696. Why does tapping the face of the barometer sometimes cause the hand to move?

Because the weight on the surface of the mercury frequently leans against the sides of the tube, and does not move freely. And, also, the mercury clings to the sides of the tube by capillary attraction; therefore, tapping on the face of the barometer sets the weight free, and overcomes the attraction which impedes the rise or fall of the mercury.

[Fig. 21] illustrates the mechanism at the back of the barometer. A is a glass tube; between A and E there exists a vacuum, caused by the weight of the mercury pressing downwards. This space being a vacuum, makes the barometrical column more sensitive, as there is no internal force to resist or modify the effects of the external pressure. E represents the height of the column of mercury; C the open end of the tube; F the weight resting on the surface of the mercury; P the pivot over which the string passes, and upon which the hand turns; W the weight which forms the pulley with the weight F.

697. Which is the heavier, dry or vaporised air?

Dry air is heavier than air impregnated with vapours.

698. Why is dry air heavier than moist air?

Because of the extreme tenuity of watery vapours, the density of which is less than that of atmospheric air.

699. Why does the fall of the barometer denote the approach of rain?

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.