92 Scientific Proceedings, foyal Dublin Society. 
times which light must have to enable it to reach us from the dis- 
tances represented by a unit in each of the indicated parts of the 
table. The information there recorded may be supplemented by 
that added in fig. 5 on the preceding page. 
On THE MEASUREMENT OF TIME. 
The same table may be employed for measuring time. In- 
tervals of time for the purposes of physical inquiry are best 
measured by the distances over which light in the open ether 
would travel in those periods. In this way measures of distance 
become measures of duration upon that scale upon which a metro- 
eight (which is the same as the centimo-ten) represents one-third 
of a second—a scale which in practice is found to be very con- 
venient, especially for the study of molecular physics. To represent 
a second of time on the diagram, insert the digit 3 instead of the 
eipher which occupies the middle place in the planetary group of 
positions. In this way of measuring time 300 metres of time 
(1000 feet)’ is the same as the millionth of one second. 
Or Moxecutar EVENts. 
In molecular physics, the periods of time which have to be 
dealt with are almost inconceivably shorter than any to which we 
are accustomed. ‘The unit of time which the present writer has 
found the most generally convenient is the micron of time—the 
time which light takes to advance one micron forward in the open 
ether. It is the hundredth part of the jot (or fourth-metret of 
time), which unit he found it convenient to use in his memoir on 
the production of double and multiple lines in spectra by pertur- 
bating forces acting on the electrons. (See Sc. Trans. R. D.S., 
vol. iv., p. 565.) 
1'That is, 1000 metric feet. In Science, the yard of 9 decims, the foot of 3 decims, 
and the inch of 25 millims should always be used instead of the so-called ‘‘ imperial’’ 
measures of the same names. Here the old or imperial measures are to the new or 
metric measures in the ratio of 101-6 to 100, or in the ratio of 633 to 623, or in the 
ratio of 127 to 125. It may be useful to point out that Lathes and Dividing Engines 
provided with Whitworth screws, the pitch of which is known in imperial inches, may 
be made to produce screws or graduate scales in the metric measures, by simply 
introducing two change-wheels, one with 127 and the other with 125 teeth. 
