June 27, 1918J 



NATURE 



327 



be willing- to stand the racket of bringing metric 

 units into practical use. 



Some years ago, before the war, I represented 

 to an authoritative committee that for aeronautical 

 purposes a* dynamical unit of wind velocity was 

 practically essential, and asked for a judgment 

 between metres per second and feet per second, 

 and was told that "feet per second" was the 

 more appropriate unit. The reason has been 

 voiced for me by one of my own staff — that he 

 himself could manage metric units well enough, 

 but the weaker brethren would understand feet 

 per second better ; and for the same reason I find 

 we are drifting back again to miles per hour out 

 of kindness to the less well-informed. The 

 superior person will not look at milliwatts per 

 square centimetre as a unit for solar radiation ; 

 people might think he was unacquainted with the 

 literature of the subject. 



This supposed consideration for the weaker 

 brethren is a mere will-o'-the-wisp. It is the 

 teacher with years of experience who finds it 

 really hard to change his habit. Well-meaning 

 people tell me that atmospheric pressure expressed 

 in millibars has no meaning for beginners, and 

 there is the same tendency to slip back, into 

 inches, because, forsooth, people will understand 

 them better. The supposed simplicity is quite 

 fallacious. The majority of mankind who use 

 barometers do not understand pressure measured 

 in inches ; in fact, they have never thought about 

 pressure at all, but simply about barometric read- 

 ings, which are another matter. From the founda- 

 tion of the Meteorological Department of the 

 Board of Trade in 1854 millions of barometer 

 readings have been reported to the Meteorological 

 Office from the sea, from ships of the Navy and 

 the mercantile marine ; but until reports by wire- 

 less were introduced about ten years ago, so far 

 as I know, not a single barometric pressure — 

 only the raw materials for getting- it. And the 

 result of requiring pressure instead of reading^s 

 in the wireless telegrams was as instructive for 

 the observer as it was for the office. Under the 

 old plan the observer read the barometer ; it was 

 marked in inches, but did not read in inches, of 

 course; he gave the number of the barometer, so 

 that we might look out its index error, and the 

 reading- of the attached thermometer, so that we 

 might get the correction, and some clerk in the 

 office had to make out the pressure. It is to be 

 noted that in meteorology the difference between 

 the first reading and the pressure is not a mere 

 trifle, but sometimes more than the differences of 

 pressure which we map. After these sixty-four years 

 of reading barometers without knowing the mean- 

 ing of what was read, some jolt is necessary to per- 

 suade people to understand what we really mean, 

 not in theory, but in actual working practice, by 

 the pressure of the atmosphere ; and the use of a 

 real pressure unit is by far the best form of jolt. 

 When the Israelites crossed the Jordan they were 

 ordered to set up a pillar of stones so that 

 their posterity might ask the question, "What 

 mean ye by these stones? " And so the question, 

 NO. 2539, VOL. lOl] 



"What mean ye by millibars?" is the first step 

 in the enlightenment of many practical men about 

 the realities of atmospheric pressure in meteoro- 

 logy. And, strange as it may seem, our marine 

 observers, who are practical men, ar'e not in the 

 least unwilling lo regard the innovation in that 

 light. No objection to the change has come from 

 them. What other and more ordinary educational 

 institutions think about it I have never heard ; 

 they have probably not considered it at all. It is, 

 of course, deplorable that there are so few people 

 within reach of any inquirer who can give him 

 the answer to the question that leads to enlighten- 

 ment ; but that is another story. 



So with temperature : the schools and universi- 

 ties use the Centigrade scale and zero. I defy, 

 anybody who is used to the practical convenience> 

 for meteorological purposes of the Fahrenheit 

 scale to face the change to the Centigrade scale 

 and zero, which would flood our tables with nega- 

 tive quantities, without asking himself : " What 

 rheans this zero? " And if he does ask himself the 

 question he must realise that, besides avoiding for 

 ever negative values in meteorological work of every 

 kind, the adoption of the so-called absolute scale 

 sets up a pillar round which much information of 

 the utmost interest and importance is twined. The 

 expansion of gases, the transformation of heat 

 into work, the radiation into open space, all 

 depend "upon the absolute temperature, which, 

 therefore, has some reality about it.- There may 

 be small discrepancies, as Prof. Marvin has 

 pointed out ; but they are not of the first order. 

 In these days, when temperature is of real signifi- 

 cance in all sorts of ways, I cannot imagine how 

 a teacher has the courage to face a pupil Avith a 

 neg-ative temperature, and to go on doing it seems 

 to me to be either inveterate habit or simple ob- 

 scurantism. And yet I find, now that flying- men 

 are beginning to record temperatures in the upper 

 air, they are on the way to g-ive us negative tem- 

 peratures on the Fahrenheit scale because (I sup- 

 pose) they understand them ! To me the practice 

 appears to be based, not on thought, but on 

 simple thoughtlessness. 



There is a time for everything, and the present 

 seems to be the time for a change of units. Take, 

 for example, the decimalising of our coinage. Let 

 it be granted that we cannot change the pound 

 because that would mean translating all the re- 

 corded values from the time of the introduction of 

 the sovereign. To change the shillings and pence 

 into decimals is, from that point of view, a small 

 matter. The stumblings-block has always been 

 the penny, because there were so many hundreds 

 of millions of things sold for a penny that a 

 change in the penny would ruin either the traders 

 or their customers. But now the penny has lost 

 all its significance ; everybody wants it changed. 

 A penny paper is seeking- a new price ; it costs 

 three halfpence or twopence or threepence ; a 

 penny stamp now costs three halfpence. Why 

 does not the Chancellor of the Exchequer put 

 stamps in terms of hundredths of a pound? The 

 opportunity is a golden one for the nation, and 



