THERMOMETEBS.] UNDTJLATORY FORCES. HE AT. 



17 



temperature by means of liquids. There is in every case, 

 with but one exception, a constant contraction sustained 

 by liquids on their losing heat ; and thus they afford means 

 of ascertaining temperatures which may be much below 

 those experienced in daily life. 



This exception is that of water ; and in it we find a 

 remarkable instance of an apparent suspension of a uni- 

 versal law for effecting a special end. At a temperature 

 of 40, water ceases to contract ; and as it approaches a 

 temperature of 32, its freezing point, it expands con- 

 siderably ; so much so, indeed, that if confined in a closed 

 Teasel of great strength, it will, on freezing, expand suffi- 

 ciently to rend the vessel into fragments. This singular 

 exception has an' astonishing effect in influencing the 

 climate of the polar regions. If water contracted continu- 

 ously on suffering a decrease of temperature, it would, 

 on arriving at the solid state of ice, at once sink to the 

 bottom of a vessel from its attaining an increased specific 

 gravity ; but through the expansion which takes place as 

 it cools, this cannot occur. Hence we find the ice on the 

 turfaee of the rivers and the sea, ready to receive the 

 rays of solar heat, which will again change it to the liquid 

 state a result which could never occur if, obeying the 

 usual laws, it sank, when formed, to the bottom of the 

 ocean. This circumstance may be adduced as one of 

 the most striking proofs of the existence of an OMNIS- 

 CIENT BEIXO. 



The ordinary thermometer consists of a glass tube, of 

 small diameter, one of whose ends has been blown into a 

 bulb. This and a portion of the stem is filled either with 

 men -y, or spirits of wine which has been coloured ; and 

 after expanding the fluid until it completely fills the stem, 

 the open end is hermetically sealed or closed. A scale is 

 then made, whose object is to ascertain comparative tem- 

 perature*. Various scales are employed ; such as those 

 of Reaumur, the Centigrade, and that of Fahrenheit 

 Confining our attention to the latter for the present, we 

 hall describe the mode adopted for mnlting the scale 

 which is invariably used in this country. 



On the bulb and item being completed, the former is 

 dipped into a vessel containing melting snow or ice ; and 

 when the mercury in the stem ceases to fall, a mark is 

 made on the glass. This indicates the freezing point of 

 water, which U denominated by Fahrenheit as 32. 

 Water U then heated in a metal vessel (one of glass, <tc. , 

 will not do) ; and when the water, which should be quite 

 pare, boils, another mark U made at the point to which 

 the mercury has risen in the stem. The interval between 

 the freezing and the boiling point is divided into 180 equal 

 parta, or degrees. Hence, on the scale we here refer to, 

 water boils at a temperature of 212. 



We (hall not enter, at present, into various corrections 

 which must be made for the expansion of the glass bulb and 

 tube, each of which has the tendency of inducing errors in 

 observation ; as these point* will be fully considered under 

 the bead of Meteorology. Spirit* of wine is usually em- 

 ployed for measuring very low temperatures, as it is not 

 liable to freeze an occurrence which would happen to 

 mercury, even in the temperature of the arctic regions, 

 at some periods of the year. The scale of Reaumur has 

 but 80 between the freezing and boiling points of water ; 

 nd a* this is often used in foreign scientific works, we 

 append a rule for converting the degrees of each scale to 

 that of the other. 



1. To convert the degree* of Fahrenheit's scale to those 

 of Reaumur, multiply the number of degrees of Fahren- 

 heit, from which has been subtracted 32, by 4 ; and divide 

 the amount by 9 : the quotient will bo the degrees indi- 

 cated by Reaumur's scale. 



KxampU. What number of degree* will be shown on 

 the scale of Reaumur, if the temperature indicated by 

 that of Fahrenheit U 212 ? 



tract ing 32 from 212, leave* 180. 

 Multiplying 180 by 4, gives 720 ; and, 

 Dividing 720 by 9, 80. An. 80. 



2. To convert the degrees on Reaumur's scale to those 

 of Fahrenheit, multiply those of Reaumur by 9, and 

 diviile by 4 ; to the quotient add 32, which will give the 

 number sought 



VOL. I. 



Exampk. What number of degrees on Fahrenheit's 

 scale corresponds with 80 degrees on that of Reaumur ? 

 Multiplying 80 by 9, gives 720. 

 Dividing 720 by 4, 180. 

 Adding 32 to 180, 212 Ans. 212. 



The above calculations evidently rest on the fact, that 

 nine of the degrees of Fahrenheit's scale are eqxiivalent to 

 four degrees of that of Reaumur. 



The Centigrade scale is divided into 100 between the 

 freezing and boiling points. Hence, five degrees on this 

 scale correspond with nine degrees of those on Fahren- 

 heit's scale. 



3. To convert the degrees of Fahrenheit to those of 

 the Centigrade, multiply the number of the former, less 

 32, by 5 ; and divide by 9 : the quotient will afford the 

 corresponding degrees of the Centigrade scale. 



ExampU. What is the number of degrees on the Cen- 

 tigrade corresponding to 212 Fahrenheit ? 

 212 less 32, equals 180. 



180 multiplied by 6, gives 900. 

 900 divided by 9, equals 100. Ans. 100. 



4. To convert the Centigrade to the Fahrenheit scale, 

 multiply the former by 9 ; and, dividing by 5, add 32 to 

 the quotient. 



Example. What number, on the scale of Fahrenheit, 

 corresponds with 100 on the Centigrade scale ? 

 Multiplying 100 by 9, produces 900. 

 Dividing 900 by 6, 180. 

 Adding 32 to 180, gives 212. Ans. 212. 



The following skeleton table may be of assistance in 

 many instances, and, occasionally, will save the trouble 

 of calculation : 



Comparative indication* of thermometer! of the scale of 



Fahrenheit. Reaumur. Centigrade. 

 Freezing point . . 32' 0' 



Boiling . . 212' 80" 100". 



Various forms of instruments for measuring heat will 

 be fully noticed in other parts of the work, as coming 

 more especially under other subjects : among these we 

 include the air, registering, and dew-point thermome- 

 ters ; as also Breguet's, and an interesting instrument, 

 by means of which electricity is employed for ascertain- 

 ing minute changes of temperature. We have omitted 

 to give exact directions for constructing a thermometer, 

 because long practice is necessary to produce even a 

 tolerable instrument ; and the cost of such ia very slight, 

 if purchased of the instrument-maker. 



Having thus described the thermometer as an instru- 

 ment which enables us, by the expansion of a fluid, to 

 ascertain the comparative temperature of bodies, we 

 proceed to notice some of those natural and ordinary 

 effects which depend on the expansion of liquids by heat 

 for their proximate cause. 



In every instance in wliich a liquid is heated, it be- 

 comes lighter than that of a lower temperature ; hence it 

 has a tendency to ascend. This effect is magnificently 

 illustrated in the case of the gulf stream, which sets in 

 towards our western shores from the Gulf of Mexico. 

 Its temperature is much higher than the surrounding 

 ocean, and it accordingly floats on the surface of a 

 colder, and therefore denser, fluid. In domestic life this 

 law is taken advantage of ; hence the construction of hot- 

 water apparatus, in which the hot water rises through an 

 upper pipe, and, when cooled, returns by one placed at 

 a lower leveL The character of these currents of 

 heated liquid, and their tendency to rise, may easily be 

 noticed by trying the following experiment : 



Experiment 3. Throw a small piece of litmus, or 

 indigo, into a glass flask nearly filled with cold water, 

 and placing the bottom of the flask over the flame of a 

 candle or lamp, it will be noticed, that as the lower 

 stratum of liquid becomes heated, the colouring matter 

 will rise with it; and, on the liquid cooling, it will 

 return to the bottom of the flask, again to undergo a 

 similar process. 



EXPANSION OK GASES. Gaseous bodies, such as air, 

 expand still more than liquids, on the addition of heat ; 

 and the cause and applications of this fact, form a most 



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