NEWTON S PRINCIPIA. 319 



or, 



but dx and dy are quite independent and can have any 

 ratio 



.-. X = 0, Y = 0, 



that is, Z, the only force, acts along the normal. 



Consequence III. If the fluid be elastic, Amontem has 

 shown, by experiment, that there must be the relation 



between p, ^, and the temperature, k, is a constant quan 

 tity depending on the nature of the gas, and a a numeri 

 cal quantity = ^y, nearly. Then we have 



dp_ 



f p &quot; k(l + *l) 



hence, as before,/?, 9, P are functions each of each, so that 

 All level surfaces are surfaces of equal temperature. 



