12 UNDULATORY FORCES. [INTRODUCTORY. 



With respect to Electricity, we shall, after discussing the laws of that force as obtained either by friction or 

 by chemical action, proceed to the consideration of Lightning Conductors ; the effects of Free Electricity on the 

 Animal and Vegetable Kingdom ; the Production of Minerals, Ores, and Gems ; that instructive and interesting 

 process called Electrotyping, <kc. ; and allied matters which we cannot now stop to mention. 



In the applications of Magnetism the Mariner's Compass, the Action of Iron and of Iron Vessels thereon ; 

 Electro-Magnetism as a motive power ; the construction of the Electric Telegraph ; and those beautiful phenomena 

 which have been brought to light by the apparatus invented by M. Ruhmkorf, will engage our attention. 



We propose introducing, as an appendix to our discussion of the Undulatory Forces, a treatise on Acoustics, or 

 the Laws of Sound. This may, at first sight, appear incongruous to those who are unaware of the analogy which 

 exists between the four forces we have hitherto spoken of, and the effects produced by vibrations of the air. This 

 difficulty will, however, immediately disappear, when we reflect that, as sound is certainly caused by undulations in the 

 atmosphere, and as we can measure the lengths of these with grteat accuracy, it follows that sound actually becomes 

 an explanatory key of all the phenomena which may arise from the undulations of a supposed ether. Indeed, the 

 reader may instantly combine his accurate knowledge of the laws of atmospheric vibrations, with theoretical ideas of 

 those produced in ether. We may, with advantage, dwell for a short time on their analogy. If a string be tightly 

 stretched between two points, as in the violin and harp, and be made to vibrate, the sound produced is propagated 

 by the vibration producing others in the surrounding air, just as successive waves are produced on casting a pebble 

 into a still pond of water. 



Again, if the length or tension of the string is varied, the sound produced will be proportionally altered from a 

 bass to a treble. Transferring our attention to light, we equally assume that a vibration produces that force ; and if 

 we increase the number of these vibrations in a given space, the intensity and nature of the light produced is at 

 once affected. The analogy between sound and light is therefore undeniable. 



But we may go still farther : for if we cause a set of sonorous vibrations to cross each other, their sound either 

 is doubled, or entirely disappears ; just as two waves of water, when coming in contact, either increase in size, or are 

 destroyed. So with light, we find the interference of rays, or fits of transmission and reflection, produce analogous 

 effects, which, by the aid of modern research, may be measured with great accuracy 



In connection with Acoustics, we propose to deal somewhat extendedly with the construction of several kinds 

 of musical instruments, so far as they may relate to scientific laws. Some interesting applications of the Con- 

 duction of Sound will be fully noticed. The human voice, and that of animals, will also be treated on. 



We have thus attempted to give a general epitome of the leading phenomena of what are called the Undulatory 

 Forces ; and we now proceed to discuss them in detail Our arrangement will be, first, to investigate the abstract 

 laws of each force ; secondly, to point out and enter into the details of its applications ; and, lastly, we shall furnish 

 copious Tables, which will be equally valuable to the savant, the practical man, and the general reader. 



Wo may be permitted to offer a word of advice to those who may use our pages without the aid of a com- 

 petent instructor. Whilst we have endeavoured to be as plain and as concise as possible, still we cannot conceal 

 the fact that difficulties will necessarily arise to those who have not had the advantage of a course of -scientific 

 training. On such wo urge, that they leave no subject until they have mastered its details. If they do not pursue 

 this course, they can only be compared to a general who, having passed through a hostile country, leaving behind 

 him hosts of minor foes that he may press on to give battle to the powerful one, will, on his return, find that the 

 petty enemies he had despised have combined against him, and are more to be dreaded than the jfpparently stronger 

 foe. A general and superficial study of the laws of nature may give a smattering of knowledge, but can never afford 

 solid pleasure. Of those who adopt that course, it may be safely said 



" A little learning is a dangerous thing." 



Such persons are always foremost in the rank of inventors : they strike out sparks of light ; but the genial benefit of 

 sound advantage is never a product of their career. 



Those, however, who will patiently and perseveringly devote a modicum of intelligence to Studying Nature, do 

 not only derive great personal advantage, intellectually and pecuniarily, but they form a class of men on whom the 

 world may reckon as its benefactors; and of whom it may be said, as of a Watt or of a Stephenson - 



"Si monumentum queris, circumspice." 



The following is the order in which we shall proceed in treating on the Undulatory Forces : 



1st. HEAT AND ITS APPLICATIONS. 

 2nd. LIGHT ,, ,, 



3rd. ELECTRICITY ,, 

 4th. MAGNETISM ,, ,, 

 6th. ACOUSTICS 



