The use of Trigonometric Series was first suggested by Daniel Bernouilli in 1753 in his researches on the musical vibrations Of stretched elastic strings, although Bessel's Func tions had been already (1732) employed by him and by Euler in dealing with the-vibrations of a heavy string suspended from one end; and Zonal and Spherical Harmonics were introduced by Legendre and Laplace in 1782 in dealing with the attrac tion Of solids Of revolution.