Scientific Lectures. 59 



nse our instrument to weigli the atmosphere? If the atmosphere 

 would always be in its normal condition, why should its weight vary ? 

 Why should not the column of mercury in the barometer always show 

 the same height ? In order to imderstand the variations, we can't 

 dispense with entering somewhat into the general laws which govern 

 the atmosphere, not only its weight, but the influences which disturb 

 its equilibrium. "We must know what are the conditions of the 

 equilibrium of the atmosphere, and we must find the forces which 

 interfere with that equilibrium, and which cause, therefore, the varia- 

 tions of the barometer. These variations which I have now to 

 explain, may be classified in a general way, by saying that we 

 have " regular variations " and " irregular variations." There 

 are great atmospheric tides each day, each month, and each year 

 almost. These atmospheric tides are constant in their operation, 

 though they are not the same in all latitudes. Every day there 

 is a vast tidal wave of the atmosphere ; it is found in all regions 

 of the globe, south and north, to sixty degrees of latitude ; beyond 

 that it is not very marked. Every day the barometer, after 

 discarding all the irregularities, and, by means of a long series of 

 observations, taking the averages, rises and falls regularly like a tide. 

 You will find that it will stand highest in the morning at about 

 9 o'clock. On an average, the lowest point of the twenty-four hours 

 is at 4 o'clock in the afternoon. It rises again during the evening, 

 until 10 o'clock at night ; then it sinks until it reaches the minimum ; 

 then it rises again the next morning until 9 o'clock. It is a constant, 

 tidal wave, repeated twice in every twenty-four hours. The difference 

 between the maximum and minimum is not very great ; it amounts 

 only to the 200th of an inch ; in our latitude, indeed, less than that, 

 but it increases towards the tropical regions, and about the tropics 

 and on the equator you have the greatest amount of tidal wave ; and 

 it amounts then from the 200th to the 300th of an inch. This rise and 

 fall are so clearly and specially remarkable that Humboldt said, while 

 traveling in South America, that he could tell what time of the day 

 it was by looking at the heighth of the barometer. It was a regular 

 clock. ISTow you see this is a kind of variation, the character of 

 which is to increase towards the tropical regions, or as the heat 

 increases and decreases towards the cold. The regular variation with 

 the months is of a similar character. In summer months the baro- 

 meter is generally lower than during the winter, and, on an 

 average, it is lower in all the warm regions of the globe, than 



