512 



KNOWLEDGE • 



[Apeil 14, 1882. 



galvanic l<utt<>ry in the transfcrcnco or molion of par- 

 ticli*8 of mutter from ono state of coTnliiimtion to 

 anotlirr. Thus, in tin- Duiiii'll ooll, the sulpliute of 

 copper Ik oonverU'il iiitt) Kul)>huric ocid l>y the iiieaiiH of 

 hydrojjen, which di)<]>laces the cojjper, the siilpliuric 

 acid iK-in;; likewise tonvei-ted into sulphate of zinc hy 

 particles or ittonis of zijic taking tlu; j)luce of the hydrogen. 



'lliere are, however, other forms of motion which in their 

 tuni are capaMi^ of generating; currents of electricity, 

 motion not necessarily of minute particles, hut of large 

 masses of matter. If one of two ma.ssea moves or passes 

 iK'fore the other, more particularly if magnetism or elec- 

 tricity i.s present in one of them, an electric current is 

 generated. Let us imagine, in the first place, thot 

 wo have a hollow }ieli.x or coil of copper wire, such as 

 that contained on the leg of an electro-magnet, or such as 

 we should get liy carefully winding an insulated copper wire 

 round a thin stick of wood. Now, suppose the two ends of 

 the wire are connected to a galvanometer. Then on plung- 

 ing a steel mognet into the coil, the needle of the galvan- 

 ometer will be de(lcct<d, say, to tlie right, and will indicate 

 the passage of an eli'ctric current. When the magnet is 

 withdrawn, the needle will be again deflected, this time, 

 however, to the other side, thereby indicating the passage 

 of a current in the opposite direction to the previous one. 

 Let us pursue this a step further. Our readers are 

 doubtless aware that when a piece of soft iron is placed 

 in the vicinity of a magnet, magnetism is immediately 

 induced in it If, then, we place a core of soft iron in 

 our coil of wire, and Ijring a magnet near it, it becomes at 

 once magnetised. The core being already inside the coil, 

 magnetising it in that position is equivalent to plunging 

 in a magnet instantaneousl)/. A current of electricity is 

 consequently produced, but, of course, of greater strength 

 than when the magnet is plunged into the coil. Similarly, 

 on removing the inducing magnet, the iron core becomes 

 at once ckinai/iie/ised. This is identical to withdrawing 

 tlie magnet, and therefore a strong current is generated or 

 induced, travelling in the opposite direction to the preced- 

 ing one. 



What have we done ? We moved a magnet, and obtained 

 a current of electricity. To a certain degree, the current 

 produced is a measure of the force expended in moving 

 the magnet, and it is ijiteresting to notice that when the coil 

 circuit is complete, it is more difficult to move the magnet 

 than when tlie circuit is broken. This phenomenon will, 

 however, be referred to more fully on a future occasion. The 

 experiments above described illustrate the principles of 

 ■what is known as the magneto-electric machine, such as 

 those used for medical purposes, in which a coil of wire 

 and its core of soft iron are made to approach and recede 

 from a permanent magnet, which is fixed. M'e shall, in 

 our next, give instructions for making a small machine, 

 not one of those used by the medical faculty, but one of a 

 much-improved form, which will be capable of doing all 

 tliat a galvanic battery will do. 



The afiparatus for performing the experiments described 

 in this paper may be very cheaply constructed, and our 

 readers are strongly recommended to try them, in order 

 that they may belter understand our subsetjuent papers on 

 the subject The coil may be made by winding five or six 

 layers of No. 22 cotton-covered wire* on a paper tube, 

 about half-an-inch in diameter, and three inches long. 

 After the wire is wound on the tube, it would be as well 

 to coat it with a little shellac varnish, made by dis- 

 solving sliellac in methylated spirit The magnet may 



• MosHfK. Riokarc), of Dorl.v, nrc wire makers of the highest 

 repntp ill clrcfricnl circles. Their price for No. 22 is Is. lOJd. per 

 pound. 



be of the ordinary round form, such as are nsed in 

 telephones, and should fit the tube easily. A piece 

 of soft iron of similar dimensions will complete this 

 I>ortion of the apparatus. The galvanometer may also be 

 easily made. Magnetise an ordinary sewing-needle by 

 rubbing it a few times in one direction over one pole of 

 the steel magnet, and then suspend it by a thin thread 

 (or, better still, by a fibre of unspunsilk), in an oblong coil, 

 consisting of alxiut ten tunis of the No. 22 wire, just long 

 enough to allow the needle to rotate freely. 



THE THREE COLD DAYS OF APRIL. 



Bv THE Editor. 



FEW weather phenomena in this country are more 

 remarkaVile, and seem at present less easily explained, 

 than the so-called " borrowing days," as they are called, 

 between the 10th and 14th of April, when usually the 

 temperature falls considerably below that due to the time 

 of year. The cold at this time is, at any rate, sutKciently 

 marked — first, to have attracted long since general atten- 

 tion ; and, secondly, to affect in a very obvious manner the 

 average temperature for these days during the last eighty 

 years. We find these three cold days of April, wliich 

 before the change of style were the first three days of the 

 month, thus described in doggrel lines in the north of 

 England : — 



" Majch borrows from April 



Three days, and tliey are ill : 



The first of tliem is wan and weet, 



The second it is snaw and sleet. 



The third of them is a peel-a-bane, 



And freezes the wee bird's neb tae stanc." 

 The following lines are given in the " Glossary of Scotch 

 Words and Phrases " : — 



" Said Mai'ch to April, 



Gic me three lioggs upon yon hill ; 



And in the .space of days three 



I'll find a way to gar them die. 



The first a bitter blast did blaw, 



The second it was sleet and snaw, 



The third it cam sae full a freeze 



The birds' nebs they stack to the trees ; 



But when the days were past and gane 



The three puir hoggs cam hirplin hame." 



This is manifestly an imperfect version of the lines in the 

 poem called the '• Complaynt of Scotland,'' where the re- 

 ference to the borrowing of three April days is much 

 clearer (in the above account JIarch borrows hogs not 

 days) :— 



March said to Aperill 



I see three hogs upon a hill ; 



But lend your first three days to me, 



And I'll be bound to gar them dee. 



The first it shall be wind and weet. 



The next it shall be snaw and sleet, 



The thiixl it shall be sic a freeze, 



Shall gar the birds stick to the trees. 



But when the borrowed days were gane, 



The three silly hogs cam hirplin hame. 



This is, I believe, the oldest version of the doggrel. It 

 belongs to a time when the three cold days of April really 

 were the first three days of April. The other was perhaps 

 modified to correspond with the new style, according to 

 wliich the cold days fall in the heart of the montli, and 

 cannot be very well imagined to be borrowed by March. 

 It is worthy of observation how correctly common obser- 

 vation has indicated the true position of these cold days; 

 for ill the temperature curve derived from three quarters 

 of a century of accurate observation at Greenwich, the 

 depression corresponds exactly with the days which before 

 the change of style were the 1st, 2nd, and 3rd of April. 



