LOBELIA INFLATA. 



LOCAL ATTRACTION. 



LOBELIA 1SKLATA (Bladder podded lobelia, or Indira tobacco), 

 This American annual plant foutmet in iU 



who), structure aom. very peculiar propertiea, approximating to 

 Virginian tobaeoo so oloaely as to have received a nearly similar name. 

 The plant Boners in July, and the entire plant -r herb is to be taken 

 up in August or September ; every part possessing the characteristic 

 properties. These display IheHMSITK quickly on those unaccustomed 

 to the ua of this herb, even when only a small portion i* held in the 

 mouth. " It produce* gjddiasss and pain in the head, with a trembling 

 agitation of UM whole body ; and at length bring* on extreme nausea 

 and vomiting. TUe taste resemble* that of Urtar emetic." (Bigelow's 



Medical Botany,' an American Work.) It* secondary efleot* are great 

 low of saliva and increased expectoration, with general relaxation of 

 UM muscular aysUm, and depression of the heart's action. Small 

 does are useful in Mthrna, but in hooping-cough greater relief may be 

 more safely procured by enveloping the chest with flannels steeped in 



trpid decoction of poppy heads ; renewing these frequently. The 

 seeds are very small, beautifully reticulated, so that they can be easily 

 reeognieed under the microscope. In cases of an overdose, strong tea, 

 then brandy and ammonia, should be given. 



LOBKLLXK, an oily non-volatile alkaloid found in the Lobelia 

 (Lubrlia imJUta). It is very 'Soluble in water, alcohol, and ether. 

 Administered to animals, it produces narcotic effects. , 



LOCAL ATTRACTION, a term accepted by men of science as 

 applicable to influences exerted on the compass by substances on ship- 

 board. Few subjects have taxed the patience and intelligence of 

 nautical men so much as that of the disturbance of the compass by 

 local attraction ; nor has the consideration of the subject by living 

 philosophers been productive of more than a verification of certain 

 suspected facts. 



Professor Airy baa indeed deeply investigated the causes as they 

 ordinarily appear, and has been successful in providing some alleviation 

 of the evils attendant upon a disturbed compass ; but in other respects 

 nothing of permanent benefit has accrued to the compass question 

 either Irom committees, or from elaborate experiment ; nothing which 

 aids the snaman by an infallible remedy. Were a ship to remain 

 always in nearly the same part of the globe, and her local attraction to 

 remain a constant quantity, Airy's improvements would be all that 

 could be deaired ; as it is, no other mechanical means of correcting by 

 magnete in a long voyage is so convenient or so manageable. 



That the compass was subject to changes and deflections irrespective 

 of terrestrial magnetism, was of little importance in ships constructed 

 only of wood. Irregularities of the compass were occasionally detected, 

 but these were mostly attributable to iron bolts or nails, or accidental 

 deposits of metal, such as a knife or marlinspike upon or near the 

 binnacle. Beyond these casual or accidental disturbances, the mariner 

 of olden time had little to do with local attraction. An iron cargo 

 occasionally caused him some extra watchfulness, but the error once 

 ascertained left him at ease upon the subject of his compass. No 

 sooner however had the interest of merchants increased the space 

 for stowage in their ships, and, in adopting modern improvements, 

 introduced iron knees, chain rigging, and various other metallic 

 appliance*, than the consideration of the actual compass course to be 

 Jeered became involved in new and perplexing corrections, and various 

 speculations concerning a ship's condition as altered by these improve- 

 ment* ***"""* the subject of study. For nearly a quarter of a century 

 this state of things was suffered to progress, when the establishment of 

 the present Board of Trade, and the intelligence of its naval heads, 

 introduced a new and still enduring scrutiny. 



Snipe had begun to be constructed of iron only ; some had been 

 built many years previously for the writer remembers' seeing the first 

 boa veseei which ever anchored in the Downs, a fine brig called the 

 'Tontine,' in about the year 1816. From iron hulls, iron masts and 

 even yard*, soon were to be found afloat in British waters. And as if 

 the compass were to be threatened with total rejection, iron ships were 

 soon fitted with steam engine*. Wooden steamers had already (about 

 thirty yean since) exercised the patienoe and vigilance of ship masters 

 to a large extent, but change* so enormous were gradually producing a 

 maHgnant phantom more and more forbidding in its aspect, from its 

 power of eluding the grasp of it* pursuers, and deluding them into 

 till deeper swamps of error and disappointment. Such effects then 

 had at first "local attraction." Some, to avert the evils of an 

 invisible and fickle agency, fastened magnets to their decks as palliatives, 

 but they did *o with a* many mUgivings as they still fasten the horse- 

 shoe to UM bowsprit end. Any species of "charm" which the 

 credulity of the superstitious could invoke to avert mischief from the 

 oompaes was eagerly applied to its bowl, or the binnacle, or to the 

 deck, until in 18i4, the unfortunate catastrophe of the ' Taylcur,' a fine 

 shin of above MOO ton* (actual measurement) built of iron throughout, 

 and baring eompartmenU also of iron, was lost with 200 of her crew 

 at LamUy. on the Irish coast. 



The investigation of the causes of her loss resulted in an official 

 report from Captain Walker, (one of the naval beads of the Board ol 

 Trade) attributing her destruction to the " supposition " on the part ol 

 UM Captain, that the ootnpas* before the helmsman was correct," and 

 alleging that the captain " was aware there was a difference of two 

 points between the compasses, but he knew not which was in error. 

 We see then the effect* of compaei disturbance, and proceed to 



examine the causes, but have to' deal with the circumstance of local 



M being liable to sudden and great changes, so that corrections 



which are available and eflVctivu at one period are totally useless at 



. It in this ehanifx which baffles us, and in it lie* the greatest 



amount of danger to navigation. 



l Scoresby, in his patient experiments and indefatigable ntt. 

 to the subject through many years, bos shown that some portion of a 

 ship's local attraction may be traced to her very cradle that the very 

 lammering and rolling, and bending, and riveting of her plates engen- 

 ders a mischievous influence variable in its intensity as In its duration, 

 and is thus coexistent with the origin of her frame-work iteelf. That 

 even the direction of the line of keel while building, as measured by 

 ts angle with the magnetic meridian, establishes for a time a systi 

 derangement of the action of terrestrial forces in which the effects of 

 terrestrial dip and obliquity have their united influence, which may or 

 may not affect the future navigation of the ship by means of a mariner'* 

 compass. 



\V, re the efforts of Dr. Scoresby, and Mr. J. K. Napier (the eminent 

 ron ship-builder of Glasgow), and the approach to principles developed 

 by them, not countervailed by the changes incident to every iron ship 

 on leaving her birth-place, little would remain to the sea captain - 

 receive from the hands of the builder, a diagram of the ship's magnetic 

 condition, as produced by the causes already referred to ; and indeed as 

 ralpably evident and accurate as her load and light water lines, &c., 

 would be the correcting card so handed over ; for it is not ditHeult to 

 discover a neutral point in every iron ship, at which a standard compass 

 would feel the effects of terrestrial magnetism only. But no sooner, 

 unfortunately, in the profess of launching does the ship, if built of 

 metal, plunge into a denser medium, such as water, than the sudden 

 change of temperature affecting the submerged portions of the mass, 

 communicating itself to the upper works, challenges at once the 

 stability of her magnetic condition. Nor is this, at this period, the only 

 enemy to magnetic permanency, for it has been shown by Dr. Scoresby, 

 that the very twist and strain in the framework of the ship consequent 

 upon her being, in launching, raised gradually by the buoyancy of the 

 water, is sufficient to affect materially that which other influences had 

 matured. 



The iron ship being once afloat, we shall find still other perplexing 

 disturbances at work, and especially if she be intended for a steamer, 

 with iron masts and yards. 



If the latter, the increased hammering in the fitting of the engines 

 odds its quota to the general confusion of conflicting and counteracting 

 forces. The lighting of the fires, the heating of the boilers, the pro- 

 duction of steam each of these is a fertile source of further electric, 

 and therefore magnetic, disturbance. It would seem that tli. 

 adoration of thermo-electricity, as operating upon the mass of metal 

 in an iron ship, has been somewhat neglected. The enormous change 

 of a single drop of water, which, on its conversion into steam, is 

 expanded into 1700 times its bulk, has been proved years since to 

 generate, or rather to release, a vast amount of electric or magnetic 

 power ; and as we cannot circulate electric currents without magnetising 

 iron, when in certain favourable positions with regard to it. tli 

 plication of disturbances is so great in an iron steamer that it is not 

 surprising if the wisest heads fail in providing a permanent remedy for 

 local attraction. Ships ore "swung generally at this period, and the 

 question of compass error is supposed by many to have been satisfied if 

 the amount of then existing deviation is recorded. But what follows ? 



If we test an iron pillar or bar of even a tradesman's shop-front, wo 

 shall find that in time, and from its vertical, position, it b> . 

 magnetic, and exhibits polarity : and this law affects every vertical bar 

 in the ship. It is an error to suppose that the weight of an iron mass 

 has reference to its magnetic intensity ; for, as magnetism seems to 

 collect on surfaces of metal, it follows that a 0-ineh hollow inm j. 

 funnel, having two surfaces, an inner and an outer, would i-\hibit 

 more power of disturbing a compass than if it were of solid 

 therefore stove-pipes have been often found dangerous neighbours to a 

 standard or binnacle compass much more so should we expect -.<- 

 mast to be. Hence the very ports used in the count : titling 



of an iron ship are liable to the change to which we have adverted, and 

 this change may bo either an increase or diminution of magnetic in- 

 fluence, we know not which ; as one piece of iron used in the construction 

 of the ship may and docs counteract another, partially of wholly ; the 

 change moreover may be cither rapid or protracted, until some other 

 new source of disturbance comes into controlling operation. 



In process of preparation for sea, cargo of various materials, such as 

 hardware, machinery, or iron bars themselves, loads the ship, increasing 

 the ttrain upon her plates aud knees, &c. With i . .r.ird to iron bars as 

 a cargo, let any one walk round a heap of bare lying upon a wharf, with 

 a pocket-compass, and remark their otleet <>n the needle (whether lyins 

 out of or in the magnetic meridian should be Doted). It must be 

 remembered that these are to be placed, we will suppose, in an . 

 magnetised iron ship, and subject, especially in an iron steamer, to 

 another serious source of change, a change in temjHM-ature. Now it is 

 known from experiment that the intensity of a magnet is nearly d 

 by lacing raised to a "cherry-heat :" the change of temperature in a 

 snip's hold may, even in the tropics, be considered comparatively 

 insignificant, but the fact remains that disturbance of temperature is 

 disturbance of electricity, and as no electric current can circulate 



