THE MOON AND THE WEATHER. 



fluence, which has been the subject of certain and satisfactory observation. 

 There is a daily fluctuation in the barometric column, called the diurnal vari- 

 ation, which has an obvious relation to the apparent diurnal motion of the sun, 

 and which probably is caused by solar heat. It is observed that the baromet- 

 ric column falls daily, from nine in the morning till noon. In Europe, this 

 effect is frequently obliterated by other disturbing causes ; but ii is always ob- 

 servable when a mean is taken of observations, continued for any considerable 

 number of days. This diurnal variation will be combined with the effect of the 

 lunar attraction in the results of the observations. Now at a new and full 

 moon these causes produce contrary effects on the barometric column. Du- 

 ring the three hours preceding noon, the lunar attraction has a tendency to im- 

 part to it an ascending movement ; while, by reason of the diurnal variation, it 

 would have at the same time a descending movement ; the result would con- 

 sequently be the difftrcn.ee of the two effects. If the diurnal variations were 

 equal to the effects of the moon's attraction, the motions would neutralize each 

 other, and the column would be stationary ; but if they be unequal, the column 

 will ascend or descend by their difference. At the quarters these two effects 

 will conspire in producing a descending movement of the barometric col- 

 umn during those hours before noon, and the result of observation will be a 

 descent equal to the sum of the two effects. 



Observations, therefore, made at and before noon at the times of new and 

 full moon, and at the quarters, ought to supply estimates of the sum and the 

 difference of these two physical effects ; and if such observations be continued 

 for a sufficient length of time, a mean estimate may be obtained from which the 

 effects of disturbing causes will be eliminated. M. Arago has applied this 

 method of investigation to a series of observations conducted for twelve years 

 in Paris, and he has found that the effect of the lunar attraction on the barom- 

 eters produced between the high and low states of the atmosphere, correspond- 

 ing to high and low water, cannot exceed the six hundredth part of an inch 

 a quantity too small to be appreciated by any meteorological instruments, and, 

 certainly such as could produce no sensible effect on the atmosphere. 



It is evident, then, that if the moon has any influence on our atmosphere, 

 it does not proceed from any cause analogous to that which produces the tides of 

 the ocean ; and therefore, that the fact, that the moon does produce such tides 

 can afford no countenance to her imputed meteorological influence. 



But it may be said that although the moon may not affect the atmosphere by 

 her gravitation, yet she may influence it by her light, or by electrical or mag- 

 netical emanations, or, in fine, by some occult physical causes not yet discover- 

 ed by astronomers. This is an objection that, from its vagueness and indefi- 

 niteness, is difficult to be rebutted by any means which theory can furnish. It 

 is known that the light of the moon concentrated in a point by the most pow- 

 erful burning lenses, is incapable of producing the slightest sensible effect on 

 the most susceptible thermometer, neither is it found to produce any effects 

 of an electrical or magnetical kind. It may be assumed generally, ihat 

 the effects commonly imputed to the moon, in producing change of weatlur at 

 her principal phases, are so contradictory that it is impossible to imagine any 

 physical causes which could account for them. If the new and full moon ami 

 the quarters are attended by changes of the weather, the cause producing 

 this effect, under the same circumstances, has incompatible influences : it' lair 

 weather precede the phase, the supposed physical cause must be such as to be 

 capable of converting it into foul weather; and if foul weather precede the 

 phase, the same cause must convert it into fair weather. It will be admitted 

 that it is hard to imagine any physical agent whatever, which, under precisely 

 the same circumstances, shall produce upon the same body effects so opposite. 



