1821.] Dr. Apjohn on Wollaston't Thermometer. 293 



sities of the atmosphere were determined by a common baro- 

 meter. But that we may fully understand the manner in which 

 the pressure, at any station, is inferred from the temperature of 

 water boiling there, or more properly from the temperature of its 

 steams, it is necessary to recollect, that when water boils, the 

 tension of its steam is in equilibrio with the pressure of the 

 atmosphere, and, therefore, that both are susceptible of the same 

 measure. But Dr. Ure has shown, by a number of accurate and 

 well-arranged experiments, that the temperature at which water 

 boils, and the tension of its steam, are so connected that 30 in- 



ches of mercury, being its tension at 212° Fahr. j—= will be its 



30 



tension at 202, — - — — . will be its tension at 192, and so on, 



1*23 x 1'24 ' ' 



the second figure of the decimal part of the ratio increasing by 

 unity for every 10 degrees as we descend, but decreasing by 

 unity for every 10 degrees as we ascend. Thus the tension of 

 steam at 222 is 30 x 1'23 ; at 232 is 30 x 1-23 x 1'22, &c. 

 That this formula can only be true within certain limits is obvious 

 from the consideration that at all temperatures above 442, we 

 should have the elastic force of steam decreasing while its tem- 

 perature increased — a conclusion manifestly absurd. However, 

 with such high temperatures, we have at present nothing to do, 

 nor indeed was it at all intended that the formula should extend 

 to them; and for all temperatures about 212°, with which alone 

 we are concerned, I think, from the well-known accuracy of the 

 experimenter, it may be safely relied on. By means of this for- 

 mula, Dr. Wollaston has constructed a table, the small fragment 

 of which here given, is quite sufficient to enable us to calculate 

 all heights within its range from observations made with the 

 thermometrical barometer. 



Boiling points of water. ^^P^^^S 11 ' 8 ° f "to a'faU oTVdegreeTnlhl 



boiling point. 



214 31-2395 



213 30-0149 526-320 



212 30-0000 528-666 



211 29-3948 531-006 



210 28-7993 533-352 



209 28-2133 535-692 



208 27-6367 538-028 



207 27-0695 540-378 



206 26-5115 543-724 



205 25-9627 545-064 



204 25-4230 547-404 



203 24-8923 549-750 



202 24-3704 552-090 



The above table might have been more easily constructed by 

 assuming that the tension of steam observes a geometric, while 



