19 



serves also for the absorption of the one wave, leaving the 

 other free to act. 



The following note, " On the Course of the diurnal Fluc- 

 tuations of the Barometer," by James P. Espy, A.M., of Phi- 

 ladelphia, was communicated by Dr. Apjohn. 



" It is a law of inertia, that if a body is forced upwards, 

 it will react and press on its support, more than its natural 

 gravity ; and if it is permitted to descend, it will press on its 

 support less than its natural gravity, and the increase and 

 diminution of pressure will be proportional to its velocity. 



" Moreover, if a body is permitted to descend with a cer- 

 tain velocity, and then retarded, it will, when retarded, press 

 more on its support than its natural gravity, and that in pro- 

 portion to the rapidity of its retardation. 



" This principle will explain the four fluctuations of the 

 barometer which occur every day. 



" Just before sunrise, when the atmosphere is neither 

 becoming hotter nor colder, the barometer will indicate the 

 natural weight of the air, which we may call a mean ; as the 

 sun rises the air will begin to expand by heat, and the whole 

 atmosphere will be lifted up by this expansion, and by its 

 reaction will cause the barometer to rise ; and this will be 

 the greatest, at the time when the air is receiving the most 

 rapid accessions of heat, which must take place before the 

 hottest time of the day, when the air is becoming neither 

 hotter nor colder. On this principle, then, the maximum 

 day fluctuation will take place between daylight in the morn- 

 ing and the hottest time of the day, and this corresponds with 

 the fact ; for this maximum, which amounts to more than 

 the tenth of an inch, takes place about nine or ten o'clock, 



A. M. 



" At the hottest part of the day, when the air is neither 

 expanding nor contracting, it is manifest that the barometer 

 will stand again at a mean. Soon after this, however, the air 



