[96] PROFESSOR STOKES, ON THE EFFECT OF THE INTERNAL FRICTION 



Section II. 

 Suggestions with reference to future experiments. 



80. I am well aware that the mere proposal of experiments does not generally form a 

 subject fit to be brought before the notice of a scientific society. Nevertheless, as it frequently 

 happens in the division of labour that one person attends more to the theoretical, another to 

 the experimental investigation of some branch of science, it is not always useless for the theo- 

 rist to point out the nature of the information which it would be most important to obtain 

 from experiment. I hope, therefore, that I may be permitted to offer a few hints with refer- 

 ence to experiments in which the theory of the internal friction of fluids is concerned. 

 I shall omit all details, since they would properly come in connexion with the experi- 

 ments. 



Experiments with which the theory of internal friction in fluids has more or less to do may 

 be performed for either of the following objects : first, to test still further the truth of the 

 theory ; secondly, to determine the index of friction of various gases, liquids, or solutions ; to 

 investigate the dependance of the index of friction of a gas on its state of pressure, tempera- 

 ture, and moisture ; or to endeavour to make out the law according to which the index of fric- 

 tion of a mixture of gases depends upon the indices of friction of the separate gases ; thirdly, 

 to measure the length of the seconds' pendulum, or its variation from one part of the earth's 

 surface to another. 



81. First object. The theory has been already put to a pretty severe test by means of the 

 experiments of Baily and others. Nevertheless there are some uncertainties in the comparison of 

 theory and experiment arising from the influence of modifying causes of which the effect could 

 only be estimated from theory, and yet was not so small as to be merged in errors of obser- 

 vation. Moreover, experiments on the decrement of the arc of vibration are almost wholly 

 wanting. The following system of pendulums, meant to be swung in air and in vacuum, would 

 afford a very good test of the theory. 



No. 1. A 2-inch or 1^-inch sphere swung with a fine wire. 



No. 2. A very small sphere swung with the same kind of wire. 



No. 3. A long cylindrical rod, a few tenths of an inch in diameter. 



No. 4. A cylinder only three or four inches long, of the same diameter as No. 3, swung 

 with the same kind of wire as No. 1. 



The vacuum tube ought to be of sufficient size to render the estimated correction for con- 

 fined space less than, or at most comparable with, errors of observation. The vacuum appa- 

 ratus used by Col. Sabine would do very well. If the vacuum tube be not of sufficient 

 size, it ought to admit of removal, and to be removed when the pendulums are swung in 

 air. 



In all the experiments the arc of oscillation ought to be carefully observed several times 

 during the motion, the observation of the arc being quite as important for the purposes of 



