It is interesting to note, however, that the two degrees were found of unequal lengths, suggesting that the earth is not a perfect sphere--a suggestion the validity of which was not to be put to the test of conclusive measurements until about the close of the eighteenth century.

The Arab measurement was made in the time of Caliph Abdallah al-Mamun, the son of the famous Harun-al-Rashid.

Both father and son were famous for their interest in science.

Harun-al-Rashid was, it will be recalled, the friend of Charlemagne.

It is said that he sent that ruler, as a token of friendship, a marvellous clock which let fall a metal ball to mark the hours.