HEAT. 



the manner of a funnel, into which a 

 quantity of quicksilver is put ; the bulb 

 is heated over a lamp to expel the air, 

 and when the portion of air which re- 

 mains cools, the quicksilver descends 

 and partly fills the bulb ; heat is again 

 applied until the quicksilver boils, this 

 occasions the more effectual expulsion 

 of the air ; on cooling a larger quantity 

 of the fluid metal is made to enter 

 into the instrument. A sufficient quan- 

 tity having been introduced, and the 

 whole having been properly boiled, heat 

 is applied until the mercury begins to 

 overflow at the extremity of the tube, 

 previously drawn to a fine point ; flame 

 is urged upon it by a blow-pipe, and it 

 is hermetically sealed, or melted and 

 joined to exclude the air. 



To regulate whatever scale may be 

 applied to the instrument, it is neces- 

 sary to ascertain the points in the tube 

 at which the mercurial column stands 

 when cooled to the freezing, and heated 

 to the boiling, of water. For this pur- 

 pose, the instrument is immersed in 

 melting ice or snow, and permitted to 

 remain there for gome time, until the 

 quicksilver becomes stationary at one 

 place, which is the freezing point; a 

 mark is made at that point upon the 

 glass. By allowing the instrument to 

 remain for some time in boiling water, 

 the mercury, after having ascended 

 through a large proportion of the tube, 

 becomes stationaiy at one place, which 

 is the boiling point ; if the calibre, or 

 bore, of the tube is equal throughout, the 

 space between these two points upon 

 the scale, to be applied, according to 

 Fahrenheit, is divided into 180 equal 

 parts, thirty-two of the same equal divi- 

 sions being placed below the freezing 

 point. In other words, the scale in- 

 vented by Fahrenheit, which is in general 

 use in this countiy, commences at 32 

 below the freezing point of water, and 

 has the boiling point at 212. It is be- 

 lieved that Fahrenheit took his zero or 

 commencement of his scale from the 

 degree of cold produced by mixing 

 snow and common salt, that being the 

 greatest degree of cold known in his 

 time ; although a considerably greater 

 degree of cold may be produced by 

 mixing the same or other ingredients. 

 If the tube is unequal in the bore, use 

 must be made in the graduation of the 

 scale, of points obtained by dividing the 

 tube into parts of equal capacity, in 

 the manner already described. 



The scale called Reaumur's, which 



has been much used on the Continent, 

 commences at the freezing point of 

 water which is marked o, between 

 which and the boiling point are eighty 

 equal divisions or degrees, the point 

 at which water boils being at the 

 80th degree. Each degree of Fahren- 

 heit's scale being equal to of a de- 

 gree of Reaumur's, to find the corre- 

 spondence between these scales, it is 

 only necessary to multiply the number 

 of degrees of Fahrenheit, above or be- 

 low the freezing point, by 4, and divide 

 by 9 ; the sum obtained will indicate the 

 number of degrees upon Reaumur's 

 scale. 



Fahrenheit. Reaumur. 



50 32= 18x4= 724-9= 8. 



185 32=153x4 = 612+9 = 68. 



To make those of Reaumur corre- 

 spond with Fahrenheit, the rule is to 

 multiply by 9 and divide by 4. 



Reaumur. Fahrenheit. 



8x9= 724-4= 18 + 32= 50. 

 68x 9 = 6124-4 = 153 + 32= 185. 



Another scale, which is now exten- 

 tensively used, particularly in France, 

 is that of Celsius, and is more simple 

 than any : it begins at the freezing 

 of water, between which and the 

 boiling of the same fluid there are 

 100 equal divisions upon the scale ; so 

 that the boiling point is at the 100th 

 degree. This instrument is now called 

 the Centigrade Thermometer, and is by 

 many preferred to the others, the divi- 

 sions on its scale being considered the 

 most natural. The graduation of Fahr- 

 enheit, however, has some important 

 advantages over the others : its divisions 

 being small, there is the less necessity 

 for stating fractional parts ; and its 

 commencement being low, it is seldom 

 required to state negative degrees. 



The degrees on Fahrenheit's scale, 

 being each equal to of a degree, on 

 the Centigrade scale, to find the corre- 

 spondence of the degrees of the former 

 with those of the latter, multiply the 

 degrees, above or below the freezing of 

 water, by 5 and divide by 9, thus : 



Fahrenheit. Centigrade. 



86 32= 54x5 = 270-f-9 = 30. 

 1 76 32= 144 x 5 = 720-^-9 = 80. 

 To reduce the degrees of the Centi- 

 grade scale to those of Fahrenheit, 

 multiply by 9 and divide by 5. 



Centigrade. Fahrenheit. 



30X 9 = 270-^-5 = 54 + 32= 86. 

 80 X 9= 720-r5 = 144 f 32=; 1 76, 



