The topic herein presented had its origin in some work preliminary to another investigation, requiring the use of a sensitive balance operated with the greatest possible degree of precision. The balance used is by Sartorius, with 18 cm. beam and sensibility of about 2.3 scale divisions per mgr., pans empty, and so constructed that the sensibility increases slightly with moderate increase of load. The writer was studying this balance with a view to getting acquainted with its peculiarities, determining its constants and learning what degree of precision to expect of it (as should always be done with a new balance about to be used in particular work), and had taken a series of consecutive zero readings by the oscillating pointer method with pans empty, when a marked progressive change in the readings attracted immediate attention and demanded explanation
