72 PNEUMATICS. 



The barometer constantly descends in a geometri- 

 cal ratio for equal ascents in the atmosphere, subject 

 to a correction for the decreasing temperature of the 

 elevation. 



Mensuration of Heights by the Boiling Point. 



In consequence of the diminution of the atmos- 

 pheric pressure as we ascend, fluids evaporate at a 

 much lower degree of heat. Near the top of Mont- 

 blanc, water boiled at 187, instead of 212, the 

 boiling point at the level of the sea.* 



Cloudless Elevations. 



Owing to its rarefaction, the air beyond a certain 

 height is incapable of sustaining clouds. The prin- 

 cipal masses of clouds are sustained in the air at a 

 height between 4500 and 7500 feet, the average being 

 rather more than a mile. 



Origin of Rain. 



Rain is produced by the mixture of atmospheric 

 currents of different temperatures. The power of 

 the air to hold water in solution does not increase in 

 the same ratio with the increase of its temperature, 

 but in a much higher ratio. Hence, when two masses 

 of air, saturated with moisture and of different tem- 

 peratures, are mixed, the resulting compound is not 

 capable of holding the whole water in solution, and 

 a part is in consequence precipitated as rain. As the 

 whole atmosphere (which is a vast laboratory in per- 

 petual action) when saturated, is calculated not to 

 hold in solution more water than would form a sheet 

 five inches in depth, while the mean annual deposit 



* With a tea-kettle and thermometer, a tolerable approxima- 

 tion to the true height of the highest mountain may be attained ; 

 but great care must be taken to keep the water free from all ex- 

 traneous matter, the presence of which Dr. Bostock thinks affects 

 the boiling point 4 '5 degrees. 



