1847.] Progressive Changes of Matter. 83 



PROGRESSIVE CHANGES OF MATTER.— NO. II. 



BY A. OSBORN. 



Winds and Rain. 



Winds are caused by an uneven temperature of the atmosphere 

 at the earth's surface. The rays of the sun falling upon this 

 surface, cause the inequality; the colder portion of the air being 

 heavier than that which is more rarified by M-arrath, rushes in to 

 occupy its place, thereby producing a current. Every day's ex- 

 perience teaches us that local heat, when coming in contact with 

 the air, produces a current in this fluid. A heated stove and the 

 fire-place of our dwellings are suitable places to observe these 

 facts. A fire placed in a stove causes a current of air to rush in, 

 which in common speech is called a draft, and some may have 

 an idea that the fire or heat within draws in the air from without, 

 whereas, in fact, the colder air without rushes in to occupy its 

 place. This fact is owing to the inequality of atmospheric den- 

 sity, produced by heat and cold. The same fact becomes appa- 

 rent in cold winter weather when our dwellings are surrounded 

 by a freezing atmosphere, and within the comforts of warmth 

 prevail. The exterior air is sighing at every crevice for admit- 

 tance; but when the summer returns and the temperature of the at- 

 mosphere, both within and without are equal, no complaint is heard, 

 no ingress sought. Bring two fluids in contact where the degrees 

 of density in each make a still greater contrast, to wit: air and 

 water. The latter fluid, when standing in a tube, if an orifice be 

 made at the base, rushes into the air with a velocity in proportion 

 to the height of its column. In this case we do not embrace the 

 idea that the air draws the water out of the tube. A cork placed 

 in water will rise to the surface; the same fact appears in rela- 

 tion to an air bubble in which cases we see an illustration of the 

 forcing propensity of a denser fluid, rather than a draioing pro- 

 pensity in a less dense to produce a current. The motion in the 

 atmosphere known as winds or currents, is caused by that subtle, 

 ever active and universal law, gravitation. It is the great pres- 

 sure of the atmosphere that forces a cold jet of air into a warmer 

 medium, and when this cold air is forced into a heated stove, it 

 becomes almost instantaneously rarified and forced on by a rear 

 column; hence, a current of air, or draft in common speech. 

 Having glanced briefly at the cause producing a disturbance, in the 

 atmosphere we are next to cast about ibr proofs illustrative of the 

 principle. 



It is known to every one, that when the sun's rays fall upon a 

 surface at right angles, the greater heat is produced at that point. 

 For this reason the south side of hills have a warmer temperature, 



