ESTKODCtTORY.] 



UNDULATORY FORCES. 



11 



By the term "refraction," we mean that the rays of either heat or light are bent out of their straight course when 

 passing from a rare into a dense medium, or vice versa. A very simple illustration of refraction is witnessed when a 

 stick is put obliquely into a vessel containing water. The stick will appear bent, or broken, owing to the rays of 

 light which pass from it being bent or refracted from a straight line. In a similar manner, if light is made to pass 

 through a piece of glass of a triangular shape, such as a prism, the ray is refracted, and seven beautiful colours are 

 produced. A ray of heat may be refracted or broken up by passing it through a prism of rock-crystal ; and although, 

 of course, no colours are visible to the eye of the observer, still effects exactly like those produced by the glass prism 

 on light are at once afforded. 



The effects of polarisation are not so easily understood without a lengthened description, which we shall give in 

 its proper place. We may, however, state, that when a ray of light is passed through, or reflected from, a bundle of 

 glass plates, or a piece of tourmalin, it is divided into two parts, one of which proceeds onwards, and the other is 

 deflected at a certain angle. Our description would be more scientific, if we were to say that one ray is transmitted 

 and the other reflected. The very same result happens to heat, if its rays are treated in a similar manner. 



For these and many other reasons, we may arrive at the conclusion, that heat and light proceed from the same 

 cause, modified by circumstances with which we are at present but slightly acquainted. This assumption, though 

 most likely true, is not based, however, on an unexceptional foundation. 



Each of the forces of which we are now speaking, are termed central or radial, because their effects are observed 

 to extend in straight lines from a centre. As we shall subsequently prove, their intensity diminishes as the square 

 of the distance from the source ; and having any data of the amount of force first excited, its intensity becomes a 

 matter of easy measurement. 



With respect to the speed with which these forces travel, little is known, except in reference to light ; and here 

 we may speak with tolerable certainty. By observing the eclipses of Jupiter's satellites, it is found that their light 

 does not appear so soon after an obscuration as it should do, according to exact astronomical calculations. Now, 

 as the latter are so accurate as not to vary in many instances to the extent of a second, even if the event predicted 

 shall be mouths after being calculated in coming to pass ; and as the distance of Jupiter from the earth, and other 

 data, are well known, it requires little stretch of the imagination to guess, that the error of observation is due 

 to the length of time required for the light of the satellite to reach the earth after it had passed the solid body of 

 its planet. Thus we judge that light travels at the rate of about 200,000 miles per second. To familiarise this to 

 the mind, we may add, that a ray of light would pass round the earth eight times in one second ; that it requires but 

 one second of time for light to pass from the moon to the earth ; and if the sun could suddenly be obscured on the 

 surface of its luminous part, and again exposed, we should not perceive its light, after first losing it, for the space 

 of about eight minutes. 



We have no means of ascertaining the rate at which heat travels, except when associated with light, as in the 

 rays of the sun ; and, so far, it has been judged that heat and light travel at an equal speed. 



Electricity seems to travel at a speed which varies according to the conductor through which it passes. In the 

 earlier works on this subject, an extraordinary rate was assigned ; but recent experiments by no means countenance 

 the idea. There are so many interfering causes, as to leave the best-tried results uncertain. As far as our present 

 knowledge goes, it would seem that, in a conductor 3,000 miles long, the electric effect is observed at its extremity 

 in about a second of time. There is, however, a very singular result perceived when a current of voltaic electricity 

 is sent through a very long wire. Instead of the effect being instantaneous, the delivery of the current is protracted 

 just in proportion to the length of the conductor, after the first sign of the presence of the current is manifested. 

 The electricity seems, in fact, to have lost its instantaneous, and to have acquired a gradual, mode of evidencing its 

 presence. 



Our means of producing magnetic effects are so small, as to preclude the chance of measuring the rate at which 

 they are propagated. The comparatively small distance at which magnetism is perceptible from its sources in our 

 experiments, and the impossibility of our cutting off the effects of terrestrial magnetism, present insuperable 

 difficulties in arriving at a knowledge of the speed of the magnetic force. 



After discussing the different laws which research has opened out to us, it will be our duty to present, in the 

 following pages, some outlines of the applications of wliich the imponderable agents are susceptible, whether in 

 respect to natural occurrences, or for social, economical, and domestic purposes. It will be our endeavour to impress 

 on the minds of our readers the truths of the different branches of Science ; and, presuming that those have been 

 mastered, their varied applications will necessarily become interesting and valuable. And here we have a wide 

 field opened for contemplation. There is scarcely a circumstance, or an object of daily life in our time, but wliich 

 points to Science as having something to do with it. 



Let us, for instance, take each force in the order in which we have hitherto spoken. To Heat we are indebted 

 for our steam-engines, our metallurgical processes, the infinite variety of manufactures which employ the 

 industrial population of our country, and also for the comforts, conveniences, and vital necessities of our social 

 existence. Of some of these applications we shall speak under the head of Steam, and Warming and Ventila- 

 tion. Many others we must defer to our Chemical section. The natural phenomena dependent on heat 

 such as Volcanic Agency, the heat of the Sun, Climate, <fcc. we shall enter into in the subject of Terrestrial Heat, 

 on which we shall have still further to expand when we treat on Meteorology. 



The Applications of Light such as the Cause of Colour, Optical Instruments, Artificial Illumination, Pho- 

 tography, <fec. will immediately follow our consideration of the Laws of Optics ; and thus what has been by many 

 considered to be a comparatively difficult subject, will be found to present a large amount of interesting and useful 

 matter to the general reader, and valuable results to the practical man. 



