 2. How does gravity vary, both below and above the surface of the Earth? In the absence of confirming data, Newton's answer — to a first approximation, linearly with distance to the center below the surface, and inversely with the square of the distance above it — presupposed uniform density in the first part and a spherical Earth with spherically symmetric density in the second, and therefore left open the possibility that gravity is constant near the surface of the Earth, just as Huygens continued to claim in his response to the Principia [HD, 153], citing supporting evidence.