Rayleigh's Auciress to the British Association. 91 



escaping gases carry away with them the larger part of the whole 

 heat developed. It was to meet this difficulty that the regene- 

 rative furnace was devised. The products of combustion, before 

 dismissal into the chimney, are caused to pass through piles of 

 loosely stacked fire-brick, to which they give up their heat. After 

 a time the fire-brick upon which the gases first impinge, becomes 

 nearly as hot as the furnace itself. By suitable valves the burnt 

 gases are then diverted through another stack of brickwork, which 

 they heat up in like manner, while the heat stored up in the first 

 stack is utilized to warm the unburnt gas and air on their way to 

 the furnace. In this way almost all the heat developed at a high 

 temperature during the combustion is made available for the work 

 in hand. 



As it is now several years since your presidential chair has 

 been occupied by a professed physicist, it may naturally be 

 expected that I should attempt some record of recent progress in 

 that branch of science, if indeed such a term be applicable. For 

 it is one of the difficulties of the task that subjects as distinct as 

 Mechanics, Electricity, Heat, Optics and Acoustics, to say nothing 

 of Astronomy and Meteorology, are included under Physics. Any 

 ■one of these may well occupy the life-long attention of a man of 

 science, and to be thoroughly conversant with all of them is more 

 than can be expected of any one individual, and is probably 

 incompatible with the devotion of much time and energy to the 

 actual advancement of knowledge. Not that I would complain of 

 the association sanctioned by common parlance ; a sound know- 

 ledge of at least the principles of general Physics is necessary to 

 the cultivation of any department. The predominance of the 

 sense of sight, as the medium of communication with the outer 

 world, brings with it dependence upon the science of Optics ; and 

 there is hardly a branch of science in which the effects of tem- 

 perature have not (often without much success) to be reckoned 

 with. Besides, the neglected borderland between two branches 

 of knowledge is often that which best repays cultivation ; or, to 

 use a metaphor of Maxwell's, the greatest benefits may be derived 

 from a cross fertilization of the sciences. The wealth of materia 

 is an evil only from the point of view of one of whom too much 

 may be expected. Another difficulty incident to the task, which 

 must be faced but cannot be overcome, is that of estimating 



