156 On the Influence of the Sun 



ohfervatlons *, This ftate of the barometer takes place when 

 the mercury ceafes to fall and afterwards begins to rife, or 

 before rifing is ftatlonary. When it does not rife after noon, 

 but finks continually or by intervals, the loweft Itate after 

 noon muft be at midnight. The cafe is the fame when it 

 firft rifes after noon, and afterwards falls towards the end of 

 the natural day. But when it continuallv rifes after noon, 

 though this movement may be interrupted by its once be- 

 coming llationary, the loweli; ftate when it falls earl v, or when 

 this docs not happen, is to be confidi.rcd as takir.o; place 

 about twelve at noon. When in this cafe a frequent falling 

 takes place in the morning, I have obferved that it was nearell 

 Doon and the 4 of the daj- : when the quickfdver rofe and 

 fell fevcral times between noon and evening, I marked feveral 

 hours, without further examining at which the ftate of the 

 barometer was loweft. 



Now it appears from thefe tables, that in 222 days the 

 loweft ftate of the barometer correfponded only ten times 

 with the 1 of the day ; on the other hand, it deviated 

 from that period ten times about half an hour, at other 

 times a whole hour, and often half a day; and confequently 

 De Luc's rule, at leaft according to my obfervations, is not 

 correft. 



We may therefore aflc thofe who afcribe the falling of the 

 barometer to heat, Why fhould this caufe be fo clofely con- 

 fined to the period between 11 in the mornin<r and the middle 

 of the firft hour after midnight? Whv does it not take place 

 earlier or later, when the mercury rifes continualiy the whole 

 day, or feveral days either fall a little towards noon^ or at 

 leaft become ftationary or rife ftowly ? and, Whv does not 

 this rather take place feveral hours after noon, the time at 

 which the heat generally is the greatell ? 



But that heat is not the caufe of this phaenomenon is proved 

 by the falling of the mercury at midnight; which is no lefs 

 common and certain than the falling at noon : on account of 

 the cold, it ought then rather to nle or to fall. 



We may aftert the fame thing of vapour as of heat, as 

 thefe have the elofsft connexion with each other. As a 

 certain accumulation of vapours diffblved in the atmofphere 

 makes the air lighter, and obliges the barometer to fall, the 

 power with which it afts on the mllrnmcnt muft be in pro- 

 portion to its quantity. When it obliges the barometer, 



• This table has been omitted in this tranflauon, becaufe the refults here 

 given are lufFicient. 



therefore. 



