10 



40 



10 



: 46 



10 



37 



1834.] on Atmospherical Phenomena. 351 



Let us next compare the day of the moon's opposition, (viz. the 9th 

 before the new moon,; and three days before and three days after, as 

 was done in the former case. 



The ratio of the amount to the whole lunar period was 



In the first four months, 



Summer months, 



Oct., Nov., Dec, . . . . , 



But taking the 13th before, instead of the 9th, (for the last quarter,) we get a 

 ratio of : : 10 : 28 



We may observe then that the amount which falls in these days near 

 the full moon is greatest in winter, when the moon near the full has 

 north declination. On the contrary, in summer, the amount which 

 falls near the new moon, when the moon at that season, and that age, 

 has north declination, is the greatest. We may recollect that in the 

 theory of the tides the height of the tide is said to vary as cos 2 x 

 (where .ris the angular distance between the moon and zenith of the 

 place). The above observations seem to point to a law somewhat 

 similar. But of this I have yet to offer some further probability. I 

 have not here compared the number of rainy days as well as the quan- 

 tities of rain fallen, but they tend to the same conclusions, though less 

 decisively. Nor have I said any thing respecting the two other maxi- 

 ma on the 3rd day before, and 12th after, the new moon, as I have no 

 probable cause to allege for them. 



Let us then dismiss from our minds the idea of a sphere covered 

 with a homogeneous fluid, and substitute that of a surface partly of dry 

 land, and partly of water, the first covered with a stratum of air nearly 

 dry, the last with a stratum saturated with moisture ; and to carry on 

 the comparison with the tides of the ocean, let us remember that we 

 cannot measure the actual height of the tide, as in that case, but that 

 if an observer, situated on the border of an estuary, were to endeavour 

 to estimate the relative intensity of the currents flowing in from the 

 open sea, by the quantity of salt contained in the water before him, 

 then his case would be somewhat similar to ours, when we attempt to 

 draw a like reference respecting the aerial currents from the heights of 

 the dew-point. If he were to endeavour to conjecture the force of the 

 floods from the country above, by measuring the quantity of earthy 

 matter precipitated from the water, then he might expect to approxi- 

 mate to the truth about as much as we do when we attempt to infer the 

 force of the current of air flowing in from the regions of the ocean, by 

 the quantity of water precipitated. In both cases an approximation only 

 can be expected. 



Having gone thus far, the next step to be desired was to make a 

 comparison between the heights of the dew-points at different ages of 



