260 FOURTH REPORT 1834. 



would be everywhere equal to the adherence of two particles ; 

 and the phaenomena of capillary tubes would then present no»- 

 thing which could not be determined by analysis." The process 

 here indicated Monge did not follow up by mathematical calcu- 

 lations. The main purpose of the memoir, from which the above 

 sentence is extracted, is to give an explanation of the apparent 

 attraction and repulsion observed to take place between small 

 substances when they float near each other on the surfaces of 

 fluids. These phspuomena are of three kinds. (1.) If two float- 

 ing bodies are each surrounded by a depression of the fluid sur- 

 face, and are separated at first by a small interval, they will 

 move towards each other as if mutually attracted. (2.) When 

 the fluid rises up around them, they will in this case also appear 

 to be attracted when brought near each other. (3.) When one 

 is surrounded by an elevation of the fluid, and the other by a 

 depression, they will appear to be mutually repelled. The motion 

 of the bodies towards each other in the first case, is owing to the 

 circumstance that the depression of the fluid about one is in- 

 creased on their mutual approach by the depression about the 

 other, at those parts of each that are neighbouring ; which occa- 

 sions an unequal hydrostatic pressure against each of the bodies 

 in the horizontal direction, the pressure being greatest where 

 the depression is least. This explanation was first given by 

 Mariotte. To explain the second phaenomenon, Monge reasons 

 as follows. If a plate of any substance be dipped with its plane 

 vertical in fluid, the fluid will rise by capillary attraction on 

 each side of it. The surface of the raised water, being stretched 

 like a chain, will draw the plate in a horizontal as well as verti- 

 cal direction, but equally on both sides, so that it will remain in 

 its vertical position. If now another plate of the same matter, 

 and exactly alike circumstanced, be brought near the other, the 

 fluid, it is well known, will rise between them. The total quan- 

 tity of fluid raised above the ordinary level will remain the same 

 as if the actions of the two plates did not interfere with each 

 other, because the raising forces will be the same. But the 

 weight of water raised between the plates, being suspended from 

 a diminished quantity of fluid surface, the superficial tension 

 within will become greater than that without, and will more 

 than counterbalance the latter. The plates will consequently be 

 drawn together. The third phaenomenon is explained by saying, 

 that when a body, surrounded by an elevation of the fluid, is 

 brought near one surrounded by a depression, there is occasioned 

 a diminution of depression on the side of the latter nearest the 

 other, and a consequent inequality of hydrostatic pressure, the 

 excess being on the side where the depression is least. This 



