It could be that we have nowadays a different sensitivity to the subject (we have made a little poll among friends and colleagues, and our impression has been unanimously confirmed), but we find it hard to be rationally convinced by the arguments of the following kind:
Once it has been chosen as base, will either the whole meridian or a sensible part of it be taken as a unit? The wholeness? Out of question! The half, that stretches from one pole to the other, may not be easily conceived by our mind because of the part which is located ``below'', in the other hemisphere. This is not the case of the quarter of the meridian that, on the contrary, can be easily imagined: it stretches from ``one pole to the equator''. In the future it will be said: France opened the divider and pointed it on one pole and the equator, a sentence that will be greatly successful. There is another reason, that is really scientific and supports the meridian: its quarter is the arc intersected by the right angle. That's right: however, why should it be considered as a further advantage? Simply because the right angle is considered as the natural angle, the angle of the vertical and the gravity. It is the unit-angle, the degree is nothing but its subdivision. (Ref. , p. 55 of the Italian translation)What would be the alternatives? As an exercise, we show in table 5 some possible `natural' choices of units of length based on the dimensions of Earth, together with a reasonable decimal sub-multiple as practical unit.
|one radiant along the meridian||1/10000000||64||0.803|
|(same as radius)|
|1 degree of Earth's arc||1/100000||111||1.057|
|1 minute of Earth's arc||1/1000||185||1.367|
|1 second of Earth's arc||1/100||31||0.558|
|Equal to 1 nautical mile, that is 1852m.|
We see from table 5 that the 10000000 part of the quarter of meridian is the closest to the length of the seconds pendulum. So, when the French scientists proposed the new unit of length, we think it is possible, among the many `defensible natural units', they chose the closest to the seconds pendulum. The reason could be a compromise with the strenuous defenders of the seconds pendulum. Or it could have happened that, since they had in mind some `cooperation' between the new unit and ``a pendulum having a determined length'', choosing a unit close to the well studied seconds pendulum would have simplified the intercalibrations.