ON THK MATHEMATICAL THEORY OF FLUIDS. 249 



from such a law the above-mentioned properties would result ; for 

 the state of things would thus be nearly the same as if the fluid 

 were supposed to consist of perfectly smooth spherical balls in 

 contact, whose radii are all equal to the radius of the sphere of 

 activity of the molecular repulsion, and whose centres conse- 

 quently in the state of equilibrium are equidistant. This mode 

 of accounting for the characteristic property of fluids is not in- 

 consistent with the principal inference M. Poisson draws from 

 his calculation of the pressure of fluid in motion, viz., that the 

 pressure is not the same in all dii-ections from a given point. 

 For this deviation from the law of equal pressure may be reason- 

 ably ascribed to the circumstance that the molecules take titne 

 to fulfil the condition of similarity of arrangement, being made 

 to assume their positions relatively to each other by the action 

 of the repulsive and attractive forces. I may here observe that 

 although the inequality of pressure of fluids in motion is a legi- 

 timate deduction from the molecular hypotliesis, yet as theoi'y 

 cannot determine the amount of error committed by considering 

 the pressure equal, it seems unnecessary to take account of the 

 inequality unless some error should be detected by experiment, 

 especially as we know beforehand that the amount must be very 

 small. 



In closing this communication, I beg leave to add a few no- 

 tices respecting subjects contained in my former reports ; and 

 first, with regard to capillary attraction, it will be right to ob- 

 serve that some remarks made in the last report, in accordance 

 with the strictures of Dr. Young on the equation in art. 12 of 

 Laplace's Theory, I afterwards saw reason to conclude were in- 

 correct, and in a communication to the Philosophical Magazine 

 and Journal of Science for February 1836, explained that the 

 proper inference from that equation, though Laplace omits to 

 draw it, is, that the angle of actual contact of two fluids, or of a 

 solid and fluid when the specific gravities are not very diff'erent, 

 is an exceedingly small angle*, if the contact be perfect. It does 

 not appear that any exception can be taken to the reasoning in 

 any part of Laplace's Theory. The principles may indeed be 

 objected to on the ground that Poisson takes up, viz., that if 

 the molecular constitution of bodies be admitted, there must be a 

 superficial variation of density which that theory takes no ac- 

 count of : as, however, experiment has not yet detected any such 

 variation, and we have no means of assigning the amount of its 



* A phasnomenon I clianced to observe presented by oil floating on water 

 seems to favour this inference. See Phil. Mag. and Journal of Science for 

 April 1836. 



