470 Dr. Gr. Johnstone Stoney : Suroey of that part oj 



insert the digit 3 instead of the cipher 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 Molecular 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 pether. 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 perturbating forces acting on the electrons. 

 (See iSc. Trans. R. D. S. vol. iv. p. 565.) 



One of the conveniences of the proposed way of representing 

 time is its perfect flexibility. In each investigation we may 

 select as our unit of time that of the whole decimal series 

 which happens to be the most convenient to use in the investi- 

 gation. In the above-mentioned inquiry it happened that a 

 relatively large unit was the most convenient. In other 

 inquiries the micron, which is 100 times briefer, is a more 

 convenient unit, and in some few, in which very much smaller 

 periods of time were under consideration, the tenth-metret of 

 time was employed. 



The micron of time is the XIV fc (fourteenthet) of the third 

 of a second, that is, the 300th part of the billionth of a second. 

 To magnify it till it becomes one second of time is the same 

 process as to magnify the fifth part of the thousandth of a 

 second until it becomes 1900 years, i.e. the whole duration of 

 the Christian era. It is instructive to bear this in mind when 

 dealing with molecular events. 



In dealing with molecular events, it is well to conceive a 

 magnified model of what is really going on, in which all 

 lengths are so enlarged, and all times so much prolonged, as 

 to bring both within the range of what we can conveniently 

 perceive. In order to do this, the magnification with respect 



* That is, 1000 metric feet. In Science the yard of 9 decini3, the foot 

 of 3 deciuis, and the inch of 25 millims should always be used instead of 

 the so-called " imperial " measures of the sauie 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 63 J to 62^, 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 kuown in impe.-iil 

 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 12-5 teeth. 



