226 



KNOWLEDGE 



[December 1, 1891. 



carbons, whicli are amongst the best, may be mentioned. 

 These carbons are made from tlie following ingredients : 

 15 parts of jiurc very finely powdered coke, 5 parts of 

 calcined lamp l)laek, with 7 to 8 parts of a syrup made 

 from cane sugar and gum. This mixture is made into a 

 paste with water, pressed, and then forced through a die 

 which gives to the rods the form and size required ; after- 

 wards they are repeatedly baked at a high tempei-ature. 

 After one bakmg the rods are plunged into a hot and 

 concentrated syrup of sugar, in which they are left some 

 time, taken out and again immersed until they are 

 thoroughly saturated with the liquid. After washing and 

 drying, a similar process is gone through again until the 

 carbons are as dense as is required. They are then dried 

 for a prolonged period in stoves, the result being an 

 excellent and tough form of carbon. 



During the working of the lamp the two carbon rods 

 gradually consume away, and the mechanism of the various 

 forms of arc lamp is contrived to bring up and keep the 

 carbon poles at the requisite distance from each other, so 

 that a steady and unvarying arc may continue between 

 them. As the positive carbon, as has been said, con- 

 sumes away twice as fast as the other, it must be moved 

 up towards the negative carbon with double its speed, 

 and the distance between the carbon points must be kept 

 approximately uniform. There are various devices in use 

 for attaining this object, and without going into complete 

 details, one of the forms of Crompton's lamp may be 

 taken as a specimen. This is one of the examples which 

 gives the best results and avoids the flickering, which is 

 apt to be the great fault of arc lamps. In Crompton's 

 apparatus, here described (which is one of his earlier forms, 

 but may serve as an illustration of the general methods 

 adopted), the weight of the upper carbon and its holder 

 acts as a motive power, which sets a tram of wheelwork in 

 action, and causes the carbons to move towards each other 

 in proportion as they are worn away by disintegration of 

 the carbon points. The operation of " striking the arc " 

 is the following : — The upper carbon descends through its 

 own weight, touches the lower carbon, and the current 

 passes through the circuit. In the circuit is an electro- 

 magnet, and, as the current passes round it, this imme- 

 diately attracts a plate of metal which has a spring 

 attached to it, and this causes the train of clockwork, 

 which was set in motion by the falling upper carbon, to 

 set off in a reverse direction and raise the carbon again to 

 a certain distance, thus forming the arc. When the arc 

 has reached a suitable length and the current attained a 

 certain strength, the spring attached to the armature of the 

 electro magnet presses against a wheel in the clockwork and 

 stops its motion. When the current becomes weaker owing 

 to lengthening of the arc the electro-magnet also de- 

 creases m strength, and a spiral spring which is attached 

 to its armature causes it to rise and leave the magnet, and 

 this causes the stoppage of the clockwork and allows the 

 carbons to come nearer together. As the length of the 

 arc is constantly tending to increase as the carbons wear 

 away, the clockwork is rarely at rest for more than a 

 second or two at a time, but the ingenious device of the 

 electro-magnet with its armature regulating the motion 

 of the wheelwork insures automatic adjustment, and 

 thus produces a satisfactory degree of steadiness. The 

 regulation is self-acting, for the same current which 

 produces the arc circulates round the coils of the electro- 

 magnet, and the variations in the arc and the controlling 

 magnet occur together. In other forms of regulating 

 mechanism which difl'er in detail, the electro-magnet is 

 placed in a shunt circuit, so that when the arc becomes too 

 long and its resistance increases, more of the current 



goes round the shunt, strengthening the electro-magnet 

 and causing it to act so as to produce the approach of 

 the carbons. 



Arc lamps are most suited for out-of-door illumination 

 and for lighting wide areas. For indoor purposes the 

 incandescent lamps are much preferable. The extent 

 lit by these is more circumscribed and the light less 

 intense, which is an advantage when they are to be used 

 in houses. The ai>plications of arc lamps to various 

 purposes other than street-lighting are very numerous and 

 increasingly so. Their use for lighthouse purposes, where 

 the intensity and concentration of their light, as well as 

 its space-penetrating power, renders them valuable, as 

 also their application to photography, in which their rich- 

 ness in the chemical rays is very useful, may be mentioned. 

 It is worthy of note that the immediate source of the light, 

 viz., incandescent particles of carbon, is the same whether 

 candles, oil-lamps, gas, or ' electric arc lamps are the 

 means of illumination. 



ON HUMAN PEDICULI.-I. 



Uy E. A. Butler. 



IF occasional parasites, such as fleas and bugs — crea- 

 tures which simply visit our bodies at intervals, and 

 spend only a small proportion of their lives actually 

 on our persons — excite repugnance and disgust, what 

 can be said of the feelings with which we contem- 

 plate those hideous pests that make men's bodies their 

 life-long home, born and bred thereon, generation after 

 generation, living there and there alone, and, as units of 

 life, almost, if not entirely unknown, apart from such 

 association ? And yet, though cleanly people nowadays 

 hold them in such utter abhorrence that they can hardly 

 be named in polite society, they were not always objects of 

 loathing and disgust. In former times people were more 

 inclined to joke about them than to shudder at them, and 

 some, it is said, even went so far as to be proud of their 

 guests. In Hooke's " Miorographia," which, as we have 

 already seen, was written some 230 years ago, there is a 

 brief account of the head-louse, accompanied with an 

 enormous figm-e representing a specimen magnified to the 

 length of nearly two feet. Hooke introduces his descrip- 

 tion with the following highly suggestive passage : — 

 " This is a creature so officious, that 'twill be known to 

 every one at one time or other, so busie, and so impudent, 

 that it will be intruding itself in everyones company, and 

 so proud and aspiring withall that it fears not to trample 

 on the best, and affects notliing so much as a Crown ; 

 feeds and Uves very high, and that makes it so saucy, as 

 to pull any one by the ears that comes in its way, and will 

 never be quiet till it has drawn blood." Whatever we 

 may think of the good taste of this passage, and the quaint 

 conceit it contains, it is evident that personal cleanlmess 

 was not considered in the days of the Stuarts a matter of 

 such vital importance as it has come to be regarded by 

 respectable society in the Victorian era, and visions of the 

 shady side of domestic hfe in the time of the " Merry 

 Monarch " are called up, which it is as well to draw a 

 veil over. 



Man is not exceiJtional amongst mammals in harbouring 

 these vermin, he is but in the same category with the rest ; 

 for it seems to be the rule, from elephant to mouse, 

 largest to least, that some member of this group of 

 parasites should be attached to each species, and even 

 aquatic mammals, such as the seal and walrus, do not 

 escape their attacks. But, just as the human flea is not 



