In the eighteenth century, theoretical research on fluid motion was at the forefront of mathematical physics. Especially, the problem of fluid resistance attracted wide attention because of its practical significance. Isaac Newton first sought to explain fluid resistance in a mechanical manner. He adopted the so-called ‘impact model’, which turned out to be very useful to estimate the practical value of resistance. He obtained result that the resistance exerted on an object moving through a ‘rare medium’ is proportional to its density, as well as the square of the speed of the object. As for resistance acting on a plane obliquely placed to a uniform flow, he argued that it should be proportional to the square of the sine of the angle of incidence. He further compared the resistance induced on a cylinder moving uniformly in a medium in a direction parallel to its axis with that of a sphere with the same diameter as the cylinder moving in the same medium with the same speed, calculating that the ratio of the former to the latter should be 2:1. Newton’s followers, including the Dutch natural philosopher Willem Jacob 's Gravesande, adopted this theory. Later, 's Gravesande realised that the impact model was not a realistic assumption, and he sought to find another way out. He pointed out the difference between the impact of separate particles and the pressure of continuous fluid. According to him, fluid resistance, resulting from the pressure exerted by flow, could not be represented as the aggregate of impacts of free particles, but should rather be caused by the inertial force of moving fluid, analogous to centrifugal force. In fact, 's Gravesande sought to explain the phenomenon of fluid resistance by unifying Newton’s concept of inertial or innate force and the concept of vis viva originated by Huygens and Leibniz. He then argued that the resistance acting on a plane obliquely placed to a uniform flow was proportional to the simple sine of the angle of incidence, not to its square. As for the ratio of resistances acting on a cylinder and a sphere, he concluded that it should be 3:2, a different result from that of Newton. A quarter century later, D’Alembert discussed the discrepancy between the results of Newton and 's Gravesande. Although he was rather suspicious of 's Gravesande’s theory, he had to refer the ultimate conclusion to further experimentation.