HORAEY VARIATIONS OF THE BAROMETER. 



BY MARSHAL VAILLANT. 



[Translated for the Smithsonian Institution, from the "Archives dc la Commission Scien- 



tifiqucduMexique," 18G5. ] 



When, in favorable weather, and especially under intertropical latitudes, we 

 observe with attention the height at which the column of mercury stands in the 

 barometer, it cannot long escape noticQ that this height undergoes, at different 

 hours of the day, variations which pursue a quite regular course, and which are 

 30 well established that Humboldt (we believe) has said the elevation of the 

 mercury in the barometric tube might serve as a timekeeper. This, in the domain 

 of physics, is something analogous to what is presented to the botanist by the 

 leaves or flowers of certain plants which open or close, droop or again become 

 erect, at a particular period of the day, a fact which suggested to Linnaeus the 

 idea of his horologium plavtan/m. 



The study of meteorological phenomena under the torrid zone presents a 

 peculiar interest, and affords much more readily than in temperate zones a 

 knowledge of the general laws which govern those phenomena. Between the 

 tropics, in effect, they are disengaged from almost all the disturbing causes 

 which, in our own climates, render their examination so difficult. It was under 

 the equator that the horary variations were first recognized. Bouquer, La Oon- 

 damine, and Godin observed them more than a century ago, (173G and 1737,) 

 when travelling together in South America. This phenomenon, to which is now 

 given the more simple name oi diurnal period, was described by Humboldt in the 

 work which he published in 1807, and in which he has given the precise hours 

 at which occur the maxima and minima of height of the mercurial column, as 

 well as the extent of the oscillation. M. Boussingault has, by his own obser- 

 vations made at Guayra, (Colombia,) confirmed those for which science is 

 indebted to Humboldt ; and long calculations, embracing a large number of 

 years, have resulted in mean quantities which place the existence of the diurnal 

 period beyond doubt, even in latitudes as high as the north of France. " From 

 facts ascertained by Bouvard, at the instance of Laplace, during the seventy- 

 two months of the six years which elapsed from the 1st of January, 1817, to 

 the 1st of January, 1823, it results that the mean diurnal variation at Paris is 

 equal to 0'"™ .801 ; and also that the mean variation of each month is always 

 positive." 



I was not aware of the results at which Bouvard had arrived, when, availing 

 myself of the tables furnished by the " Annalcs de Physique et de C/iimie," I 

 verified, to my own satisfaction, that the oscillations of the mercury at Paris 

 were so perceptible as to be wholly incontestable, especially during the mild 

 season. The meteorological tables, comprising the year 1825, I discussed en- 

 tirely, and those for 1824 in part. I shall give further on, and in detail, the 

 results at which I arrived ; for the moment, suffice it to say, that We are not to 

 expect to realize, under the latitude of Paris, deviations like those which are 

 manifested under the tropics, where their extent surpasses two millimetres. 



The column of mercury attains the maximum of its elevation about nine or 

 ten o'clock in the morning, from one of which hours to the other there exists, 



