It has been found that the " carbons " in drills can safely be subjected to a pressure of over 60 kilograms per square millimetre, and a speed of 25 metres per second.
As a rule flies are of small or moderate size, and many, such as certain blood-sucking midges of the genus Ceratopogon, are even minute; as extremes of size may be mentioned a common British midge, Ceratopogon varius, the female of which measures only 14 millimetre, and the gigantic Mydaidae of Central and South America as well as certain Australian robber-flies, which have a body 1-11n.
They range from a millimetre or so (smaller species of Aeolosoma) to 6 ft.
With a period of 18 seconds, and the record-receiving paper at a distance of about 15 ft., a deflection of I millimetre of the light spot may indicate a tilting of AD part of a second of arc, or I in.
37); the opposed faces of these plugs are perfectly smooth, and are placed within a millimetre of each other.