422 HEXRY A. ROWLAND 



On the rag., mm., second system, we have -# = 10-935 X 10 10 , c = 

 001047, H = 23993, g = 9800-5 at Baltimore. 



/. = 444160 mm. = 444-2 metres. 

 9 



Using Kohlrausch's value for absolute resistance, he finds 456-5, 

 which is much more in error than that from my determination. I do 

 not give the calculation from the Grove battery, because the Grove 

 battery is not reversible, and action takes place in it even when no 

 current flows. 



Thomson finds the difference of potential between the poles of a 

 Daniell cell in electro-static measure to be -00374 on the cm., grm., 

 second system. 40 Using the ratio 29,900,000,000 cm. per second, as I 

 have recently found, but not yet published, we have 111,800,000 on 

 the electro-magnetic system or 11-18 X 10 10 on the mm., mg., second 

 system. This gives 



= 474.3 metres. 



g 



General Criticism 



All the results so far obtained, except those of Joule, seem to be of 

 the crudest description; and even when care was apparently taken in 

 the experiment, the method seems to be defective, or the determination 

 is made to rest upon the determination of some other constant whose 

 value is not accurately known. Again, only one or two observers have 

 compared their thermometers with the air thermometer, although I 

 have shown in ' Thermometry ' that an error of more than one per 

 cent may be made by this method. The range of temperature is also 

 small as a general rule and the specific heat of water is assumed con- 

 stant. 



Hence a new determination, avoiding these sources of erfor, seems 

 to be imperatively demanded. 



(6.) Description of Apparatus 

 1. PRELIMINARY EEMARKS 



As we have seen in the historical portion, the only experiments of a 

 high degree of accuracy to the present time are those of Joule. Looked 

 at from a general point of view, the principal defects of his method 

 were the use of the mercurial instead of the air thermometer, and the 

 small rate at which the temperature of his calorimeter rose. 



40 Thomson, Papers on Electrostatics and Magnetism, p. 246. 



