ESSAYS 317 



of Princetown, we may obtain a better idea of what it means. The sensation of 

 pain travels along a nerve with a definite speed. If you put your finger on a hot 

 surface, the pain will be felt some little time after the moment of contact. We do 

 not realise this, as the interval is so small, the rate of propagation being roughly 

 100 feet a second. By a calculation which can easily be verified, we find that 

 if a child were born with an arm long enough to reach the sun, it would die of old 

 age long before it could feel that its fingers had been burnt. The distance to the 

 nearest star can be brought home by a slight variant of an illustration used by the 

 late Sir Robert Ball. If we could travel to the sun or the nearest star for the same 

 fare as we travel on a railway — i.e., one penny per mile — the journey to the sun 

 would cost a little less than ,£400,000, but the journey to the nearest star would 

 require a sum sufficient to pay for the cost of the late war two or three times over. 

 (The exact amount would be ^72,000,000,000.) 



Lord Kelvin used to speak scornfully about persons who spoke of such magni- 

 tudes as the distances of the stars as " inconceivably large." To him everything 

 that could be expressed in figures was conceivable, and indeed one's powers of 

 conception are greatly a matter of practice, and of the choice of suitable units 

 or the selection of intermediate standards. Some men may be able to form an 

 image of a thousand men grouped together, but would fail if the number were 

 extended to a million, while this might be easy to the commander of an army. 

 Most of us, especially those who have travelled, can form some idea of the size of 

 the earth. A sphere of 4,000 miles radius is certainly not inconceivably large ; 

 using the earth as an intermediate standard, some assistance is given to a plausible 

 representation of large figures. The number of molecules in a cubic inch of gas 

 at the freezing-point and under atmospheric pressure is 46 followed by 19 zeros, 

 and we may well at first sight look upon such a number as inconceivably large. 

 But let us spread those molecules uniformly over the whole earth, and we shall 

 find only 14 on each square inch. One further example may be given of a quantity 

 which may be made to look either very small or very large according to the 

 manner of expression. Assuming the average annual rainfall over the world to 

 be 50 inches, a quantity which we can easily picture to ourselves, it would mean 

 that in each second of time the average rainfall produces a layer of water over 

 the earth of a thickness a little more than one 1,500,000th of an inch ; an incon- 

 ceivably small figure, some may say. But working out the total weight, we find 

 that over 30,000,000 tons of water falls on the earth in each second of time. 

 Calculate from this the weight of the annual rainfall over the world, and you get an 

 enormous figure to represent the same fact which previously gave us what seemed 

 an inconceivably small number. 



Well-chosen illustrations are frequently helpful in bringing scientific teaching 

 into relationship with every-day life. Nature seldom presents to us problems in 

 their simplest form, and one of the minor worries of a teacher anxious to spread 

 the knowledge of science is that he is confronted with intelligent questions which 

 he is unable to answer, because the questioner does not possess the preliminary 

 knowledge necessary to understand the answer. What is the cause of the blue 

 colour of the sky ? or the red of the sunset ? Why does the high-water in tides 

 occur every twelve hours instead of only once a day ? Why does the barometer 

 fall at the approach of stormy weather ? These are very intelligent questions, but 

 what answer can we give without assuming a considerable previous knowledge of 

 science ? 



To obtain a knowledge of the forces of nature in their simplest form we must 

 have recourse to specially selected experiments, which generally seem to have 



