experimenter. Considering, however, that four inches is a long striking distance through dry air, and not happening to be successful when attempting to repeat the experiment according to Mr. H.'s directions, I have been induced to suggest to that gentleman the necessity of his repeating the experiment, under the following cir cumstances: First. Let the table on which the jar and chain are placed be perfectly dry, and of hard wood (say an old mahogany table that has frequently been rubbed with bees'-wax, or with anything else to render it a nonconductor). Let this jar of 160 square inches of (interior) coated surface, be charged to the intensity of 80° per quadrant electrometer; every other part of the circuit being as he has described it. Discharge the jar through this circuit. Secondly. Let the same arrangement again be made, only with this difference: draw on the table a narrow line of water (four inches long), from the outside of the jar to the extremity of the chain. Discharge the jar through this circuit. Should there occur different results in those experiments (as I am persuaded there will), why did not Mr. Howldy mention such essential particulars? Or are we to conclude from his silence, that he entertained just notions of the nature of the experiment, and that this circumstance was too trivial to notice? 66 With respect to the method in which the wooden peg is used, I am aware that it will answer very well if regulated upon the principles stated in my paper: that is, when the moisture it contains in a transverse section is proportioned to the charge transmitted; and that, whether the point be sharp or blunt, and in whatever part of the circuit it may be placed. If its being very dry" were essential to the success of the experiment, it appears strange, that when it possessed that qualitity in a superior degree, by "two or three experiments," it should be "rendered useless.' Its having a sharp point, too, may possibly be the means of its possessing properties which I have not seen. * I should, however, recommend those persons who use a piece of wood, to have it somewhat longer than that described by Mr. Howldy, for fear of meeting similar disappointments to those which he met with when varying the distance between the outside of the jar and the extremity of the chain with high intensities; as it is possible that the peg may be shorter than the striking distance. Whenever an electrical discharge is transmitted through water, for the purpose of igniting gunpowder, the length of the aqueous column ought always to exceed the striking distance; for if the wires which enter the extremities of the column are brought too near each other, the electric fluid will dart from one to the other with very little interruption, and in all probability will scatter rather than ignite the gunpowder. To insure success, the column or train of water, or any substance containing it, such as wood, twine, silk, paper, &c., should never be shorter than six inches; and the thickness or quantity of water contained in a transverse section must always be regulated according to the nature of the charge; that is, to the quantity and intensity of the electric fluid employed. When a small jar is used for this purpose, then the strip of water must be very thin, or narrow: if a larger jar be used, the thickness of the aqueous column may be increased. When the same jar is used with different intensities, then the lowest intensity requires the thinnest strip of water; not because the intensity is lower, but because the quantity of fluid is less and although a column of water that answers for a low intensity will answer for every intensity that is higher, nevertheless a column that would answer very well for high intensity, might be far too large to insure success with an intensity that is very low. As glass tubes are both convenient and elegant for regulating the diameter of a column of water; about seven or eight inches of barometer tube, of an inch diameter, answers the purpose very well, with any jar containing more than half a square foot of coated surface on each side, with any intensity of charge above 50° per quadrant electrometer.* By employing a glass tube of the above dimensions filled with water, and a jar containing 120 square inches of coated surface on each side of the glass, gunpowder was ignited in the circuit at every trial, with any intensity above 20°. When two such jars were employed at the same time, gunpowder ignited with every intensity above 10°; sometimes with an intensity as low as 7° or 8°. The gunpowder was fine grained, and of the best quality. If the gunpowder is bruised to a fine powder, it will answer still better. When moistened thread, twine, or wood is used, any of them may be cut in two, and separated a little, and the gunpowder will take fire at this interruption of the watery part of the circuit. Thus we learn that it is not necessary for metallic conductors to be in contact with the gunpowder to insure its ignition by the electric fluid. A chain may be made of alternate links of copper wire and tailor's thread, in such a manner as not to be easily distinguished from one that is all of metal. If such a chain be dipped in water, and made a part of the electrical circuit, gunpowder may be ignited in any part of that circuit, as though the thread were one continued piece. When one of the above-mentioned jars was employed, and a copper conductor formed the circuit, ether was fired at an interruption, with every intensity above 20°. When the water tube formed a part of the circuit, ether could not be fired, though both jars were employed together, with an intensity of 90°. As quadrant electrometers do not afford uniform measures of electrical intensity, but differ from one another according to the weight of the ball, delicacy of suspension, &c., an intensity is here given, which, it is expected, will answer with the generality of them. When four such jars were charged to the highest possible intensity, and discharged through the water tube, gold leaf placed in the circuit was powerfully attracted, but not deflagrated. One jar with an intensity of 60°, powerfully magnetized a sewing needle, placed in a spiral forming part of a complete metallic circuit. With two jars charged to the highest possible intensity, no magnetic effect was produced when the tube of water formed a part of the circuit. How do these experiments accord with the doctrine of quantity and intensity? In the galvanic process, a needle may be powerfully magnetized by a single pair of small copper and zinc plates; by which process, it is said, we have quantity, but not intensity. But by the experiments I have just now mentioned, it appears that quantity has not the power of magnetizing needles, without intensity. For when the electric fluid was not retarded by inferior conductors, a very small quantity produced the effect; but when the intensity of the discharge was reduced, although three times the former quantity, at least, was employed, not the slightest magnetic effect was produced. The truth is, we want another and a more appropriate term in electricity that term is momentum. : Intensity answers very well to express the relative degrees of concentration on the surface of jars, &c., but momentum is the proper term when the fluid is in motion. Hence, then, although the less quantity of the fluid produced on the needle an effect which the greater was not able to accomplish, this effect was probably owing to the momentum of the former exceeding that of the latter. When tow or cotton is moistened with spirit of turpentine, or mixed with powdered rosin, it will ignite by a very small electrical discharge when the whole circuit is of metal. But if the tube of water form a part of the circuit, the tow, or cotton, prepared as above, will not ignite, although two of the before-mentioned jars charged to the highest possible intensity be discharged through the circuit. When a narrow slip of gold leaf was placed between two pieces of glass, and made to form a part of a complete metallic circuit, one jar with an intensity of 8° discharged through that circuit, completely exploded the gold leaf. When two jars were employed at the same time, and charged to a still higher intensity than the former, and the water tube entered into the circuit, a similar slip of gold leaf subjected to the discharge, remained uninjured. When gold leaf and gunpowder were subjected at the same time to a discharge similar to the above, and the whole circuit of metal, the former was completely exploded, but the latter substance was scattered only. When the water tube formed a part of the circuit, every other part arranged as before, the gunpowder ignited, but the gold leaf was undisturbed by the discharge of the jars. Similar experiments were made with gunpowder and needles, gunpowder and ether, gunpowder and tow, prepared as above. When the circuit was completely metallic, the needles were magnetized, or the ether, the tow, &c., were fired; but the gunpowder was in no instance ignited. When the water tube formed a part of the circuit, the gunpowder was, in every case, ignited; but the other substances remained unaffected. Hence we may conclude, that in order to magnetize pieces of steel, to explode metals, to ignite ether, or tow, with resin, &c., by electricity, quantity and velocity, or momentum of the fluid is required; but to ignite gunpowder, quantity and time are indispensable: that is, when the quantity is constant. To produce the former effects requires velocity, to produce the latter effect, time. Artillery Place, Woolwich, I remain, Sir, W. STURGEON. ELEMENTARY LECTURES IN ELECTRICITY, ETC. LECTURE XVIII. IN persuing electrical enquiries from the productions of the machine to those of nature, we find every principle that is exhibited from the one source, corresponds with those displayed from the other from the simple attractions and repulsions, to the most magnificent phenomena producible by electric action. The air surrounding this globe of earth being intermixed with the electric fluid, as decidedly as the materials of the earth itself are charged with that subtle agent, it is reasonable to suppose that electro-fluctuations take place in the atmosphere as well as in the earth, by every change of temperature, hydrometric condition, &c., to which it is exposed; and as these changes are frequent, rapid, and occasionally great, they are attended by corresponding sudden, and great changes in the electric condition of the aerial shell. It is a remarkable fact that, in a still, cloudless atmosphere, the electric condition of it, as high as has been explored, is that of a gradually increasing charge, from the earth's surface upwards; and as this is uniformly the case, under these circumstances, it becomes an establised fact in atmospheric electricity, which may be conveniently employed as a standard, or normal condition, in studies of this subject. The ratio of electric increase with increasing atmospheric altitudes having never yet been determined, we are not in possession of any definite law; and as different results have been obtained by different observers, under circumstances apparently similar, the data hitherto collected are too uncertain, and too scanty, to form any basis whereon mathematical assistance could be available in the establishment of any correct law on this interesting topic. Notwithstanding, however, the want of precise theoretical laws on the electro-distribution in the atmosphere, being an acknowledged fact, the desideratum is pleasingly, though partially supplied to the observer, by the unfading picture which is formed in his mind, of the certain increase of electric agency at increasing altitudes from the earth's surface; every succeeding stratum being electro-positive to every stratum beneath it; and the whole atmosphere electro-positive to the globe which it surrounds. The easiest and surest mode of ascertaining the different electric conditions of the atmosphere at different altitudes, is by means of a series of kites, with a wire strand in each string The kites, which ought to be four or five in number, are to be floated at the same time, and with different lengths of string, from a hundred to a thousand or more yards. Let, for instance, the lowest of a series of five kites have only one hundred yards of string, and the highest one thousand yards. Under favourable circumstances the former will attain about seventy yards, and the latter between eight and nine hundred yards of altitude; and the intervening three kites will float at different altitudes between these two extreme ones of the series. If now, we place the ball of a Leyden jar to the insulated string of the lowest kite, it will become charged to a low intensity in a certain period of time; and by applying the ball of the jar to an electroscope, and testing the character of the electric action, either by sealing wax or by glass, in the usual way, we find that it is positive. We The same operations are to be proceeded with at the other kite strings, and it will be found that the whole of them display positive electric action; and with an increase of intensity in the charge of the jar, from the first to the highermost kite in the series. next proceed to ascertain the relative difference of the electric actions of the kite strings, by bringing two of them at a time in their insulated state, close to each other, and we observe a spark pass between them; and after satisfying ourselves by these means that, although the whole series are positive to the ground, they are positive and negative with respect to each other: we next insulate a jar, and by connecting one kite string with its exterior surface, and another with its interior one, the jar becomes charged, and in such a manner, that the highest or longest string of a pair, invariably communicates the positive charge to the glass, whether it be the inside or outside of it with which it is in contact. There are, however, frequent cases, whilst experimenting with kites, in which that with the longest string will not be the highest; under such circumstances the intensity of the charge of such a string, is not so great as that of a shorter, whose upper end is much higher; although the quantity obtained by a discharge from the former is occasionally somewhat greater than that discharged by the latter. Now, since there is a general law in electricity by which we are enabled to understand that the fluid cannot be transmitted from one body to another, unless the former be positive to the latter, it becomes obvious that the exploring wires in the kite string |