1871] WKmXGS OF JOSEPH HENRY. 467 



phere are due I think to its instability produced by the for- 

 mation and condensation of vapor. It is not impossible that 

 when the air is in a very unstable condition on account of 

 the heat and moisture of the lower strata, the aerial tide may 

 induce an overturning of the tottering equilibrium at some 

 one place in the northern or southern hemisphere more un- 

 stable than the others, and thus commence a storm which, 

 but for this extraneous cause, would not have happened. 

 To detect any such influence of the moon however, it will 

 be necessary to compare simultaneously the records of the 

 weather from day to day throughout all the northern and 

 southern temperate zones, and to ascertain whether the 

 maximum of these changes have any fixed relation in time 

 to the changes of the moon. 



The changes of the moon take place at a given moment 

 on every part of the earth: the greatest effect of a lunar tide 

 ought therefore to be felt in succession entirely around the 

 earth in the course of about twenty-four and one-half hours 



The problem cannot be determined however by such casual 

 observations as those which you narrate. I have not the 

 least idea that the attraction of Venus produces any appre- 

 ciable effect It is too small to produce a result which would 

 be indicated by any of our meterological instruments. 



I am far from subscribing to the justice of your remarks 

 in regard to Mr. Espy, since I have a great respect for his 

 scientific character, notwithstanding his aberration, in a 

 practical point of view, as to the economical production 

 of rain. The fact has been abundantly proved by observa- 

 tion that a large fire sometimes produces an overturn in the 

 unstable equilibrium of the atmosphere and gives rise to the 

 beginning of a violent storm, but it was not wise in him to 

 insist on the possibility of turning this principle to an eco- 

 nomical use. 



