tin 



rrro U placed not at, but below the freezing point of 

 water. 



nc.it 



WV..T . :. 



Witrr nwm 



Cold produced by mlitura of ic 

 ml Mil 



Otmtiermon o/ .FaArrnAeii to Centigrade Degrees. The 

 proposition U evident that one Fahrenheit decree is 

 equal to five-ninths of a centigrade degree, inasmuch as 

 the number of Fahrenheit degrees, between the fruoxin^ 

 and the boiling point, U to the number of centigrade 

 degrees for the same space as unity to five-ninths, as is 

 seen to be established by the following proportion : 



F. C. F. C. 



(212 32) = 180 : 100 : : 1 : 188 = J, 



whence it appears that we may convert Fahrenheit 

 decrees into their centigrade equivalents, by first sub- 

 tracting 32, which leaves 180 ; then multiplying this 

 number by five, and dividing by nine, as in the following 

 proportion : 



212-33-180and^ 9 *- 5 - 



100. 



Conrenion of Centigrade to Fahrenheit Degrees. Inas- 

 much as each centigrade degree is longer than a degree 

 of Fahrenheit in the ratio of to 1, therefore the 

 former may be reduced to the latter by multiplying by 

 9, dividing by 5, and adding 32. The truth of this 

 operation may be readily demonstrated by working 

 on the number 100, which should give, if the rule just 

 enumerated be correct, 212 : 



100 X 9 

 j.^ - 180, and 180 + 32 = 212. 



The examples just given, illustrate the process of cal- 

 culation when positive degrees, or degrees above zero, 

 thus (-)-), are concerned. Exactly the same rule has to 

 be followed when negative degrees ( ) are in question, 

 although the rule, when stated in common terms, appears 

 to be different, inasmuch as following the diction of 

 arithmetic, the operator must be told to subtract 32. An 

 example will render this more' evident. Suppose we 

 require to represent 6 below zero, or 6 of C, by its 

 equivalent F. Now the number 32, with 9 subtracted, 

 gives 23 for remainder. Viewing all the steps of the 

 calculation involved algebraically, it will be found that 

 the rule of adding 32 has been implititly followed ; a 

 negative 5 however ( 5) yields in the following opera- 

 tion a negative 9 ( 9), which, being added to + 32, is 

 equivalent to subtracting a positive 9 (+ 9). For 

 example 



The various steps for the reduction of one system of 

 tharmoawtria degrees to another, are comprehended in 

 the appended formula; : 



Fahrenheit to Centigrade, F " ~ 32 X 5 _ C. 



y 



Centigrade to Fahrenheit, SL*J? + 32 - F. 



Fahrenheit to Reaumur, F -l ~ 32 x 4 _ R. 



;i 



Reaumur to Fahrenheit, R ' X !> -f 32 - F 



The : 1'htrmomtter. Tho greatest difficulty 



attendant upon the use of the thermo. ^ ;<l _ 



meter for metcorologic observations, is "^ ^ 



n-fcraMe to the necessity of frequent 

 examination. To obviate in some 



:!io necessity for th 

 in tmni.-nt called tin 

 momtttr lias been devised an in-tin- 

 .\liieh depends for its action on 

 the tr two steel bars in the 



bore of tho tube, each pressed forward 

 by the expansion of a liquid column. 

 An instrument of this kind is repre- 

 sented in Fig. 20. The register ther- 

 mometer is accurate eiioiuh ii' its in- 

 dications for some rough purposes, 

 but it is by no means adapted to super- 

 sede the use of thermometers of ordi- 

 nary construction. 



The Thermometer nf Brerpi' 

 very delicate thermometer has been 

 invented by M. Breguet. It dinYrs 

 from all which we have hitherto de- 

 scribed, in the fact of its dispensing 

 altogether with mercury, or other 

 expansive liquid, and utilising tho 

 expansion or uncoiling of a compound 

 metallic bar. 



The illustration of the different rates 

 of expansion by heat of 'two different 

 metals (for instance, iron and brass) 

 is often made by the following con- 

 trivance : (FL f . -7) is a compound 

 bar of this kind, perfectly straight 

 when cold ; but if this same bar bo heated, it becomes 

 curved, as represented by Fig. 28. 



Fig. 27. 



Breguet's thermometer is merely an amplification of 

 the preceding experiment ; instead of a straight corn- 

 Fig. 23. 



Fig. 29. 



pound bar, a compound bar twisted into the spiral form is 

 employed. One eud of the spiral is fixed ; the other end 

 is freo, and is attached to an index. The mode of action 

 of the instrument will be obvious. Variations of tem- 

 perature producing variations 

 of curve, will cause the spiral 

 to unfold, or contract, accord- 

 ing as the variations are to- 

 wards the direction of increased 

 heat or increased cold. The 

 metals employed in making the 

 spiral of Breguet's thermome- 

 ters, are platina, gold, and sil- 

 ver. Experiment has demon- 

 strated that the needle of this 

 intruincnt travels over equal 

 arcs for equal increments of 

 temperature ; hence '. 

 thermometer is not only com- 

 parable with itself, Imt with all other instruments on tho 

 same construction. I'nfortuiKitely, this ileheate ii> 

 ment has no great range of application, its Indie.-. 

 being limited between the freezing and tho boiling points 

 of water (Fig. 29). 



/hermoscove cf AbbiK. Uy far the most delicate 

 indicator of minute increments and decrements of 







