112 



BRITISH ASSOCIATION FOR THE ADVANCEMENT OF SCIENCE. 



1854.J 



hibited to the Section, or they must arrange to test effects in the 

 interior of hollow electro-magnets without seeing them at the time 

 they are taking place. Interesting questions, which the mathematical 

 theory answers decisively, had also been asked regarding the minimum 

 condition of the central line in a field between opposite flat poles, of 

 two cylindrical soft-iron bar magnets, and the etfeots of rounding off 

 the edges of these poles. It appears that, if we consider the intensity 

 of the force in a plane perpendicular to the magnetic axis through the 

 centre of the field, we find it increasing from the central point to a 

 certain circle of maximum intensity, beyond which it diminishes grad- 

 ually and falls to nothing at an infinite distance. If the edges of the 

 cylinders be rounded off, the circle of maximum intensity contracts, its 

 centre always being a point of minimum intensity, until a certain de- 

 gree of convexity of the poles is attained, when the circle of maximum 

 intensity becomes comtracted to a point — the central point of the field 

 — which will then be a point of maximum intensity (the central mini- 

 mum being eliminated), and will continue a maximum, as regards all 

 points in the plane through it, perpendicular to the axis, for any less 

 flat or more prominent or pointed forms of poles. No form of rounded 

 poles, by doing away with maximum or minimum points, can possibly 

 give a uniform distribution of intensity through ever so small a finite 

 bulk of the field. 



On the Deviation of the Magnetic Needle peculiar to Liverpool, by 

 SiK John Ross. 



Ever since the year 1799, when my attention was first directed to 

 the deviation of the magnetic needle, I have lost no opportunity of 

 making observations in many parts of the globe, on the interesting 

 phenomena appertaining to that influence, — a statement of which has 

 been published by me in the narrative of my first two voyages of discovery 

 to the Arctic regions. Since which my attention was called to the 

 frequent losses of ships consequent on the fallacious system adopted 

 by the Admiralty, called "adjusting the compass," at Gravesend 

 and other places ; and after the loss of the Birkenhead, I felt it my 

 duty to publish a pamphlet, which, although dedicated by permission 

 to the First Lord of the Admiralty, did not at once obtain their 

 Lordships' approbation, inasmuch as it exposed the absurdity of the 

 system then in practice under the superintendence of a naval oflicer 

 attached to the Admiralty. But I maintained the truth of my state- 

 ment ; and, after some correspondence on the subject, my assertions 

 were found to be correct, and, consequently, the ofiice of Superintendent 

 of Compasses was abolished, and circulars issued by the Admiralty, 

 not only ordering a monthly examination of the deviation, but that 

 auch observation should be instituted at every change of the ship's 

 position, and on every circumstance which was known or supposed to 

 affect the ship's deviation, or local attraction, which is now admitted 

 to be of infinite service. But my attention has for some time been called 

 to the fact of ships sailing from the port of Liverpool, after having 

 been swung in the Mersey to obtain the amount of deviations, or as it 

 is called, to have their compasses adjusted, that immediately on their 

 proceeding on their voyage it was found ^at the deviation observed in 

 the Mersey was incorrect, and there have been lamentable instances of 

 shipwreck in consequence. It has occurred to me that this untoward 

 circumstance is very easily explained. The fact is, that the Mersey is not 

 locality eligible for ascertaining the true deviation of the magnetic 

 needle, the ships being in a position between establishments in which 

 large masses of iron are deposited, which must have an influence on 

 the magnetic needle during the evolution of swinging the ship, while 

 the embarkation of passengers with their luggage, or anything else 

 subsequent to that process, cannot but have the effect of producing a 

 false and dangerous result to the observations. But this evil is not 

 without an effectual remedy, which is within the power of every 

 captain of a ship after he has left the port of Liverpool, and which 

 will be found in the following proposals. — It is proposed that the 

 present method of swinging the ship in the Mersey shall be continued ; 

 and, in order to obtain a verification or a correction of results observed 

 at that time, it is proposed to place on the san(Hiills of Rockland (near 

 the Rock Lighthouse), two posts or beacons, true north and south of 

 each other, in the positions best seen near the red buoy of the Rock 

 Channel, when the ships passing will be steering about true west, or 

 west-north-west, by compass. When these two objects can be brought 

 into one, i.e. due north of each (both being south of the ship), either a 

 verification of the deviation that was observed in the Mersey, or the 

 amount of difference to be taken into consideration or account on that 

 particular point of the compass, will be shown, from which a calcula- 



tion may he made in approximation of the other points; and if, 

 further on, two other posts were erected on the magnetic meridian, the 

 ships, on passing them, when in one with each other, could observe 

 the exact amount of the deviation either in the increase or the diminution 

 of the variation on the course of the ship, keeping in mind that it will 

 be on the south point of the compass that the observations will be 

 made. Posts placed due south of Lizard Lighthouse would be useful, 

 and also on the magnetic meridian. But all posts or beacons denoting 

 the true north or south bearings, and those further off denoting the 

 magnetic meridian, should be painted of different colours. The former, 

 that is, the true or nearest, should be red ; while the latter, showing 

 the magnetic meridian, should be chequered. Great Ormshead and 

 Holyhead should have beacons placed on them, which would be ob- 

 servable to ships both outward and homeward bound. 



On a Neio Refractometer : by Prof. Bennabd. 



This instrument was founded on the principle of passing a ray 

 through a medium bounded by two parallel surfaces, and might be 

 called the refractometer of separation (refractometre de tran.'sport). 

 "When a ray passes through such surfaces, if it be incident perpen- 

 dicularly, it emerges in the same course. If it be incident obliquely, 

 its emergent course is parallel to that of its incidence. Then tha 

 relations which connect the perpendicular distance between the 

 incident and emergent rays — the angle of incidence — the thickness of 

 medium, or distance between the surfaces bounding it — the index of 

 refraction is known — the first two can be observed, the third measured, 

 and then the foui'th — which is what we seek — is a matter of simple 

 calculation. 



Dr. Whewell expressed the pleasure he experienced at seeing these 

 very beautiful instruments ; and was particularly struck with the clear 

 proof arrived at by Prof. Bennard, that the light at the several parts 

 of the solar spectrum was simple, and not compounded light ; and 

 that thus the view, which had been some years since propounded, and 

 which was still enterfained by some, that the spectrum obtained by 

 the prism was composed of several superimposed spectra, is proved to 

 be unfounded and must be abandoned. 



On the influence of the Solar Radiations on the Vital Powers of Plantt 

 growing under different Atmospheric Conditions : by 

 Mr. J. H. Gladstone. 



This was the second Report given by the author under the same 

 title, and commenced by describing accurately what portions of the 

 prismatic spectrum were cut off by the various coloured glasses em- 

 ployed in his experiments. A series of observations followed on 

 hyacinths grown under very varied influences of light, and solar heat and 

 chemical agency. Among the results may be mentioned the power on 

 the yellow ray to diminish the growth of rootlets, and the absorption 

 of water ; the power of the red ray to hinder the proper developement 

 of the plant ; and the effect of total darkness in causing a rapid and 

 abundant growth of thin rootlets, in preventing the formation of the 

 green colouring matter, but not of that of the blue flower, nor of the 

 other constituents of a healthy plant. A series of experiments on 

 germination was then detailed. A\'heat and peas had been grown with- 

 out soil under large colourless, blue, red, yellow, obscured colourless and 

 obscured yellow glasses, and in perfect darkness. The effects resulting 

 from these varied conditions were very marked ; and the description of 

 them occupies a considerable space in the report. The two plants ex- 

 perimented on — being chosen from the two great botanical divisions — <■ 

 exhibited a wide diversity, sometimes amounting to a direct opposition, 

 in their manner of being affected by the same solar ray ; but in the 

 case of both the plants, under the circumstances of the experiment, 

 the following effects were observed : — The cutting off the chemical 

 ray facilitates the process of germination, and that both in reference 

 to the protrusion of the radicles, and the evolution of the plame ; the 

 stem grows unnaturally tall, and there is a poor developement of 

 leaves in darkness, becoming more manifest as the darkness is more 

 complete ; and the yellow ray exerts a repellant influence on the roots, 

 giving the wheat a downward and the pea-roots a lateral impulse. A 

 few experiments on the germination of other seeds were then narrated; 

 and the report concluded with an account of experiments on the ger- 

 mination of wheat and peas in oxygen, hydrogen, and carbonic acid 

 gases, as well as in ordinary atmospheric air, and in air from which 

 carbonic acid was at all times certain to be removed. The results 

 confirmed former observations on the necessity of oxygen. 



