146 
POPULAR SCIENCE liEVIEW. 
all directions tlirougli space. For a long period it vras sup- 
posed by all who bowed before the shrine of Xewton, that 
material particles of infinite tenuity were launched forth from 
the Sun_, and travelled at the rate of nearly 200_,000 miles in 
each second of time towards the earth and the planets. But 
noWj in the place of the theory of emission^ we have the theoiy 
of undulations. This view supposes the existence of an infi- 
nitely elastic ethereal medium filling space^ and interpenetra- 
ting all forms of matter. A disturbance originating on the 
Sun sends a wave forth from its shores, the pulsations of 
which are presently felt on the shores of this island. Earth, 
which floats at a distance of 91,328,600 miles in the ethereal 
ocean. According to the rate of these pulses — or, rather, 
according to the vibratory excitation which they produce in 
matter — so the senses recognize heat or light, and either 
chemical or electrical phenomena are manifested. A bell, says 
Mayer, is sounded in the Sun, and its notes are sensible upon 
the Earth and all the planets. Presuming that it will be allowed 
to be, at all events, probable that like effects are the results of 
like causes, and that the development of heat and light on this 
planet presents some analogy to operations which are ever 
active in the Sun, it v/ill elucidate our subject if we briefly 
examine the evidences afforded by the terrestrial (artificial) 
production of heat and light. 
A body at rest shows no increase of temperature. If it is 
screened from the influence of heat radiations, it remains, — in 
ordinary language, — cold. If the mass of matter be set in 
motion, its temperature rises. This is familiarly proved by 
rubbing a metal button on a piece of cloth or on a board, when 
the heat is determined by the friction — resistance to motion — 
to which it is subjected. A body falling through empty space 
— a void — would not acquire heat, because there would be no 
disturbance of its particles ; but if suddenly stopped in its fall 
it would be subjected to a disturbance, equivalent to the velocity 
it had attained, and a certain increase of heat would be the 
result. A rifle-ball in passing through the air, is heated by 
the resistance offered to its passage, and its velocity being 
known, it is easy to calculate how much heat it will acquire in 
its flight. Dr. Joule has determined that the heat generated 
by a body falling through the air is proportional to the height 
through which it falls — that the quantity of heat which would 
raise one pound of water one degree Fahr. in temperature, is 
exactly equal to what would be generated if a pound weight, 
after having fallen through a height of 772 feet, had its moving 
force destroyed by collision with the earth. The application 
of this will by-and-by be seen. 
By mechanical attrition, so large an amount of heat may be 
