10 



THERMOMETER AND PYROMETER. 



containing" a fusible alloy of two parts 

 of bismuth, one of lead, and one of tin. 

 In the temperature of the air, it remains 

 solid in the tube; it becomes fluid 

 about the boiling point of water ; then, 

 as a fluid, expands by increase of tem- 

 perature ; and its expansion being seen 

 through the semitranslucent tube, 

 which is divided into equal parts or de- 

 grees, becomes an indication of the 

 temperature applied to the ball. 



This invention promises to be of con- 

 siderable utility, and is capable of ex- 

 tension by the employment of less 

 fusible metals. From the simplicity of 

 its construction, it is rather surprising 

 that it has not been more generally 

 known, and employed in potteries, 

 where the instrument could be easily 

 made. An instrument on this con- 

 struction would be a better method of 

 uniting the scales of the common ther- 

 mometer and pyrometer than any here- 

 tofore employed. 



Of these various thermometric scales 

 there are but three in very general use, 

 viz. that of Fahrenheit, Celsius, and 

 Reaumur. Fahrenheit's is chiefly used 

 in Britain, North America, and Hol- 

 land : the scale of Celsius was adopted 

 by the French, and is now employed in 

 most parts of the north and middle 

 of Europe: Reaumur's was the only 

 one used in France before the Revolu- 

 tion, and is still that best known in 

 Spain and in some other continental 

 states ; but it is further important, as 

 affording the terms in which numerous 

 very valuable observations are recorded. 



For these reasons it is useful to have 

 formulae for readily converting one 

 scale into the equivalent degrees of the 

 other two. The freezing point of water 

 on Fahrenheit's scale is at 32, and on 

 those of Celsius and Reaumur at 0, 

 while it boils on each respectively at 

 180, 100, and 80, above that point. 

 Hence the degrees of Fahrenheit are to 

 those of Celsius as 180 : 100 = 18 : 10 

 = 9:5, and to those of Reaumur as 

 180: 80 = 18:8 = 9:4 , or 9 of Fah- 

 renheit are equal to 5 of Celsius and 

 to 4 of Reaumur. Therefore, when we 

 wish to convert the degrees of Celsius 

 into those of Fahrenheit, we have to 

 multiply the number of the former by 9, 

 divide by 5, and add 32; to reduce 

 the degrees of Fahrenheit into those of 

 Celsius, the converse of the proposition 

 will give the required result; that is, 

 from the degree of Fahrenheit subtract 

 32, then multiply by 5, and divide by 9. 



When we wish to convert the degrees of 

 Reaumur into those of Fahrenheit, we 

 have to multiply by 9, divide by 4, and 

 add 32 ; and subtracting 32 from the 

 given degree of Fahrenheit, multiplying 

 the remainder by 4, and dividing by 9, 

 will give the equivalent degree of Reau- 

 mur's scale. 



The following short formulae will 

 apply to each case : 



1. 



3. 



4. 



(F - 32) X 5 



(F- 32) x 4 



These formulae apply to all degrees 

 above the freezing point of water ; but 

 when negative degrees of Celsius are to 

 be converted into the equivalents on 

 Fahrenheit's scale, multiply the degree 

 of Celsius by 9, divide by 5, and the 

 difference between the quotient and 32 

 is the required degree of Fahrenheit : 

 or when negative degrees of Fahrenheit 

 are to be reduced to their equivalents 

 on the scale of Celsius, add 32 to the 

 given degree of Fahrenheit, then multi- 

 ply by 5, and divide by 9. By substi- 

 tuting 4 for 5, the same formulae will 

 apply to Fahrenheit and Reaumur, all 

 which may be thus expressed : 



1. 



2. 



3. 



4. 



C = 



-F 



R = 



(F + 32) X 5 



9R 



4 



32. 



(F + 32) X 4 



The formulae are convenient for re- 

 ducing a few examples from one scale 

 to another ; but when they perpetually 

 occur in reading or writing it is very 

 useful to have comparative tables, fron 

 which, by one glance, the desired infor- 

 mation may be obtained. 



2. Precautions necessary to be ob- 

 served in constructing accurate Ther- 

 mometers. 



A general idea has been already 

 given of the mode of constructing a 



