438 Explanation of the Theory, S^e. [June, 



directly as the pressures, and reciprocally as the volumes or 

 heights. 



Variation from Temperature.— 20. The dry air, uniformly of 

 the temperature of 32° F. contained in the cylinder C being 

 exposed to increased temperatures, will have its volume, elasti- 

 city, and height, augmented, without regard to the pressure it 

 supports at the viniform rate of -,-1^ per degree. 21. Diminution 

 of temperature will occasion a corresponding decrease of volume, 

 elasticity, and height, in the same ratio. 



JLxamphs. 



Temperatures +32°F. +8° +80° + 512° 



Volumes and heights 1-00000 0-95000 1- 10000 2-00000 

 Densities 1-00000 1-05263 0-90909 0-50000 



Mr. Daniell has adopted, in his barometrical tables, &c. a most 

 erroneous method of calculating the alterations of density from 

 variation of temperature. Calling the volume of dry air under a 

 given pressure (30 inches), and of the temperature of 32° F. = 

 rO, he proceeds to find the densities at other (more elevated) 

 temperatures by subtracting the corresponding increase of 

 volume from 1*0, the assumed density at 32°. Had Mr. Daniell 

 extended the table to 512°, at which temperature the original 

 volume becomes doubled, the incorrectness of the method would 

 have been detected, — the density would have come out ! The 

 heights computed from his table will consequently exceed the 

 truth, especially when the mean temperature of the air was high, 

 or the elevation of the mountain considerable. The altitude 

 being 5000 feet, the error at 80° F. would be about 50 feet. 

 Well might the author of the Traitc de Fhijsique make the 



remark, "semblabies a un riche malaise qui n'a point 



d'ordre, au milieu de nos theories les plus briilantes, nous maix 

 quons souvent du plus simple necessaire." 



(To be continued.) 



