wallace suggested that she should study the writings of the french mathematician pierre-simon laplace, which summarised the theory of gravity and collected the mathematical results that had been established in the 50 years since "principia" had been published. she said that studying laplace's work gave her the confidence to persevere in her mathematical studies. somerville extended her studies into astronomy, chemistry, geography, microscopy, electricity and magnetism. at the age of 33 she purchased for herself a library of scientific books, including: louis-benjamin francœur's "elements of mechanics", sylvestre françois lacroix' "algebra" and "calculus treatise", jean-baptiste biot's "analytical geometry and astronomy", siméon denis poisson's "treatise on mechanics", joseph-louis lagrange's "theory of analytical functions", leonhard euler's "elements of algebra" and "isoperimetrical problems", alexis clairaut's "figure of the earth", gaspard monge's "application of analysis to geometry", and françois callet's "logarithmus".