HEAT OR CALORIC. 113 



Apparatus for showing the influ- 

 ence of Relaxed Pressure, on 

 the capacity of Mr for Heat, 

 or Moisture. 



" A glass vessel with a tubulure 

 and a neck, has an air thermom- 

 eter, fastened air tight, by means 

 of a cork into the former, while 

 a gum elastic bag is tied upon the 

 latter, as represented in this fig- 

 ure. Before closing the bulb, 

 the inside should be moistened. 

 Under these circumstances, if 

 the bag, after due compression 

 by the hand, be suddenly releas- 

 ed, a cloud will appear within 

 the bulb, adequate in the solar 

 rays, to produce prismatic colors. 

 At the same time the thermome- 

 ter will show that the compres- 

 sion is productive of warmth 

 the relaxation of cold." 

 " The tendency in the atmosphere to cloudiness, at certain el- 

 evations, may be ascribed to the rarefaction which air inevitably un- 

 dergoes, in circulating from the earth's surface to such heights."* 



* In connexion with this effect on the transparency of the atmosphere, it may be 

 interesting to recollect, the important influence of barometrical pressure on our 

 health and comfort. If we were to regard (a supposition which is not exactly true, 

 but which may be made for the sake of illustration,) the muscular power of the heart 

 and arteries as a constant force, propelling the blood regularly in the circulation ; 

 then it is obvious, that the varying pressure of the atmosphere must necessarily af- 

 fect both our feelings and our safety. With a diminished pressure, there must be a 

 more rapid and hurried circulation, and with it we might expect faintness and op- 

 pression as is experienced on high mountains. The oppression and lassitude expe- 

 rienced in what is called a heavy air, (which is really a lighter air, our feelings alone 

 being heavy,) is probably owing, in part, to this cause. At moderate elevations, we 

 do not experience oppression, for there is generally a clearer and a cooler atmos- 

 phere, and our moral energy is invigorated by the scenery, and our physical force 

 by the exercise. The subject is perhaps worthy of some attention in selecting situa- 

 tions for invalids, but many other causes must be taken into view, such as the exha- 

 lations, the temperature, &c. 



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