Measurement of Time. 369 



circle of the hand-screw, gives the time-interval between 

 spark and spark. This is the "space-measurement"; and 

 the "uniformity 5 ' was most emphatically expressed in a 

 letter of Siemens and Halske accompanying the apparatus, 

 to this effect : If you wish to obtain satisfactory results, do 

 not start the sparks at once but only after the drum was 

 already spinning for a good while, a prescription, no doubt, 

 based upon the makers' dynamical knowledge of the driving 

 machinery, but at any rate a direct appeal to what had to be 

 trusted to be uniform beforehand, without in this case the 

 least possibility of checking the uniformity by investigating, 

 as Dr. Campbell says in his concluding paragraph, whether 

 the " body covered equal distances in equal times/' the 

 " equality" of these minute intervals being in this case not 

 otherwise actually definable, unless one appealed to yet 

 another uniformity, viz. that of the propagation of electro- 

 magnetic waves along the wire-systems*. I have dwelt 

 upon this example, not only because it shows the two prin- 

 ciples in their neatest form, but also because in writing my 

 first paper on the time-scale, I had this spark-chronograph 

 incessantly in my mind. The same remarks can literally be 

 repeated with regard to all the familiar devices in which the 

 drum is replaced by a light rotating mirror used as reflector. 

 Very minute time-intervals are thus being measured and 

 give well consistent results. 



Bat intervals still much shorter, the periods of light- 

 oscillations, are measured again on the same principles. 

 The propagation of light is declared to be uniform, and then 

 linear segments (translated more or less indirectly on a 

 magnified scale into an interference pattern) associated with 

 this propagation are measured in the Euclidean fashion. 

 And there is even an ever-growing a priori confidence into 

 the uniformity of light-propagation and a tendency to make 

 it the highest court of appeal for all properly mechanical 

 uniformities. 



In short, every precise chronometry is kinematical (motion 

 of bodies or propagation of light), and the foremost concept 

 of kinematics is that of "uniform motion, " exactly so as is 

 that of "straight line " in geometry. Both are, theoretically, 

 undefined terms, and in application things are declared to 

 he good approximate samples of either or pointed out (with 

 the finger, as it were) — this or that is uniform, this or that 



* The interval between the sparks was, in the application of the 

 chronograph I have in mind, due to the difference of the two corresponding 

 circuits, one very short and the other about 3 km. long. 



Phil. Mag. S. 6. Vol. 39. No. 231. March 1920. 2 B 



