M. Coulomb, who had great experience in making such observations, cautions those who may repeat his experiments against being deceived by such circumstances: "Je prie" (says he) "ceux qui voudront les repeter, s'ils n'ont pas le temps de mesurer les resultats après plusieurs jours d'un travail continu, d'observer les ouvriers à différentes reprises dans la journée, sans qu'ils sachent qu'ils sont observés. L'on ne peut trop avertir combien l'on risque de se tromper en calculant, soit la vitesse, soit le temps effectif du travail, d'après une observation de quelques minutes."—Mémoires de l'Institut. Tom. II. p. 247. It frequently happens, that in a series of answers to such questions, there are some which, although given directly, may also be deduced by a short calculation from others that are given or known; and advantage should always be taken of these verifications, in order to confirm the accuracy of the statements; or, in case they are discordant, to correct the apparent anomalies. In putting lists of questions into the hands of a person undertaking to give information upon any subject, it is in some cases desirable to have an estimate of the soundness of his judgment. The questions can frequently be so shaped, that some of them may indirectly depend on others; and one or two may be inserted whose answers can be obtained by other methods: nor is this process without its advantages in enabling us to determine the value of our own judgment. The habit of forming an estimate of the magnitude of any object or the frequency of any occurrence, immediately previous to our applying to it measure or number, tends materially to fix the attention and to improve the judgment.
