II. THERMOMETRT. 227 



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 l(r 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 5-4-2 + 100. 



To convert milligrade into centigrade degrees - n-^100 x 2-h5. 



To convert Fahrenheit into milligrade degrees - n + 40x25-f-18. 



To convert milligrade into Fahrenheit degrees - n x 18-4-25 40. 



John Williams, F.C.S. 



1055a. Thermometer, with 19 differently graduated scales, 

 traced on a silvered metal plate ; the midst is taken in by the 

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



Professor Buys-Ballott, Utrecht. 



1055b. Four Registering Thermometers. 



E. Cetti and Co. 



1055c. One Siemgan's Pyrometer. E. Cetti and Co. 



1055d. Metallic Thermometer, indicator of two tempera- 

 tures. Francis Pizzorno, of 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 a ring bell placed in action. Two small pearls 

 carried by a thread stretched between the extremities of the graduated arc, 

 and which are displaced by the index in its movements, serve to indicate the 

 maximum and minimum temperature. 



lO55e. Five Thermometers, various. E. Cetti and Co. 



1067. Wedgwood's Pyrometer, consisting of pieces of clay, 

 contracting according to the heat to which they are exposed. These 

 are afterwards slid along the gradually diminishing and graduated 

 groove in the brass plate, and so indicate the degree of heat to 

 which they have been exposed, Robert Garner, F.R.C.S. 



1068. Pyrometer, of iron and copper, for lecture illustration. 



Yeates $ Sons. 



The above consists of a compound bar of iron and copper, bent into the 

 form of U> one arm of which is firmly attached to the stand ; the other arm is 

 free, and carries a long index. If the compound U De immersed in a beaker 

 of boiling water, the index will move over several degrees of the scale. 



1069. Reflecting Pyrometer, for showing the difference of 

 expansion of different metals. Yeates $ Sons. 



1070. Wedgwood's Pyrometer, invented in 1782. 



Edinburgh Museum of Science and Art. 



P 2 



