Have you noticed that the magnitude of an earthquake is never described as, say, "5.4 Richters", but rather in somewhat convoluted terms as "measuring 5.4 on the Richter Scale"? In this it resembles wind strength when the latter is described, for example, as being "force seven on the Beaufort Scale". In both cases there is no such unit as a "Beaufort" or a "Richter" in the sense that we have "degrees Celsius" or "miles per hour"; the divisions on the Beaufort and the Richter scales are non-uniform, and unlike the other two already mentioned, are not directly or conveniently convertible to other units.
The eponymous Charles Francis Richter was born in 1900, and devised his scale in 1935 while working at the Californian Institute of Technology. It is a measure of the total energy released by an earthquake, which is in turn deduced from seismographic readings. For practical purposes the scale runs from 0 to 9 - although in theory there is no upper limit - and as we have noted it is not a scale of equal increments; on the contrary, an increase of one unit signifies an earthquake about 30 times as great as one corresponding to the lower number. Richter's scale measures the magnitude of an earthquake, and provides a unique figure for each such event. Also used from time to time, however, is the Modified Mercalli Scale, originally devised in 1902 by Guiseppe Mercalli, which tries to assess the intensity of a tremor at a particular place; it is not dissimilar in concept to the Beaufort Scale, in that the number chosen is based on observation of the local damage.
Each "strength" on the Mercalli Scale is defined in terms of its observed effect on the structural and human environment, ranging from Strength I, detectable only on very sensitive instruments, to Strength XII, where there is complete devastation in the region. In between comes Strength VI, for example, a "Fairly Strong Shock", accompanied, we are told, by "the awakening of those asleep, general ringing of bells, oscillation of chandeliers, stopping of clocks, visible agitation of trees and shrubs, and many startled persons leaving their dwellings".
The disadvantage of the Mercalli Scale is that, unlike the Richter, it does not provide a unique figure for a given seismic disturbance; the "strength" will vary depending on the distance of the epicentre from the observation point. To put it another way, an earthquake may have an infinite number of intensities, but has only one magnitude.