132 Professor Osborne Reynolds [June 2, 



Amongst the ordinary phenomena of motion there are many 

 which render evident the internal motion of fluids. Small objects 

 suspended in the fluid are important, and that their importance has 

 long been recognised is shown by the proverb — straws show which 

 way the wind blows. Bubbles in water, smoke and clouds, afford the 

 most striking phenomena, and it is doubtless these that have furnished 

 philosophers with such clues as they have had. But the indications 

 furnished by these phenomena are imperfect, and, what is more im- 

 portant, they only occur casually, and in general only under circum- 

 stances of such extreme complexity that any deduction as to the 

 elementary motions involved is impossible. They afford indication 

 of commotion, and perhaps of the general direction in which the com- 

 motion is tending, but this is about all. 



For example, the different types of clouds ; these have always been 

 notioed and are all named. And it is certain that each type of clouds 

 is an indication of a particular type of motion in the air; but no 

 deductions as to what definite manner of motion is indicated by each 

 type of cloud have ever been published. 



Before this can be done it is necessary to reverse the problem, 

 and find to what particular type of cloud a particular manner of 

 motion would give rise. Now a cloud, as we see it, does not directly 

 indicate the internal motion of which it is the result. As we look at 

 clouds, it is not in general their motion that we notice, but their 

 figure. It is hard to see that this figure changes while we are 

 watching a cloud, though such a change is continually going on, but 

 is apparently very slow on account of the great distance of the cloud 

 and its great size. However, types of clouds are determined by their 

 figure, not by their motion. Now what their figure shows is not 

 motion, but is the history or result of the motion of particular strata of 

 the air in and through surrounding strata. Hence, to interpret the 

 figures of the clouds we must study the changes in shape of fluid 

 masses, surrounded by fluid, which result from particular motions. 



The ideal in the method of colour bands is to render streaks or 

 lines in definite position in the fluid visible, without in any way 

 otherwise interfering with these properties as part of the homogeneous 

 fluid. If we could by a wish create coloured lines in the water 

 these would be ideal colour bands. We cannot do this, nor can we 

 exactly paint Hues in the air or water. 



I take this ladle full of highly coloured water, lower it slowly into 

 the surface of the surrounding water till that within is level with 

 that without ; then turn the ladle carefully round the coloured water ; 

 the mass of coloured water will remain where placed. 



I distribute the colour slowly. It does not mix with the clear 

 water, and although the lines are irregular they stand out very beauti- 

 fully. Their edges are sharp here. But in this large sphere, which 

 was coloured before the lecture, although the coloured lines have 

 generally kept their places, they have, as it were, swollen out and 

 become merged in the surrounding water in consequence of molecular 



