817 



LOCI. 



LOCK. 



318 



without its attendant magnetism acting at right angles to the direction 

 of such current, it follows that totally unexpected and most intricate 

 and untraceable influences may be set in operation, even by lighting the 

 furnace fires for a few hours in order to take still further precautions 

 in the usual " trial trip " to test both engines and compass. This is 

 another stage at which it is deemed by the majority prudent to finally 

 adjust the " correcting card " for the voyage. This is especially the 

 period of " swinging" her majesty's ships, with what certainty of per- 

 manent advantage may be still further illustrated. 



We have mentioned her majesty's ships as an extreme case we now 

 will confine our deductions to the incidents attending their proceeding 

 to sea. It is fair to assume that ships are fitted in still waters, either 

 in harbours or estuaries ; whether such ships be new, or have been 

 lying dismantled in ordinary, affects our main scrutiny as to changes in 

 a very immaterial degree. In either case, whatever changes do occur 

 they are due to what Scoresby has called retentive magnetism. Now 

 guns are formed of a metal which, from partial carbonisation, rapid or 

 unequal cooling, and a tendency to crystallisation, or at least a peculiar 

 arrangement of its particles, has in the ordinary form of cast-iron a 

 peculiar aptitude for the absorption of magnetic influence : and, indeed, 

 excellent permanent magnets may be formed of it, inferior only to steel. 

 These guns have moreover to undergo a process of dbncussion in firing 

 still further calculated to derange local attraction, by increase or 

 diminution, we cannot tell which : and even the training of these guns 

 may, in their altered position, affect the local attraction of the whole 

 ship. 



And again, independent of the evolution of magnetism in the boiler, 

 the very act of combustion of coal has its influence on the condition of 

 an iron steamer. For not only does the combination of carbon with 

 the oxygen of the air (which is decomposed in the furnace in the act 

 of combustion) in the form of carbonic acid gas increase the magnetic 

 intensity of the funnel, but the decomposition of the coal itself is a 

 source from which magnetism is as it were poured into the very mass of 

 the ship herself in abundance, perhaps to saturation, if such can exist. 

 In addition to the electro-magnetic disturbance incidental to all de- 

 compositions, coal is composed of carbon and " earthy matter." Now 

 pure carbon is of itself capable of polarity, inasmuch as crystallisation 

 is found in the diamond : but the " earthy matter " is worthy of serious 

 examination. Coal varies greatly in the quantity of earthy matter it 

 contains, and for the following reasons : the lower the specific gravity 

 of the coal used, so much the less probability of magnetic disturbance 

 one might look for. 



Earthy matter in coal amounts to, in general, from 1 to 10 or 12 

 per cent., such being either pure oxide of iron, or sulphuret of iron 

 with occasional traces of alumina. How if we, for experiment, burn 

 off the sulphur by the blowpipe from a small portion of sulphuret of 

 iron, or iron pyrites, we shall find the residue strongly magnetic ; and 

 this may be verified by our taking some of the dust of the ash-pit of a 

 furnace, or of a common house-grate, and placing it on a piece of 

 writing-paper ; holding a magnet under the paper will at once show 

 polarity in the particles, and moving the magnet along the surface and 

 Kick again will abundantly show that combustion of coal therefore 

 produces constantly a large number of small distinct magnets, the power 

 of which is increased not only by their being in proximity to each 

 other, but by the high temperature at which they are maintained until 

 usually removed from the ash-pit. When we further consider that in 

 a large steamer some 100 to 200 tons of coal are consumed in a day, it 

 is not too much to say that some tons of magnets are manufactured 

 daily on board these vessels : we say manufactured, for in the form of 

 coal, or even as sulphuret of iron, such is not magnetic. 



And again, with all these tendencies to produce a disturbing effect 

 on the known or once ascertained local attraction of the ship, there 

 remains yet the possibility of further inconvenience to the compass 

 arising from heavy pitching or rolling in a sea way : even her " heeling 

 over " is well known to affect the compass to a considerable degree. 



Such then is " local attraction " on shipboard ; an influence from 

 which no ship nearing the land would be safe, as the changes re- 

 ferred to cannot be anticipated or averted by any process yet known. 

 It remains, therefore, to renew " corrections " as often as possible, 

 using a heavenly body as the sole standard. Under the article COMPASS 

 CORRECTIONS it has already been shown how readily and completely 

 this may be done by means of Saxby's spherograph and Professor 

 Airy's arrangement of soft iron and magnets. Local attraction on ship- 

 board is, therefore, a subject for anxious vigilance on the part of every 

 commander. Many ships are found veiy unsusceptible of changes ; 

 others, after years of inactivity, suddenly become excited. As a general 

 rule, local attraction is expected to diminish after a ship has been a 

 year or two at sea : but no ship or vessel, especially a steamer, ought 

 to be navigated by means of a correcting card, unchecked by frequent 

 celestial observations, for at least the first week or two after leaving 

 port. A prudent commander will never trust to his compass for a 

 single day if azimuths can be taken. 



(Nautical Magazine ; Dr. Scoregby, Magnetital Inreiliyationt ; &o.) 



LOCI. [Locus.] 



LUCK, a fastening in which a moveablo b'llt may be projected by 

 the action of a key, introduced into the lock in such a manner that the 

 bolt cannot be returned to its original position, so as to release the door 

 or OOY*T, excepting by another application of the key. There are, 



however, many locks in which the bolt is projected by the action of a 

 spring, without the aid of the key, and in which it may be drawn back 

 by means of a handle ; although it cannot be moved from the other 

 side without the application of the key. In other locks, again, there 

 are two or more bolts, one of which only is under the control of the 

 key, the others being moved by handles ; while in another class two 

 or more bolts are shot or projected by the action of the key alone. 

 In addition to such differences, the varieties in the form and size of 

 locks, and in the arrangement of their parts, are almost infinite. In 

 door and closet locks, the bolt is usually projected into a fixed socket. 

 In various kinds of box and cabinet locks the bolt is not projected 

 beyond the casing of the lock, but is caused to shoot into or through a 

 staple which drops into the lock to receive it. In desk-locks and some 

 others the bolts project permanently, and are of a hooked shape, 

 adapted to catch, by a lateral . movement, into staples provided to 

 receive them. Padlocks are a kind of detached lock in which a curved 

 bar of iron, pivoted to the lock at one end, may be passed through a 

 staple, and then so secured by shooting the bolt into a cavity in its 

 free end, that it cannot be removed from the staples through which it 

 has been passed. 



By far the greater number of the contrivances for rendering locks 

 inviolable may be classed under one of three systems of security. 



Warded Locks. The key of an ordinary lock consists of a cylindrical 

 shank with a loop-shaped handle at one end, and a piece called the bit 

 projecting from it at or near the other. The bit end of the shank is, 

 in the keys of locks which are to be entered by the key from one side 

 only, made hollow or tubular, to fit on to a pin fixed in the lock ; but 

 in locks which may have to be opened from either side, such as ordi- 

 nary room-door locks, the shank is made solid, and is prolonged beyond 

 the bit, so as to enter the upper part of the key-hole of that plate of 

 the lock which happens to be farthest from the person applying the 

 key. The projecting bit, after being introduced through the key-hole, 

 is turned round within the lock by a rotatory motion imparted, until 

 it comes in contact with a part of the bolt so shaped that the bit 

 cannot pass it, to complete its revolution, without shooting the bolt 

 either backwards or forwards, as the case may be. When thus moved 

 the bolt is retained in its position by a spring, or some other means, 

 until it is again moved by the reverse action of the key. 



The first and simplest means by which the entrance of a false key 

 may be rendered difficult, is by giving a peculiar form to the substance 

 of the bit, and either adapting the form of the key-hole exactly to it, 

 or inserting pieces of metal in the lock in such a way as to prevent the 

 admission of a bit of different shape. Fig. 1 represents several forms 



Fiff. 1. 



1 1 1 1 I 



of bit commonly used. Of these a, b, e, and d are adapted for key- 

 holes of various corresponding forms ; while e and /, though suitable 

 for key-holes of the same general form, admit of further security by 

 forming projections upon the sides of the key-holes. 



The next and principal means of security is the use of pieces of 

 metal of various forms, fixed within the lock in such a way that no 

 key can be turned round within it unless corresponding notches are 

 cut in its projecting bit. fig. 2, which represents a portion of the 



Fig. 2. 



interior of a lock, with the bit end of the key in its placo, will illus- 

 trate this. Attached to the back-plate are two concentric prominent 

 rings, of different degrees of elevation. These prominent rings are the 

 Kard8 t which impede the introduction' of a false key. No key could 

 be put into a lock provided with them, unless a notch were made in its 

 bit to correspond with the larger of the two rings ; and it is evident 

 that no key could be turned round without having also a notch to 

 correspond with the smaller circle. In the commoner kind of locks, 

 the wards seldom form a complete circle, but their effect is the same 

 if they occupy only a small segment. Many wards are of a more 

 complicated character, such as what are termed L, T, or 2 wards, from 

 the resemblance of their sectional form to those letters respectively. 



