THE EEASON WHY. 159 



" Thou visitcst the earth, and waterest it: thou greatly enrichest it with the 



river of God, which is full of water : thou preparest them corn, when 



thou hast so provided for it." PSAXM nr. 



GOG. WTiy docs 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 uercury. 



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. 



G97. W7iich 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. 



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 

 Eain. 



700. Why does the rise of the barometer denote the approach 

 cffine 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. 



