Thumb rules in general give only an average, which may not apply to the case in hand. They should not be relied upon for design and construction, but may serve for rough preliminary estimates. Neither will tabulated figures cover the requirements, if based on a fixed set of conditions, such as the pressure loss for a certain length, which hardly ever coincides with a particular instance. Such tables ought to be used only for. Approximate sizing. For methodical calculation of a conduit it is essential to be aware that the fluid to be moved, or kept under control, must fol low the laws of nature. All calculations relating to the ?ow should therefore be based, first of all, on formula giving adequate expression to-the forces coming into play. This means that each case should be studied, not only with a view to applying the proper formula, but also in order to size up the various factors on which to base the computation. Theory and practice will be found to agree if this is done. Whenever they seemingly do not, some element has been left out Of consideration or'a factor has been misjudged.