compared ivith the Motions of Waves of Light. 139 



wave-length of the ray D. From this value and from the mean 

 velocity* of the molecule, it follows that its path is deflected 

 about 



7 unit-nines of times in a second, 



and accordingly that the mean duration of each of its little rec- 

 tilinear excursions is about 



14 eleventh-seconds, — 



meaning by a unit-nine a unit multiplied by 10 9 , and by an 

 eleventh-second a second of time divided by 10 11 . 



This fragment of time, tiny as it is, is nevertheless more thau 

 50,000 times the vastly shorter period that suffices for a double 

 vibration of red light, and than 100,000 times the duration of a 

 double vibration of the extreme violet ray. And as the periods 

 of all the motions within a molecule which give rise to visible 



0-000,008,56 and 0-000,029,4. They may be further abbreviated into 

 8-56 Vlth and 29'4 Vlth. 



A similar and equally convenient nomenclature for multiples is found in 

 the use of affixes. Thus the decimal multiples of the metre are — 



the decametre, 



the hectometre, 



the kilometre, 



the metre-four, metre IV, or m. IV, 



the metre-five, metre V, or m. V, 



and so on ; 



a metre-five meaning, of course, a metre multiplied by 10 5 , 



Thus the mean distance of the sun is 14-/4 metre-tens, which is equiva- 

 lent to 147,400,000,000 metres. The distances of the fixed stars are to be 

 measured in metre- sixteens, the distance corresponding to a parallax of 1" 

 being 3-04 m. XVI, which means the same as 30,400,000,000,000 kilo- 

 metres. Light takes about T06 year to travel a metre-sixteen, using 

 Foucault's determination of the velocity of light, viz. 29'8 m. VII. (metre- 

 sevens) per second. 



So, again, using the term ohmad to designate the absolute electromag- 

 netic unit of electrical resistance referred to the metre and second, the B.A. 

 unit is the ohmad-se^en, or ohmad multiplied by 10'. Resistances will be 

 conveniently measured in ohmad-sevens or ohmad-nines, insulation in 

 ohmad-XIII or ohmad-XV. If this nomenclature were adopted, it would 

 keep constantly in view how the measures we may in practice find it most 

 convenient to use are related to one another and to the absolute unit. 



So, again, high numbers may be as conveniently dealt with as ordinary 

 numbers. Thus the eye receives per second 5 unit-fourteens, or 5 .XIV, 

 of waves of yellow light. This number is the same as 5 x 10 u . 



It is an advantage of this nomenclature that it is equally adapted to any 

 language, and is equally intelligible in all. 



* The mean velocity for this purpose ought to be the arithmetic mean of 

 the values of v, which would be somewhat less than the mean given above ; 

 but the ratio of the two means is such (nearly 11 to 12 according to Max- 

 well, see Phil. Mag. 1860, vol. xix. p. 23) that to use one for the other can- 

 not prejudice an inquiry like the present, which does not admit of or 

 attempt exactness. 



