Personal Errors. These are. Errors characteristic of the individual observer. If we swing a pendulum before a class of students and ask them to indicate the times of greatest displacement by tapping with their pencils, the result will well illustrate the personal equation. In the experiment in wireless telegraphy by which time messages were sent from Paris to Arlington, each observer was carefully rated in order that this error should be guarded against. Mistakes are, unfortunately, too common in students' laboratory work to call for an extended explanation. Errors in reading scales, in computation, and in tabulation may be classed as mistakes, and these should become less and less frequent as the student gains experience.