426 THE PRINCIPLES OF SCIENCE. 



provided that we have a series of observations sufficiently 

 numerous and long continued to enable us to balance all 

 the larger effects against each other. For this purpose 

 the observations should be continued over at least one 

 complete cycle, in which the effects run through all their 

 variations, and return exactly to the same relative position 

 as at the commencement. If casual or irregular disturbing 

 causes exist, we should probably require many such cycles 

 of results to render their effect inappreciable. We obtain 

 the desired result by taking the mean of all the observa- 

 tions in which a cause acts positively, and the mean of all 

 in which it acts negatively. Half the difference of these 

 means will be the desired quantity, provided indeed that 

 no other effect happens to vary in the same period. 



Since the moon causes so considerable a movement of 

 the ocean, it is evident that its attraction must have some 

 effect upon the atmosphere. The laws of these tides were 

 investigated by Laplace, but as it would be impracticable 

 by theory to calculate their amount, we can only determine 

 them by observation, as Laplace predicted that they would 

 one day be determined m . But the oscillations of the 

 barometer thus caused are far smaller than the oscillations 

 due to several other causes. Storms, hurricanes, or changes 

 of weather produce movements of the barometer some- 

 times as much as a thousand times as great as the tide in 

 question. There are also regular daily, yearly, or other 

 fluctuations, all greater than the desired quantity. To 

 detect and measure the atmospheric tide it was desirable * 

 that observations should be made in a place as free as 

 possible from irregular disturbances. On this account 

 several long series of observations were made at St. 

 Helena, where the barometer is far more regular in its 

 movements than in a continental climate. The effect of 

 the moon's attraction was then detected by taking the 



' i Philosophique sur les Probability,' pp. 49, 50. 



