There is a persistent myth of `irrationality' of `virial' Liapunov functionals used in proofs of fluid instability. The aim of the paper is to address that issue and to produce the unifying and systematic concept on introducing of `virials'. Our `virial' can be introduced via the second variation of Lagrangian, and correspondent `virial equality' is closely linked to the Jacobi equation for deviations of geodesics. This link explains the nature of `virial' as Liapunov functional. It may be considered as the `distance' between two geodesics corresponding to the main flow and perturbed flows. New applications of `virials' to instabilities both states of rest (`converse Lagrange theorems') and flows (`horseshoe instability' and `Arnold's instability') are considered and further perspectives are discussed.