The Avondale Disaster. MESSERS EDITORS :My suggestion will be too late to help the unfortunate miners at Avondale, but may be of use in some future disaster. The plan I suggest is, to have close by each mine two or more complete diver's suits, with all attachments ready for instant use. To enable the miner to drag the necessary air hose along the levels, he would require a hose carriage similar to that used for the compressed Itir coal cutters in England. For some points, when the hose would be liable to entanglement, small vessels of compressed air, or compressod oxygen, would enable a man to work for a limited time. Some of the suits should be the ordinary deep- sea dress so that assistance could be carried through drowned parts of a level. The suits used merely to protect against gas conld be made almost as light as an ordinary suit of clothes. By this means a few men could be supplied with air and could get at once to points that now require days to reach in safety. My suggestion may not be practical in its present shape, but the principle of carrying the limited supply of air to the point when it will give its maximum useful effect in cases of di saster will bear working out by those who are practical miners.J. G. S. Philadelphia, Pa. Marks Produced by Lightning Strofte. MESSRS. EDITORS :1 noticE^.N article on page 170, current volume of the SCIENTIFIC AMERICAN, on the subject of marks produced upon the body by lightning stroke. I wish to state an instance of the kind which occurred in the upper part, or western portion of Washington county, Md., some time in July, 1851, to which I, with several others, was an eye-witness. In an open lot near the residence of the late Captain John Ressley, about Iwo and a half miles northwest of the town of Hancock, in the county named, stood an oak tree of medium size, under which, during a thu nder storm, a sheep had taken shelter from the rain. On a limb or branch of the tree sat a robin, directly over the sheep ; a flash of lightning struck the tree, the robin, and the sheep ; killing both of the latter. We saw the sheep lying under t,h<j, tree and went to see if it had been killed. When we arrived, we found the sheep dead, lying upon the left side, and found thedead body of the robin lying upon the right, or upper side of the sheep. Capt. Ressley, who owned the sheep, ordered his servants to skin the sheep, which they did immediately, and when they came to the spot on the right, and which was the upper side, where the body of the robin had fallen and where we had found it, they noticed a strange appearance, and Called our attention to it. To our no small astonishment, we found on the inside of the skin of the sheep and also on the flesh of the body of it, a perfect picture of the robin, even to the fine fringes of the feathers of its wings. Now it could not have been the falling of the body of the robin upon the body of the sheep that caused the impression, as the figure or picture thus formed was not that of a dead bird lying sidewise on the sheep as we fotmd it, but it was a perfect picture of the robin while sitting' on “the limb of the tfea above the sheep. This cirell.mstaliee at the time Elicited it good deal of oontfoYewy among ll; “'malt elide of educated fiuiit-k'- mm ill, theonly electric fluid, and in that way caused the picture (darkened as it was) upon the inside of the skin and upon the flesh of the sheep, I am not able to prove, but that has always been my theory of it. I do hot desire to intrude upon your time and patience, but I will say that upon several other occasions I have witnessed effects of electrical action which I regard as totally outside of all infomIation that I have yet been able to gain in regard to electricity, in any and all books written on the subject, or from any other source, having been somewhat' of an experimenter myself with it, for the last twenty years. And at some future time when more expedient, I will endeavor to give several instances of strange productions by electrical force or power, which, I believe, will account for certain curious formations in nature in certain localities, which, up to this time, have not been accounted for on any scientific principles.THOS. J. LOGAN. Washington, D. C. [We certainly hope our correspondent will favor us with any other facts in his possession relative to this interesting subject.EDS. Has the Pacific Railroad Changed the Climate of the Plains 1 MESSRS. EDITORS :The scouts, guides, and hunters all agree in stating that on the Plains, as far back as their experience goes, little or no rains have fallen during the summer; but the experience of last summer and this one is, that we have sufficient rain for farming purposes, and the crops of hay and other produce raised here now attest it. The hunters with whom I have conversed all agree in stating that the rains only fall inside of a belt across the Plains of fifty miles in width, of which the railroad track is the center, that when they go beyond that belt the grass is red, crisped, ana burnt- looking, while all vegetation inside is luxuriant. Has the iron of the rails or the upturned ground the credit of the change?JOHN WHITFORD. Pond City. (Pope's Modern Practice of the Telegraph.) Conducting Powers of' Materials. According to the experiments of Mr. M. G. Farmer, made some years since, the relative electrical resistance of different metals and fluids at ordinary temperatures is as follows, pure copper being taken as 100 : ific conductivity, as maybe seen by the following table, which gives the result of careful determinations by Dr. Matthiessen, the conducting power of pure copper at 59^9° Fah. being taken as 100. Lake Superior, native, not fused98's at 5\)'9° "” fused (commercial) \12'0 at 59'0() Burra Burra88'7 at 57'2° Best selected81'3 at 57-5" Bright copper wire2 at (S0-r Tough copper71-0 at 631° Demidoff59-3 at M's° Rio Tintoi! S8-6" Thus Rio Tinto copper possesses no better conducting power than iron. This shows the great importance of testing the conductivity of the wire used in the manufacture of electro magnets, cables, etc. e Zinc 3-70 Brass “ 3'83 German silver wir e11'sO Nickel” 7-70 Cadmium” 2'()1 Aluminum “ 1'75 Copper wire 1'00 Silver '; '98 G old “ 1 '13 Iroii “ 5'6;5 Lead “ 10-7(i Mercury “ 50'00 Pallad'm “ 5'GO Platinum “ 6'78 His experiments with fluids gave the following results : Pure rain water 40,653,723'00 Wate,r, 12 parts ; sulphuric add, 1 part 1,305,467-00 Sulphate of copper, 1 pound per gallon1 8. 150.000'00 Saturated solution of common salt 3,173.000'00 "” of sulpliate of zinc 17,330,000-00 Nitric acid, 30 B 1,606,OUO'OO The following table gives the specific resistance in ohms (an ohm is an amount of resistance equal to that exerted by of a mile of common galvanized iron telegraph wire No. 9) ot various metals and alloys, at 32° Fah. , according to the most recent determination of Dr. Matthiessen: NAME OF METALS. Res! starce of wirel foot long weigh ing 1 grain. Silver annealed ” hard drawn Copper annealed ” hard drawn Gold annealed ” hard drawn Aluminum nnnealed Zinc pressed Platinum annealed Iron annealed Njekel annealed Tin pressed Lear! pressed Mercury liquid Platinum silver aiioy, hard or annealed, used for standard resistance coils German silver, hard or annealed, commonly used for resistance 0-2214 O'24'Zl 0-:,0()4 0'2108 0-5310 O'R050 0-068;B 0-5710 3 -530 1-2125 l'07S5 1'317 3-236 18*746 2-653 2-391 Resistance of wire 1 foot long 1 - l,OOOth incli II diam eter. 9-936 9-151 9-718 9'910 12-52 12' 7! 17''72 32-22 55'09 59'10 75 '78 80'36 11SP39 600'00 127-32 66” 10 Approximate per cent. variation in resistance per degree temperature at 20 degrees. O-377 6*388 0:365 0-365 0-365 0-38? 0-072 0-014 0-065 Gold silver alloy, 2 parts gold, 1 part silver, hard or annealed... The use of this table is as follows : Suppose it is required to find the resistance at 32” Fah. of a conductor of pure hard copper, weighing 400 lbs. per knot. This is equivalent to 460 grains per foot. The resistance of a wire weighing one grain is found by the table to be 0 2108, therefore the resistance of a foot of wire weighing 460 grains will be but the resistance of one knot will be 6,087 times that of one foot, therefore the resistance required will be iUL8-Xxjp..1JiJi = 2^79 ohi us. If the diameter of the wire be given instead of its weight per knot, the constant is takon from the second column. Thus the resistance at 32° Fah. of a knot of pure hard drawn copper wire 0'1 inch in diameter would be ^TwVir-- = 0 '05. The resistance of wires is materially altered by annealing them, and a rise in temperature increases the resistance ot all metals. Dr. Matthiessen found that for all pure metals the increase of resistance between 32° and 212° Fah. is sensibly the same. The resistance of alloys is much greater than the mean of the metals composing them. They are very useful in the construction of resistance coils. '['he highest value which has probably been found for the conducting power of pure copper ia sixty times that of pure mercury, according' to Sabine, Commercial copper may be con»Wej!ed of good quality wliea its conducting power is o?or fifty,()fwr groatlf in sy<?<;.- Agassiz on Humboldt, ” Referring to Humboldt's career as an embassador at Paris, Professor Agassiz described a personal interview he had with him : ” His official position and his rank in society, as well as his great celebrity made him everywhere a cherished guest, and Humboldt had the gift of making himself ubiquitous. He was as familiar with the gossips of the fashionable and dramatic world as with the higher walks of life and the abstruse researches of science. He had at this time two residences in Paris; his lodging at the Hotel des Princes, where he saw the great world, and his working room in the Rue de la Harpe, wliere he received with less formality his scientific friends. It is with the latter place I associate him ; for there it was my privilege to visit him frequently. There he gave me leave to come and talk with him about my work and consult him in my difficulties. I am unwilling to speak of myself on this occasion, and yet I do not know how else I can do justice to one of the most beautiful sides of Humboldt's character. His sympathy for all young students of Nature was one of the noblest traits of his long life. It may truly be said that towards the close of his career, there was hardly one prominent or aspiring scientific man in the world who was not under some obligation to him. His sympathy touched not only the work ot those in whom he was interested, but extended also to their material wants and embarrassments. At this period I was twenty. four; he was sixty-two. I had recently taken my degree as Doctor of Medicine, and was struggling not only for a scientific position, but for the mean of existence also. I have said that he gave me permission to come as often as I pleased to his room, opening to me freely the inestimabfo advantages which intercourse with such a man gave to a young investigator like myself. But he did far more than this. Occupied and ^surrounded as he was, he sought me out in my own lodging. The first visit he paid me at my narrow quarters in the Quartier Latin, where I occupied a small room in the Hotel da Jardin des Plantes, was characteristic of the man. <After a cordial greeting, he walked straight to what was then my librarya small book shelf containing a few classics, th” meanest editions bought for a trifle along the quays, so ne works on philosophy and history, chemistry and physic s, his own 'Views of Nature,” ' Aristotle's Zoology,' 'LinnIT'us' Systema Naturre,' in several editions; Cuvier's ' Regne Animal,' and quite a number of manuscript quartos, copii s which with the assistance of my brother, I had made of> works I was too poor to buy, though they cost but a few frmcs a volume. Most conspicuous of all were twelve volumes of the new German Cyclopedia, presented to me by the publisher. I shall never forget, after his look of mingled interest and surprise at my little collection,his half sarcastic question as he pounced upon the great Encyclopedia : ' Was mochera Sie den rnit diescr E.s- e?s5rilcke.' What are you doing with this ass's bridge ?the somewhat contemptuous name given in Germany to similar compilations. ' I have not had time,' I said, • to study the original sources of learning, and I need a prompt and easy answer to a thousand questions I have, as yet no other means of solving.' ” It was no doubt apparent to him th at I was not over familiar with the good things of this world, for I shortly afterwards received an invitation to meet him at six o'clock in the • Gallerie V?tre«' of the Palais Royal, whence he led me into one of those restaurants, the tempting windows of which I had occasionally passed by. When we were seated, he half laughingly, half inquiringly, asked me if I would order the dinner. I declined the invitation, saying that we should fare better if he would take the trouble. And for three hours, which passed like a dream, I had him all to myself. How he examined me, and how much I learned in that short time! How to work, what to do, and what to avoid; how to live, how to distribute my time; what methods of study to pursue these were the things of which he talked to me on that de- • lightful eveningI do not mention this trivial incident without feeling that it may seem too familiar for the occasion; nor should I give it at all, except that it shows the sweetness and kindness of Humboldt's nature. It was not enough for him to cheer and stimulate the student; he cared also to give a rare indulgence to a young man who could allow himself few ENGLISH TELEGRAPHS.The Postmaster-General of England asks for £6,750,000 to buy up the electric telegraphs of the kingdom, and seems likely. JiQ get the money. In return, he not only promises cheap messages, a large increase in the number of offices, and other facilities, but after paying interest on the sum expended, he will have, in the first year, a surplus of £77,000. This surplus will of course increase with the increase in the number of messages dispatched. As regards the scientific part of the question, it is thought that the demand for improvements will be 00 eonstant, that invention will bo stimulated! and that We Bliall see a BUCCe88ion of methods fa,' applying the wondrous power of,Dl(1ctro=mag>wt« htn mtxtoHtog All hit]i,(.'Ho compared. 148-35 © 1869 SCIENTIFIC AMERICAN, INC.