JTJ.JT-ILI T JJVJ VV 



Comparing this with the simplified form of Darcy's formula 



flv* 4flv* 

 h = 4 - or -. 



% y m 2 g d 



4 f 



we have coincidence if '000709 = 5-^, 



64*4* 



i.e., if/ ='0114, 



which is the value given by Darcy's formula for rough pipes 7 J inches 

 diameter. 



It will be noted that in the general formula, the sum of the indices of 

 d and v is always 3. 



Variation of Resistance with Temperature. 



From the form of equation (3), it appears that if n is less than 2, the 

 resistance depends on the temperature, and varies as P 2 ~ n . Experi- 

 ments by the Author on the resistance of rotating discs (p. 181) indicate 

 the accuracy of this deduction, and this is substantially confirmed by 

 experiments by Mr. Mair l on a brass pipe, 1J inches diameter (n = 1*79). 

 The relative resistances as experimentally obtained and as calculated are 

 as follow : 



For values of n in the neighbourhood of 2 the effect of temperature 

 variation is very small. When w = 1/8, the resistance alters about one- 

 third of one per cent, per degree Fahrenheit, while when n = 1*9 the 

 change is one-seventh of one per cent, per degree Fahrenheit. 



It is worth noting in passing, that while with sinuous motion the 

 difference between the discharge at 5 and 35 centigrade is negligible, 

 below the C. V. the discharge at 5 centigrade is only one-half that at 

 35 centigrade under the same head. 



Other Exponential Formulae. On applying the Reynolds formula in its 

 original form to the results of other experiments, it is found to be some- 

 what lacking in adaptability, while experience tends to show that in the 

 particular case where n = 2 the resistance does not, as indicated, vary 

 inversely as the first, but as a slightly higher power of the diameter. 



Proc. Inst. C.E.," vol. 84, 1886, p. 424. 



