Because it shows that as the air _cannot support the full weight of the column of mercury_, the atmosphere must be thin with watery vapours. The fall of the mercury in the long arm of the tube would cause the weight F to be pressed upwards. This would release the string to which the weight W is attached; it would, therefore, fall, and turn the hand down to Rain or Much Rain. 700. _Why does the rise of the barometer denote the approach of fine weather?_ Because the external air becoming dense, and free from highly elastic vapours, presses with increased force upon the mercury upon which the weight F floats; that weight, therefore, sinks in the short tube as the mercury rises in the long one, and in sinking turns the hand to Change, Fair, &c. [Verse: "He caused an east wind to blow in the heaven; and by his power he brought in the south wind."--PSALM LXXVIII.] 701. _Why does the barometer enable us to calculate the height of mountains?_ Because, as the barometer is carried up a mountain, _there is a less depth of atmosphere above to press upon the mercury_; it therefore falls, and by comparing various observations, it has been found practicable to _calculate the height of mountains by the fall of the mercury in a barometer_. 702. _To what extent of variation is the weight of the atmosphere liable?_ It may vary as much as _a pound and a half to the square inch_ at the level of the sea. 703. _When does the barometer stand highest?_ When there is a _duration of frost_, or when _north-easterly winds_ prevail. 704. _Why does the barometer stand highest at these times?_ Because the atmosphere is exceedingly _dry and dense_, and fully balances the _weight of the column of mercury_. 705. _When does the barometer stand lowest?_ When _a thaw follows a long frost_; or when _south-west winds_ prevail. 706. _Why does the barometer stand lowest at those times?_ Because _much moisture exists in the air_, by which it is rendered less dense and heavy. 707. _What effect has heat upon the barometer?_ It causes the mercury to fall, _by evaporating moisture into the air_. 708. _What effect has cold upon the barometer?_ It causes the mercury to rise, by _checking evaporation_, and _increasing the density of the air_. [Verse: "For so the Lord said unto me, I will take my rest, and I will consider in my dwelling place like a clear heat upon herbs, and like a cloud of dew in the heat of harvest."--ISAIAH XVIII.] In noting barometrical indications, more attention should be paid to the _tendency_ of the mercury at the time of the observation, than to the _actual state of the column_, whether it stands _high_ or _low_. The following rules of barometric reading are given as generally accurate, but liable to exceptions:-- _Fair weather_ indicated by the _rise_ of the mercury. _Foul_ weather by the _fall_ of the mercury.