For general service in aërodynamic experimentation two kinds of anemometer may be useful, one measuring air velocity, the other air pressure. Sometimes the velocity alone is required; again the pressure, either static or impactual, is mainly to be determined.

In either case an accurate and faithful instrument is most desirable. Of course the impact can be calculated from the velocity, and vice versa, providing all the data are at hand for computing the density of the moving fluid. But when many exact determinations have to be made, it is well to evade such calculations by choosing the appropriate anemometer; that is to say, a velocity instrument for measuring velocity, and a pressure instrument for measuring pressure, or impact. One thereby saves much labour, and avoids tho errors that may result from misreading the elements of density.