1867.] 



Atmospheric Tide at Melbourne. 



491 



Professor Kreil, and others. This much may be stated, however, that the 

 reading of the barometer (6), after being reduced to 32°, was invariably 

 increased by 1 inch, in order that it may always exceed the mean pressure 

 for the respective hour (b), thereby avoiding negative results. This has 

 certainly the disadvantage of not exhibiting at a glance the excess or defect 

 of atmospheric pressure at any time ; on the other hand, there is no doubt 

 that a mistake with regard to the algebraic sign of the remainders is not 

 likely to occur. In the subsequent Tables it was made a rule to reduce the 

 values b-~b to their mean value for a month, year, or whatever other 

 period of time they may refer to. 



The remainders b — b were deriA^ed, in the manner just pointed out, for 

 every month throughout the period of five years for which the observa- 

 tions were continued. Then the means for every month and hour were 

 taken, thus obtaining normal values for the several months ; a general 

 mean for every month formed the basis to which those normal values were 

 referred. The subjoined Table shows the result of this proceeding. 



The values of the above Table have been thrown into curves, and Plate I. 

 shows the results. The actual mean values are indicated by dots ; a full- 

 drawn curve is made to pass through and between them in such a manner 

 as to eliminate the greater irregularities. Some of those irregularities are 

 so large as to cause the respective dots to be disconnected whh the series to 

 which they belong, and it became therefore necessary to indicate this con- 

 nexion by slight dotted hues. 



On glancing over this series of carves we cainict fail to observe a great 

 regularity, pointing at some cause common to all ; and as the remainders 

 b—h have been arranged according to the moon's hour-angle, we may 

 justly look to the moon as the primary cause. But it is nevertheless true 

 that those curves apparently poiat to some other influence, most likely due 

 to the combined action of the sun and moon. The monthly curves for the 

 several years of observation have also been drawn, though we refrain from 

 adding the results here ; and the fact that they correspond in the main 

 points with those shown on Plate I., seems to justify our attaching parti- 

 cular weight to the evidence of the moon's influence on our atmosphere, as 

 conveyed to our minds by the above Tables. Put prior to entering fully 

 upon the various points bearing on the question at issue, we need to form 

 of the monthly results quarterly and semiannual groups. If we call March, 

 April, May the first, June, July, August the second, September, October, 

 November the third, and December, January, February the fourth quarter, 

 we obtain the quarterly means inserted in the following Table. It was 

 furthermore considered serviceable to the purpose to group together those 

 quarters in which the epochs of solstices and equinoxes respectively occur, 

 under the collective names "solstitial and equinoctial quarters." The semi- 

 annual periods comprise, as usually, the nicnths from April to Septcmher, 

 and those from October to March. The mean of all the various hourly 

 values represents the mean lunar-diurnal variation in pressure of air fur the 

 year. 



