IN CLOUDS AND RIVERS, ICE AND GLACIERS. 



load of vapor is in great part precipitated, 

 dense clouds are formed, their particles coa- 

 lesce to rain-drops, which descend daily in 

 gushes so profuse that the word " torren- 

 tial " is used to express the copiousness of 

 the rainfall. I could shoTv you this chilling 

 by expansion, and also the consequent pre- 

 cipitation of clouds. 



75. Thus long before the air from the 

 equator reaches the poles, its vapor is iu 

 great part removed from it, having rede 

 sccnded to the earth as rain. Still a good 

 quantity of the vapor is carried forward, 

 which yields hail, rain, and snow in noUhem 

 and southern lands. 



ILLUSTRATIVE EXPERIMENTS. 



76. I have said that the air is chilled during 

 its expansion. Prove this, if you like, thus. 

 With a condensing syringe, you can force air 

 into an iron box furnished with a stopcock, 

 to which the syringe is screwed. Do so till 

 the density of the air within the box is 

 doubled or trebled. Immediately after this 

 condensation, both the box and the air within 

 it are wajm, and can be proved to be so by 

 a proper thermometer. Simply turn the cock 

 and allow the compressed air to stream into 

 the atmosphere. The current, if allowed to 

 strike a thermometer, visibly chills it ; and 

 with other instruments the chill may to 

 made more evident still. Even the hand 

 feels the chill of the expanding air. 



77. Throw a strong light, a concentrated 

 sunbeam for example, across the issuing cur- 

 rent ; if the compressed nir be oidinary 

 humid air, you sec the precipitation of a liltlc 

 cloud by the chill accompanying the expan- 

 sion. This cloud-formation may, however, 

 be better illustrated in the following way : 



78. In a darkened room send a Mrong 

 beam of light through a glass tube three feet 

 long and three inches wide, stopped at its 

 ends by glass plates. Connect the tube by 

 means of a stopcock with a vessel of about 

 one fourth its capacity, from which the air 

 has been removed by an air-pump. The ex- 

 hausted cylinder of the pump itself will an- 

 s\ver capitally. Pill the glass lube with 

 humid air ; then simply turn on the stopcock 

 which connects it with the exhausted vessel. 

 Having more room the air expands, cold ac- 

 companies the expansion, find, as a conse- 

 quence, a dense and brilliant cloud imme- 

 diately fills the tube. If the expeiiment be 

 made ^for yourself aione, you may see the 

 cloud in ordinary daylight ; indeed, the brisk 

 exhaustion of any receiver filled with humid 

 air is known to produce this condensation. 



79. Other vapors than that of water may 

 be thus precipitated, some of them yielding 

 clouds of intense brilliancy, and displaying 

 iridescences, such as are sometimes, but not 

 frequently, seen in the clouds floating over 

 the Alps. 



8'J. In science, what is true for the small is 

 true for the large. Thus by combining the 

 conditions observed on a large scale in 

 nature we obtain on a small scale the Dhe- 

 nomena of atmospheric clouds. 



8. MOUNTAIN CONDENSERS. 



81. To complete our view of the process of 

 atmospheric precipitation we must take into 

 account the action of mountains. Imagine 

 a south-west wind blowing across the At- 

 lantic towaid Ireland. In its passage it 

 charges itself with aqueous vapor. In the 

 south of Ireland it encounters the mountains 

 of Kerry : the highest of these is Magilli- 

 cuddy's Reeks, mar Killatne}'. Now the 

 lowest stratum of this Atlantic wind is that 

 which is most fully charged with vapor. 

 When it encounters the base of the Kerry 

 mountains it is tilted up and flows bodily 

 over them. Its load of \apor is therefore 

 cairied to a height, it expands on reaching 

 the height, it is chilled in consequence of the 

 expansion, and comes down in copious show- 

 ers of rain. From this, in fact, arises the 

 luxuriant vegetation of Killainey ; to this, 

 indeed, the lakes owe their water supply. 

 The cold crests of the mountains also aid in 

 the work of condensation. 



82. Note the consequence. There is a 

 town called C.diirciveen, to the south-west of 

 Mcigillicudely's Reeks, at which observations 

 of the rainfall have been made, and a good 

 distance further to the north-east, right in 

 the course of the south-west wind, there is 

 another town, called Pottariington, at which 

 observations of rainfall have also been made. 

 But before the wind reaches the latter station 

 it has passed over the mountains of Kerry 

 juid left a great portion of its moisture be- 

 hind it. What is the result? At Cahirci- 

 wen, as shown by Dr. Lloyd, the raini'all 

 amounts to 51) inches in a year, while at 

 Portarlington it is only 21 inches. 



80. Again, you may sometimes descend 

 from the Alps, when the fall of rain and 

 snow is heavy and incessant, into Italy, anil 

 find the sky* over the plains of Lombardy 

 blue and cloudless, the wind at the sam* 

 time blowing over tJie plain toward the Alps. 

 Below the wind is hot enough to keep its 

 vapor in a perfectly transparent state ; but it 

 meets the mountains, is tilted up, expanded; 

 and chilled. The cold of the higher summits 

 also helps the chill. The consequence is that 

 the vapor is precipitated as rain or snow, 

 thus producing bad weather upon the 

 heights, while the plains below, flooded with 

 the same air, enjoy the aspect of the un- 

 clouded summer sun. Clouds blowing from 

 the Alps are also sometimes dissolved over 

 the plains of Lombardy. 



84. In connection with the formation of 

 clouds by mountains, one particularly in- 

 structive effect may be here noticed. You 

 frequently see a streamer of cloud many 

 hundred yards in length drawn out from an 

 Alpine peak. Its steadiness appears perfect, 

 though a strong wind may be blowing at the 

 same time over th3 mountain-head. Why is 

 the cloud not blown away? It is blown 

 away ; its permanence is qnly apparent. At 

 ene end it is incessantly dissolved, at the 

 other end it is incessantly renewed : supply 

 and consumption being thus equalized, the 



