores all over the surface of their bodies and live in 

 air which takes off continually the perspirable part of their 

 substance naturally require a continual supply of food to main- 

 tain their health. Toads shut up in solid stone, which prevents 

 their losing anything of their substance, may perhaps for that 

 reason need no supply, and being guarded against all accidents 

 and all the inclemencies of the air and changes of seasons are, 

 it seems, subject to no diseases, and become, as it were, im- 

 mortal." 



The length of the time that dead bodies will retain infection 

 after sepulture is discussed in one of the letters,* and cases are 

 cited of smallpox caught from bodies thirty years dead ; fever 

 from an Egyptian mummy ; cold from a mummy of Teneriffe 

 three hundred years old ; and fevers mentioned in a newspaper 

 account resulting in fifteen funerals, from the bodies of persons 

 who had died of the plague one hundred years before. 



The early experiments with balloons were observed with 

 interest by Franklin. He was hopeful but not sanguine as to 

 the outcome, and he wrote, in 1784, that the beginning of the 

 art of flying would be a new epoch. The construction and 

 manner of filling balloons were improving daily. Remarking 

 that some of the artists had lately gone to England, he sug- 

 gested to his correspondent: "It will be well for you philoso- 

 phers to obtain from them what they know, or you will be 

 behindhand, which in mechanical operations is unusual for 

 Englishmen." He, however, discouraged one of his friends from 

 attempting to cross from France to England in a balloon, tell- 

 ing him that means had not yet been found to keep a balloon 

 up more than two hours. He saw the embryo of a steamboat, 

 of which more than one notice occurs, and which he introduced 

 to the attention of Dr. Ingenhousz in 1788, speaking of it as a 

 boat moved by a steam engine, that " rows itself against the 

 tide in a river and it is apprehended that its construction may 

 be so simplified and improved as to become generally useful." 



Franklin observed that the same convexity of glass in spec- 

 tacles through which a man sees clearest and best at the proper 

 distance for reading is not the best for greater distances, and 

 for a long time used two pairs of spectacles, which he changed 



* To Felix Vicq d'Agyr. 



