558 REPOKT— 1888. 



Dublin, will long miss as a lofty example of the highest intellectual keenness and 

 honesty, and mourn as the truest-hearted friend, full of sympathy and Christian 

 charity. In 1875 Professor Stewart gave xis a striking example of the other class 

 of address in a splendid exposition of the subject he did so much to advance, 

 namely, solar physics. He brought together from the two great storehouses of his 

 information and speculation a brilliant store, and displayed them here for the 

 advancement of science. Him, too, all science mourns. Though, from want of 

 personal acquaintance, I am unequal to the task of bringing before you his many 

 abilities and great character, you can each compose a fitting epitaph for this well- 

 known great one of British science. In this connection I am only expressing what 

 we all feel when 1 say how well timed was the royal bounty recently extended to his 

 widow. At the same time the niggardly recognition of science by the public is a dis- 

 grace to the enlightenment of the nineteenth century. What chancellor or general 

 with his tens of thousands has done that for bis country and mankind that Faraday, 

 Darwin, and Pasteur have done ? The ' public ' now are but the children of tliose 

 who murdered Socrates, tolerated the persecution of Galileo, and deserted 

 Columbus. 



In a presidential address on the borderlands of the known delivered from this 

 chair the great Clerk Maxwell spoke of as an undecided question whether electro- 

 magnetic phenomena are due to a dii'ect action at a distance or are due to the 

 action of an intervening medium. The year 1888 will be ever memorable as the 

 year in which this great question has been experimentally decided by Hertz in 

 Germany and, I hope, by others in England. It has been decided in favour of the 

 hypothesis that these actions take place by means of an intervening medium. 

 A.lthough there is nothing new about the question, and although most workers at 

 it have long been jiractically satisfied that electro-magnetic actions are due 

 to an intervening medium, I have thought it wortli while to try and explain to 

 others who may not have considered the problem what the problem is and bow it 

 has been solved. A presidential address such as this is not for specialists ; it is for 

 the whole section ; and I would not have thought of dealing with this subject 

 only that its immediate consequences reach to all the bounds of physical science, 

 and are of interest to all its students. 



We are all familiar with this, that when we do not know all about something 

 there are generally a variety of explanations of what we do know. Whether there 

 is anything of which there are in reality a variety of explanations is a deep ques- 

 tion which some have connected with the Freedom of the Will, but which I am 

 not concerned with here. A notable example of the possibility of a variety of 

 explanations for us is recorded in connection with an incident said to have occurred in 

 the neighbouring town of Clifton, where a remarkable meteorological phenomenon, 

 as it appeared to an observing scientist, was explained by others as a bull's-eye 

 lantern in the hands of Mr. Pickwick. Another kind of example is the old explana- 

 tion of water rising in a pump, that ' Nature abhors a vacuum,' as compared with 

 the modern one. Nowadays, when we know as little about anything, we say, ' It 

 is the property of electricity to attract.' This is really little or no advance on the 

 old form, and is merely a way of stating that we know a fact but not its 

 explanation. There are plenty of cases still where a variety of explanations 

 is possible. For example, we know of no experiment um cruets to decide 

 whether the people I see around me are conscious or are only automata. There 

 are other questions which have existed, but which have been experimentally 

 decided. The most celebrated of these are the questions between the caloric 

 and kinetic theories of heat, and between the emission and undulatory theories 

 of light. The classical experiments by which the case has been decided 

 in favour of the kinetic theory of heat and the undulatory theory of light 

 are some of the most important experiments that have ever been performed. When 

 it was shown that heat disappeared whenever work appeared, and vice versa, and 

 so the caloric hypothesis was disproved ; when it was shown that light was pro- 

 pagated more slowly in a dense medium than in a rare, the sciences of light and 

 heat were revolutionised. Not but that most who studied the subject had 

 given their adhesion to the true theory before it was finally decided in 



