264 SEC. 8. HEAT. 



interval 250 being just one-fourth of the interval between the freezing and 

 boiling points of mercury. Many other substances also show a curious 

 relation in the interval between their freezing and boiling points to that of 

 mercury, facts which are not obvious upon other thermometric scales. 



The practical advantages of this system of graduation consist in the com- 

 parative.smallness of the degrees, thus avoiding in many cases the necessity 

 of the use of fractions to express the boiling point of substances ; also that 

 the zero point being so low the scale is a continuous one, all numbers under 

 100 M. representing temperatures below freezing water, but avoiding the 

 necessity of the use of the minus sign, and at higher temperatures as 1,000 

 is approached, giving a clear idea that the heat is arriving at the extreme 

 limit of thermometric registration. 



In practically graduating this thermometer reference is not made to the 

 freezing or boiling points of mercury, but the freezing point of water is 

 marked as 100, and the boiling point as 350, and the scale carried upwards 

 or downwards as required. 



The conversion of centigrade degrees into milligrade degrees, or vice versa, 

 is extremely simple. A centigrade degree multiplied by 2^, and 100 

 added, gives the milligrade degree, thus 40 C. multiplied by 2^ is 100, and 

 100 added gives 200, the degree on the milligrade scale. The correspondence 

 between the Fahrenheit and the milligrade graduation is not so simple, as 

 the interval on the Fahrenheit scale between the freezing and boiling points 

 of water being 180 F., higher numbers are required to be used in the 

 calculation. The following are the lowest common numbers for the scales : 

 25 milligrade, equal to 10 cent, and equal to 18 Fahr. 



Thus it follows that the following rules can be applied to calculate one 

 scale with the others 



To convert centigrade into milligrade degrees - n x 5-f-2 + 100. 



To convert milligrade into centigrade degrees - n 100 x 2-r5. 



To convert Fahrenheit into milligrade degrees - n *- 40 x 25-^-18. 



To convert milligrade into Fahrenheit degrees - n x 18-^-2540. 



lO55a. Thermometer, with 19 differently graduated scales, 

 traced on a silvered metal plate ; the centre is occupied by the 

 thermometer-tube and bulb. This instrument was made in 1754. 



Prof. Buys-Ballott, Utrecht. 



1055b. Four Registering Thermometers. 



E. Cetti and Co. 



1055c. Siemens* Pyrometer. E. Cetti and Co. 



lO55d. Metallic Thermometer, indicating two tempera- 

 tures. Francis JPizzorno, Bologna, Italy. 



The movements of the index are in this instrument produced by the dilata- 

 tion of two zinc blades which in the figure are seen edgewise. Along the 

 graduated arc can be fixed two sliding pieces ; if the index touches one of them, 

 it closes an electric circuit and rings a bell. Two small pearls carried by a 

 thread stretched between the extremities of the graduated arc, and which are 

 displace! by the index in its movements, serve to indicate the maximum and 

 minimum temperature. 



1055e. Five Thermometers, various. E. Cetti and Co. 



1055f. Mathiessen's Differential Thermometer. 



E. Cetti and Co. 



