ERRORS. In all observations, E. must be made. The best instruments have imper fections; and .ao man, however equable his temperament, can always rely on his making :a proper use of his senses. As in astronomy numerical correctness in the results of instrumental measurements is of the first consequence, it is the constant care of the .observer to detect and make allowance for errors. The three principal sources from which they may aaise are—lst, External or incidental causes, such as fluctuations of weather, which disturb the amount of refraction; changes of temperature, affecting the form and position of instruments, etc..; 2t1, E. of obSertation. being .such as arise from inexpertness, defective vision, slowness in seizing the exact instant of an occurrence,. atmospheric indistinctness, etc. ; and such E. as arise from slips in clamping and momentary derangements of the instrument; 3d, Instrumental defects, owing to E. in workmanship, and such as arise from the instrument not being properly placed—called E. of adjustment. The first two classes of E., so far as they cannot be reduced to
known laws, vitiate the results of observations to their full extent; but being accidental, they necessarily sometimes diminish and sometimes increase them. Hence, by taking numerous observations under various circumstances, and by taking the mean or average of the results obtained, these E. may be made to destroy one another to a great extent, and so far may be subdued. With regard to the third class, it is the peculiarity of astronomical observations to be the ultimate means of detection of all defects of work manship and adjustment in instruments, which by their minuteness elude every other mode of detection. See sir John Herschel's Outlines of Astronomy, s. 138 et seq. It may be mentioned, however, that the method of subduing E. of the first two classes by the law of average is not applicable in all cases. In certain cases, recourse must be had to what is known as the method of least squares. See SQUARES, METHOD OF LEAST; see also PROBABILITY.