00 



THE FORMS OF WATER 



birth to vast glaciers in a manner hereafter to 

 be explained. 



02. It is necessary that you should have a 

 perfectly clear view of this process, for great 

 mistakes have been made regarding the man- 

 ner in which glaciers are related to the heat 

 of the sun. 



63. It was supposed that if the sun's heat 

 were diminished, greater glaciers than those 

 now existing would be produced. But the 

 lessening of the sun's heat would infallibly 

 diminish the quantity of aqueous vapor, and 

 thus cut otf the glaciers at their source. A 

 brief illustration^will complete your knowl- 

 edge here. 



64. lu the process of ordinary distillation, 

 the liquid to be distilled is heated and con- 

 verted into vapor in one vessel, and chilled 

 and reconverted into liquid in another. What 

 has just been stated renders it plain that the 

 earth an 1 its atmosphere constitute a vast 

 distilling apparatus in which the equatorial 

 ocean piays the part of the boiler, and the 

 chill regions of the poles the part of the con- 

 denser. In this process of distillation /teat 

 plays quite as necessary a part as cold, and 

 before Bishop Heber could speak of " Green- 

 land's icy mountains," the equatorial ocean 

 had to be warmed by the sun. We shall 

 have more to say upon this question after- 

 ward. 



ILLUSTRATIVE EXPERIMENTS. 



05. I have said that when heated, air ex- 

 pands. If you wish to verify this for your- 

 self, proceed thus. Take an empty flask, 

 Btop it by a cork ; pass through the cork a 

 narrow glass tube. By heating the tube in 

 a spirit-lamp you can bend it downward, so 

 that when the flask is standing upright the 

 open end of the narrow tuba may dip into 

 water. Now cause the flame of your spirit- 

 lamp to play against the flask, The flam 3 

 heats the glass, the glass heats the air ; the 

 air expands, is driven through the narrow 

 tube, and issues in a storm of bubbles from 

 the water. 



66. Were the heated air unconfmed, it woull 

 rise in the heavier cold air. Allow a sun- 

 beam or any other intense light to fall upon 

 a white wall or screen in a dark room. Bring 

 u heated poker, a candle, or a gas-flame un- 

 derneath the beam. An ascending current 

 rises from the heated body through the beam, 

 and the action of the air upon the light is 

 such as to render the wreathing and waving 

 of the current strikingly visible upon the 

 screen. When the air is hot enough, anJ 

 therefore light enough, if entrapped in a 

 paper bag it carries the bag upward, and 

 you have the fire balloon. 



67. Fold two sheets of paper into two 

 cones, and suspend them with their closed 

 points upward from the end of a delicate 

 balance. See that the cones balance each 

 other. Then place for a moment the flame 

 of a spirit-lamp beneath the open base of one 

 of them ; the hot air ascends from the lamp 

 and instantly tosses upward the cone above it. 



68. Into an inverted glass shade introduce 



a little simks. Let the air come to r-?st. an 1 

 then simply place your hand at (he open 

 mouth of the shade/ Mimic hurricanes nre 

 produced by the air warmed by the hand, 

 which are strikingly visible when the smoka 

 is illuminated by a strong light. 



69. The heating of the tropical air by the 

 sun is indirect. The solar beams have scarce- 

 ly any power to heat the air through which 

 they pass ; but they heat the laud and ocean, 

 and these communicate their heat to the air 

 in contact with them. The air and vapor 

 start upward charged with the heat thus 

 communicated. 



7. TROPICAL RAINS. 



70. But long before the air and vapor from 

 the equator reach the poles, precipitation 

 occurs. Wherever a hurnid warm wind 

 mixes with a cold dry one, rain falls. In- 

 deed the heaviest rains occur at those places 

 where the sun is vertically overhead. We 

 must inquire a little more closely into their 

 origin. 



71. Fill a bladder about two thirds full of 

 air at the sea-level, and take it to the summit 

 of Mont Blanc. As you ascend, the bladder 

 becomes more and more distended ; at the 

 top of the mountain it is fully distended, and 

 has evidently to bear a pressure from within. 

 Returning to the sea-level you find that the 

 tightness disappears, the bladder finally ap- 

 pearing as flaccid as at first. 



72. The reason is plain. At the sea-level 

 the air within the bladder has to bear the 

 pressure of the whole atmosphere, being 

 thereby squeezed into a comparatively small 

 volume. In ascending the mountain, you 

 leave more and more of the atmosphere be- 

 hind ; the pressure becomes less and less, 

 and by its expansive force the air within the 

 bladder swells as the outside pressure is di- 

 minished. At the top of the mountain the 

 expansion is quite sufficient to render the 

 bladder tight, the pressure within being then 

 actually greater than the pressure without. 

 By means of an air-pump we can show the 

 expansion of u balloon partly filled with air, 

 when the external pressure has been in part 

 removed. 



78. But why do I dwell upon this ? Sim- 

 ply to make plain to you that the unconfined 

 air, heated at the earth's surface, and as- 

 cending by its lightness, must expand more 

 and more the higher it rises in the atmos- 

 phere. 



74. And now I have to introduce to you a 

 new fact, toward the statement of which 1 

 have been working for some time. It is 

 this : The ascending air is chilled by its expan- 

 sion. Indeed this chilling is one source of 

 the coldness of the higher atmospheric re- 

 gions. And now fix your eye upon those 

 mixed currents of air and aqueous vapor 

 which lise from the warm tropical ocean. 

 They start with plenty of heat to preserve 

 the vapor as vapor ; but as they rise they 

 come into regions already chilled, and they are 

 still further chilled by their own expansion. 

 .The consequence mio;ht be foreseen. Th 



