158 CLASSICS OF MODERN SCIENCE 



A sponge filled with water, and hung by a thread in the middle of 

 a room filled with dry air, communicates its moisture to the air, it is 

 true, but soon the water evaporates and the sponge can no longer give 

 out moisture. On the contrary, a bell sounds without interruption 

 when it is struck, and gives out its sound as often as we please without 

 the slightest perceptible loss. Moisture is a substance; sound is not. 



It is well known that two hard bodies, if rubbed together, produce 

 much heat. Can they continue to produce it without finally becoming 

 exhausted? Let the result of experiment decide this question. 



It would be too tedious to describe here in detail all the experi- 

 ments which I undertook with a view of answering in a decisive man- 

 ner this important and disputed question. They may be found in my 

 memoir, "On the Source of Heat excited by Friction." I have had it 

 printed in the Philosophical Transactions for the year 1798; still these 

 experiments bear too close a relation to my later researches on heat for 

 me to omit attempting at least to give the reader a clear idea of the 

 experiments and of their results. 



The apparatus which I used in these investigations is too compli- 

 cated to be represented in this place ; still it will not be difficult for the 

 reader to form a conception of the principal experiments and their 

 results. 



Let A be the vertical section of a brass rod which is an inch in 

 diameter and is fastened in an upright position on a stout block, B ; it 

 is provided at its upper end with a massive hemisphere of the same 

 metal, three and a half inches in diameter. C is a similar rod, like- 

 wise vertical, to the lower end of which is fastened a similar hemi- 

 sphere. Both hemispheres must fit each other in such a way that both 

 the rods stand in a perfectly straight vertical line. 



D is the vertical section of a globular metallic vessel twelve inches 

 in diameter, which is provided with a cylindrical neck three inches long 

 and three and three-quarter inches in diameter. The rod A goes 

 through a hole in the bottom of the vessel, is soldered into the vessel, 

 and serves as a support to keep it in its proper position. 



The centre of the ball, made up of the two hemispheres which lie 

 the one upon the other, is in the centre of the globular vessel, so that, 

 if the vessel is filled with water, the water covers the ball as well as a 

 part of each of the brass rods. 



