BOILING. 83 



takes place at 212 Fahrenheit, which is therefore called 

 the boiling point of water. 



197. EFFECT OF HEIGHT ON BOILING. 



What effect . ... 



has height on At great elevations, the atmosphere is, m 

 boiling? factj lighterj and there is i ess o f j t above 



us, and the consequence is that water boils on moun- 

 tains, at a lower temperature than in the valleys below. 

 It is found, by careful observation, that an elevation of five 

 hundred and fifty feet above the level of the sea, makes 

 the difference of one degree in the boiling point. 



198. MEASUREMENT OF ALTITUDES. 

 the This fact once established, a tea-kettle and 



mountains be a thermometer are the only requisites for 



determined? . . . . r . mi 



taking the height of a mountain. The 

 summit being reached, the tea-kettle is boiled, and 

 the heat of the water tested by the thermometer. 

 If the mercury stands at 211, it is known that the 

 height is 550 feet ; if at 210, the height is 1100 feet ; 

 and at whatever point it stands, it is only necessary to 

 multiply 550 by the number of degrees depression of 

 the mercury below 212, to ascertain the elevation. On 

 the top of Mont Blanc, water was observed by Saus- 

 sure to boil at 184. This gives us the means of calcu- 

 lating very closely the height of that mountain. 



199. EFFECT OF DEPTH ON BOILING. 



What effect 



kas depth on In mines the atmosphere is heavier, and 

 oihng? there is, beside, more of it above us, than 



at the surface of the earth. Water must, in consequence, 

 be more highly heated before it will boil. 550 feet 

 makes, as before, a difference of one degree. We are 

 thus provided with a simple means of determining the 



