212 farmers' and mechanics' journal. 



tion is more general ; and on no occasion, perhaps, is the almanack 

 so frequently consulted, as in forming conjectures upon the state 

 of the weather. The common remark, however, goes no further 

 than that changes from wet to dry, and from dry to wet, generally 

 happen at the changes of the moon. When to this result of uni- 

 versal experience we add the philosophical reasons for the exist- 

 ence of tides in the serial ocean, we cannot doubt that such a con- 

 nexion exists. The subject, however, is involved in much obscu- 

 rity. Mr. Howard is the only one who has treated it with the con- 

 sideration which it deserves. In his book may be found much in- 

 formation upon it, the result of laborious investigation. It would 

 be foreign to my purpose to enter at large upon this interesting 

 ground, but the previous investigation suggests one particular view 

 of it. which it may be useful shortly to state. 



The action of the moon upon the srial columns over which it 

 passes, mav be regarded as diminishing the force of gravity. This 

 action must be greater in proportion as the moon approaclies the 

 earth ; in proportion as it coincides with the analogous action of 

 the sun ; and in proportion as its passage over the meridian comes 

 near to the perpendicular direction. The result of this diminu- 

 tion of sjravity must be a general decrease of density ; and its effect 

 upon the lateral currents, an acceleration of the incoming, and a 

 decrease of the outgoing streams. The loss of wei;2;hf will thus be 

 compensated, and the excess of elasticity hence derived, will 

 lengthen the column. The final adjustment v. ill. therefore, be as- 

 similated to that which arises from an equal expansion by heat. 

 Now the effect of the atmospheric tide has hitherto been sought 

 for. and mea^sured upon the surface of the earth, at the base of the 

 column ; and much conjecture and disappointment have ensued 

 from not finding the effect as great, or as regular, as had been an- 

 ti<'inated. But. if this view of the subject be correct, the total 

 weifrht of the perpendicular column would not be affected so much 

 as that of its horizontal sections ; and the amount of the lunar in- 

 ■Qupnce should be sought in the variations of the differences of den- 

 sity between some high elevation and the level of the sea. The 

 mean of a series of experiments carefully conducted with this view, 

 when the moon is upon the meridian and at the horizon, would 

 possibly exhibit the amount of the dailv tides ; their weekly in- 

 crease and diminution ; the influence of the moon's apogee and 

 perigee ; and that of its north and south declination. It has, how- 

 ever, T think, been proved that the influence is still felt at the sur- 

 face of the earth ; and the barometer, upon an average, stands 

 lower at new and full moon, than at the quarters. This also would 

 naturallv be expected, when it is considered that the attraction of 

 the moon is an action upon the power of gravity, and acts instanta- 

 neously in the perpendicular direction ; while the compensating 

 effects upon the lateral currents is gradual. 



Ts if not possible that some of the remaining discrepencies of 

 barometrical mensurations mrty be traced to this influence ? 



[DanieWs Meteorological Essays and Observatiovs: 



