590 Royal Irish Academy. 



deed a proud achievement ; his character is an assurance that it will 

 be devoted, in the most unreserved manner, to the service of astro- 

 nomy ; while the energy that could accomplish such a triumph, and 

 the liberality that has placed his discoveries in this difficult art with- 

 in reach of all, may justly be reckoned among the highest distinc- 

 tions of Ireland. 



November 30. — Dr. Kane read a paper " On the Production of 

 Audible Sounds," an abstract of which will be found in the present 

 volume, p. 247. 



The following note, " On the Course of the Diurnal Fluctuations 

 of the Barometer," by James P. Espy, A.M., of Philadelphia, was 

 communicated by Dr. Apjohn. 



" It is a law of inertia, that if a body is forced upwards, it will re- 

 act 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 certain velo- 

 city, and then retarded, it will, when retarded, press more on its 

 support than its natural gravity, and that in proportion to the 

 rapidity of its retardation. 



" This principle will explain the four fluctuations of the baro- 

 meter 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 morning 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 expand- 

 ing nor contracting, it is manifest that the barometer will stand 

 again at a mean. Soon after this, however, the air will begin to con- 

 tract from diminishing temperature, and at the moment of the most 

 rapid acceleration of contraction, the barometer will stand at its day 

 minimum, which will probably be late in the afternoon ; and it is 

 found in fact to be from four to five o'clock. From this time the 

 rapidity of the downward motion of the air from contraction begins 

 to diminish, and the barometer of course begins to rise ; and at the 

 moment when it is most rapidly retarded in its contraction, the 

 barometer will be at its maximum night fluctuation, and will again 

 be above the mean, but not so much as the day maximum. 



" This maximum is found to occur about ten or eleven o'clock p.m. 

 The air will now go on contracting more and more slowly, until 



