1801 - spent all his free time making astronomical observations. 1806 - aided the Utrecht astronomer Van Beeck Calkoen in his measurement of the longitude difference between Amsterdam and Utrecht. 1809 - attained his 'candidaats' (Bachelor's) at the University of Leiden. 1810 - studied in Paris with, among others, Delambre. 1812 - appointed the successor of Van Beeck Calkoen at the Utrecht Observatory and obtained the chair of mathematics and astronomy. 1815 - became professor of physics. 1821 - made Knight of the Order of the Dutch Lion. 1823 - made measurements of the speed of sound, the results of which are within 0.2% of modern figures. 1835 - was in charge of the Dutch part of an international effort to measure the tides along the coasts of the North Sea.