458 Dr. G. Johnstone Stoney : Survey of that part of 



metre, using this word as an abbreviation for " tenthet of a 

 metre "; just as we may say half-ounce or quarter-inch *. 



Maxwell's Determination. 



In the year 1860 the late Professor Clerk Maxwell published 

 the first determination made by man of any actual molecular 

 interval f. The principles upon which he proceeded may be 

 described as follows : — In accordance with the Kinetic Theory 

 of Gas, a gas consists of an enormous swarm of little missiles, 

 all alike in each kind of gas, though differing from one gas to 

 another. These molecules dart about among one another with 

 almost incredible activity, and are, to use Maxwell's simile, 

 like the individuals of a swarm of bees which furiously make 

 short flights in every direction, while the swarm as a whole is 

 either stationary or quietly sailing along. In a gas each 

 molecule dashes forward in an almost J straight line till it 



* It is as necessary to be able to write the quantities we Lave to deal 

 ■with in some convenient form, as it is to be able to describe them briefly. 

 The usual plan is to employ positive and negative powers of 10 to express 

 decimal multiples and submultiples. Another contrivance is to repre- 

 sent them by Roman numerals in the way indicated by the following 

 examples :— 



As specimens of decimal multiples, let XVI (a uno-sixteen) mean 

 1 followed by sixteen ciphers, and let 4 VII (four uno-sevens) mean 

 4 followed by seven ciphers. In multiples the Roman numeral indicates 

 the number of ciphers. 



Similarly, to represent submultiples, let VIII 1 (an eighthet) be used as 

 the symbol for a unit in the eighth place of decimals, and let 6 XIIF (six 

 thirteenthets) mean 6 in the thirteenth place of decimals. In sub- 

 multiples the Roman numeral indicates the decimal place. 



In manuscript it is more convenient to employ a little curved line, the 

 left-hand half of the letter o, instead of the letter t, which has been used 

 in the last paragraph for the convenience of the printer. The small 

 curved line is easily written, and it is appropriate, as it is the symbol iu 

 Pitman's Phonography for the group of letters tht, or thet. 



We may extend the same convention so as to write in a condensed form 

 multiples and submultiples of the metre, &c. Thus mXVI, 15 mX, 

 IX' m, and 7 VHP m will mean a metro-sixteen, fifteen metro-tens, a 

 ninthet-metre (or ninth-metret), and seven eighthet-metres (or seven 

 eighth-metrets). 



When once we have got accustomed to this use of the Roman 

 numerals, they will be found to work more conveniently than the 

 positive and negative powers of 10, which are usually employed. 



t Phil. Mag. for 1860, vol. xxi. p. 19, and vol. xx. p. 21. 



\ The gravitation of the molecules towards the Earth must bend the 

 free paths, but the curvature is insensible until, near the boundary of the 

 atmosphere, the attenuation of the air far exceeds any that can be reached 

 in artificial vacua. This bending of the free paths keeps the atmosphere 

 that accompanies the earth from extending outwards beyond a short 

 distance. It moreover makes the denser constituents of an atmosphere 

 come to an end sooner than the lighter constituents, so that in the upper 



