KEFLECTIO*.] 



UNDULATOEY FORCES. HEAT. 



27 



and absorbing power of the earth's surface. During the 

 day-time, the ground becomes warmed by the absorption 

 of the rays of heat, and thus an upward current is 

 produced, the sea air rushing in to supply the partial 

 vacuum formed. After sunset, the reverse effect takes 

 place ; the earth then commences to radiate heat, and, 

 becoming cooler, the air sets towards the sea, and hence 

 the production of the land-breeze. The following ex- 

 tract from a work by Captain Maury, entitled The Phy- 

 sical Geography of the Sea (Jan. , 1801), will be read with 

 interest, as affording full confirmation of these applica- 

 tions of natural laws. 



Speaking of the country near the coast, in hot climates, 

 Captain Alaury observes " The inhabitants of the sea- 

 shore, in tropical countries, wait every morning with im- 

 patience the coming of the sea-breeze. It usually sets 

 in about ten o'clock. Then the sultry heat of the op- 

 pressive morning is dissipated, and there is a delightful 

 freshness in the air, which seems to give new life to all 

 for their daily labours. About sunset, there is again 

 another calm. The sea-breeze is now done, and in a 

 short time the land-breeze sets in. This alternation of 

 the land and sea breeze a wind from the sea by day, 

 and from the land by night is so regular in inter- 

 tropical countries, that they are looked for by the people 

 with as much confidence as the rising and setting of the 

 sun. In extra-tropical countries, especially those on 

 the polar side of the trade winds, this phenomena is 

 presented only in summer and fall, when the heat of the 

 sun is ufficiently intense to produce the requisite degree 

 of rarefaction over the land. This depends in a measure, 

 also, upon the character of the land upon which the 

 sea-breezo blows ; for when the surface is arid, and the 

 toil barren, the heating power of the sun is exerted with 

 most effect. In such cases, the sea-breeze amounts to a 

 gale of wind. In the summer of the southern hemis- 

 phere, the sea-breeze is more powerfully developed at 

 Valparaiso, than at any other place to which my services 

 afloat have led me. Here, regularly in the afternoon, 

 at this season, the sea-breeze blows furiously ; pebbles 

 are torn up from the walks, and whirled about the 

 streets : people seek shelter ; the Almendral is deserted, 

 business interrupted, and all communication from the 

 shipping to the shore is cut off. Suddenly the wind 

 and the sea, as if they had again heard the voice of 

 rebuke, are hushed, and there is a great calm." He 

 also remarks " Within the tropics, however, the land 

 and sea breezes are more gentle. The oppressive heat of 

 the sun and the climate of the sea-shore is mitigated, 

 and made bcth refreshing and healthful by the alterna- 

 tion of thof.o winds, which invariably come from the 

 coolest place, the sea, which is cooler by day, and the 

 land, which is cooler by night About ten in the morn- 

 ing, the heat of the sun lias played upon the land with 

 ;it intensity to raise its temperature above that of 

 the water. A portion of this heat being imparted to 

 the superincumbent air, causes it to rise, when the air, 

 first from the beach, then from the sea, to the distance 

 of several miles, begins to flow in with a most delightful 

 and invigorating freshness." 



The phenomena of the radiation and reflection of 

 heat, may be conveniently examined by employing two 

 parabolic metal mirrors, which must be highly polished. 

 They may be from eight to twelve inches in diameter, 

 and should be fixed on stands of exactly equal height, so 

 that a straight lino passing from ono shall bo in a line 

 with both their axes or centres. They may be made of 

 polished tin, or sheet zinc ; perhaps the latter is to be 

 preferred, as being more readily beaten into the proper 

 shape. They may be polished by means of rotten- 

 stone and oil, and should be finished with tripoli, rubbed 

 on by means of cotton wool. Their focus is found by 

 placing a candle-flame some distance from the reflecting 

 surface ; and if a piece of smouldering brown paper is 

 held so that the smoke shall rise between the light and 

 the reflector, the focal point will bo at once observed as 

 that place where all the rays passing from the reflector 

 meet together. 1 n the focus of one of the mirrors, the 

 i source of heat is placed on a stand ; and, in the other, 



Fig. 6. 



the thermometer 

 which is employed 

 to indicate the re- 

 flected heat. Fig. 

 6 represents a pair 

 of these reflectors, 

 arranged for the 

 purpose of an ex- 

 periment. 



a n, represent two metallic reflectors ; 6, the source of radiant heat, as 

 an iron ball ; c, a differential thermometer. The rays of heat pro- 

 ceeding from b are reflected to a, and thence to the bulb of the ther- 

 mometer at c, in the course indicated by the lines. 



Fig. 7. 



o 



An ordinary mercurial or spirit 

 thermometer may be used for many 

 experiments, its bulb of course 

 being placed in the focus of the 

 mirror. For delicate experiments, 

 however, an air thermometer must 

 be employed. A simple one may 

 be constructed by filling a thermo- 

 meter tube with any coloured liquid, 

 and then inverting the stem in a 

 vessel of water. When so inverted, 

 sufficient liquid should be allowed 

 to escape, so that the bulb may be 

 full of air, because it is by the ex- 

 pansion of the air that the presence 

 of heat is manifested, and which also 

 drives down the liquid in the stem. 

 One of these thermoscopes, as they 

 are generally called, is represented 

 in Fig. 7. 



For many experiments on ra- 

 diated and reflected heat, an in- 

 strument, constructed on similar 

 principles, but having two bulbs, 

 is always employed. This is the 

 invention of Sir J. Leslie ; and it is 

 exceedingly sensitive in indicating minute changes of 

 temperature. It consists of two bulbs, each containing 

 air, which are joined by means of a glass tube, nearly 

 tilled with any coloured liquid. The tube is bent into 



Fig. 8. 



two arms, one of which is 

 graduated. In using the 

 instrument, one of the bulbs 

 only is brought towards the 

 source of heat, and that 

 should be the graduated 

 one. Its action is 'due to 

 the expansion of the air in 

 one bulb, and the conse- 

 quent compression of tluvt 

 in the other ; and according 

 to the depression of the 

 fluid in the graduated limb, 

 so the amount of sensible 

 heat is measured. When 

 employed in one of the re- 

 flecting mirrors, the bulb 

 attached to the graduated 

 limb is so placed in the 

 focus of the reflector, that the other may be removed 

 to one side ; the Utter is thus not influenced by the 

 reflected heat. The annexed engraving illustrates the 

 usual form of one of these instruments ; and c, Fig. 6, 

 shows one in use. 



The following experiments will give an idea of some of 

 the most interesting facts, both as regards radiating and 

 reflecting substances : 



. iment 23. Place in the focus of one mirror an 

 iron ball, previously made red-hot ; and in the focus of 

 the other mirror, a thermometer bulb, which should be 

 Blackened, may be introduced. The heat reflected from 

 one mirror to the other, and thence to the thermometer, 

 will be evident by the mercury rising. The air thermo- 

 meter is preferable for these experiments. That it is not 

 the mere radiation of the heat from the ball which pro- 

 duces the effect, is certain, because the mirrors may bo 



