aphelion and perihelion, the nearest and most distant points. Thus, then, the amount of the deviation of the motion of the sun, at any point of the ecliptic, from his mean motion, is dependent on the position of the perihelion of the earth's orbit in the ecliptic; moreover, this position is varying from


to year. Here, then, is another and most important cause of the variation of the time of true from that of mean noon, by reason of which cause alone it may be calculated, that at certain periods of the year the'time of true noon would differ from that of mean noon by about 8' 20" of time.

It has been before stated that, by reason of the obliquity of the ecliptic alone, the times of true and mean noon might be made to differ 10'3:9” of time. If, then, the time of greatest variation from the one cause coincided with the time when the greatest variation takes place by reason of the other cause, then both thus conspiring, the whole variation of the time of true from that of mean noon would be not less than 18' 23:9'. But this is not the case, and the maximum interval between the time of noon as shown by a good clock keeping mean time, and the true time of the sun's passing the meridian, never exceeds 16' 17" of time. Moreover, by reason of the irregularity introduced by the elliptic motion of the earth, the coincidence of the true and mean noon at the equinoxes and solstices is destroyed, and true noon is shown by the clock, not at the periods of the equinoxes and solstices, but at the following periods,--the 15th April, the 15th June, 1st September, 24th December. The sun gains upon the clock between the two first of these periods, loses during the second, and gains again during the third. It is behind the clock by its greatest interval of 14' 37" on the 1lth of February, and before it by its greatest interval of 16' 17" on the 3d of November.

There are three methods of measuring time, commonly in use among astronomers.

1. It is measured by sidereal time, which is regular, being governed by the regular revolutions of the earth upon its axis, as shown by successive returns of the meridian to the same star.

2. It is measured by mean time, the nature of which has been sufficiently explained in the preceding pages, and the method of regulating an astronomical clock, so as to show that time (see p. 358); this time, like sidereal time, is uniform, being dependent upon the period required by the earth to make one complete revolution in her orbit.

3. Solar, or true time, as it is called, which is measured by the time between two successive noons, or actual passages of the meridian of any place over the sun; and that time not being the same at all seasons of the year, it follows that solar time is irregular, and that the solar hour, which is the 24th part of the solar day, has not exactly the same length on any two successive days.

The difference between true and mean solar time, explained in the preceding pages, is called the equation of time. Clocks, called equationclocks, have been so constructed, that whilst one of their hands shows on the dial-plate mean time, the other points to true time. The mechanism of a clock, whose hand is to follow the irregular course of the sun, through each quarter of the year, is, however, so complicated, that little dependence can be placed upon it..

The sidereal day, which, like the solar, is divided into 24 hours, commences at the instant when the meridian, at the place of observation, passes over the equinoctial point Aries, and terminates when it returns to that point.

Thus the time of the sidereal day, when the meridian passes over any particular star, is the time which it takes to revolve from the equinoctial point Aries to that star; and since it revolves regularly through 15° in every sidereal hour*, it is manifest that, allowing at the rate of 15° for each sidereal hour shown by the clock, (or, as it is called, converting the time into degrees,) we may ascertain at once the right ascension of the star, which is no other than the number of such degrees intervening between it and the point Aries, by observing the sidereal time when the meridian passes over it.

Since the right ascensions of all the principal stars have been accurately ascertained; by observing the sidereal time shown by the clock when the meridian passes over any such star, we may conversely ascertain whether the clock be right or not; and it is thus that astronomical clocks are regulated.

Solar time is found by observing the time of two successive passages of the sun over the meridian, and dividing the interval into 24 hours. It is the time shown also by a well-constructed sun-dial.

Mean time is found by observing the true time, and allowing, according to the table of equation of time, for its difference from true time. Thus, to determine the mean time of noon, we should observe by our clock the true time of noon, or the exact time of the meridian passing over the centre of the sun. If then we deduct from this, or add to it, the equation of time for the noon of that day, the result will bring us to mean noon.

There is yet another, and practically a better method. If a clock be set to true mean time, the stars will every day complete an apparent revolution, that is, the meridian itself will complete a real revolution,precisely 3' 55:9” before the hour-hand has completed its revolution of 24 hours on the dial. Observe, then, two successive transits of a star; at the first set the hour-hand at 12, and regulate it so that at the second it shall show 3' 55.9" short of 12. It will then be regulated so as to show mean time.

It only remains to set it at the mean noon, as explained in the preceding page.

* This is evident from the fact that it completes its revolution of 360° in 24 sidereal hours.





It is now my intention to introduce to your notice a most remarkable and highly-interesting gaseous element, namely hydrogen. Before, however, you can succeed properly in evolving it, there are several manipulations requisite to be performed, which are as follows:

In the first place, you must obtain a pound or two of the metal zinc, and melt this in an iron ladle over a bright fire, just as you would so much lead. When perfectly melted, pour the metal very slowly from a height of three or four feet into a large pailful of cold water; it will hiss and spatter a good deal, but you need not be alarmed at that, for there is no danger. It is best not to hold the ladle so that all the melted zinc falls exactly in the centre of the water, but to carry the ladle round and round, so that the melted metal may be more dispersed in its fall. Now having done this, empty the water from the pail, and you will find at its bottom the zinc in small irregular pieces, of various sizes; collect them together (and be careful how you do this, for some of them are very sharp, and will wound the fingers very severely), and place them on a sieve, or in a large funnel to drain.

The process of thus reducing zinc into small particles is called granulation, and the result, granulated zinc; it is a very convenient form of the metal for many operations. You would find it a matter of no small difficulty to break up a mass of zinc by mechanical means,—with a chisel and hammer it would take you a long time ; but here, by the chemical agency of heat, you destroy the cohesion of the metal in a few minutes; and by pouring it in the melted state slowly into water, it solidifies into small masses. This operation of granulation is not confined to zinc alone; many other metals may be similarly treated, as will be fully shown hereafter.

The granulated zinc must now he dried by spreading it on the hob of the grate, and whilst this is doing, you may proceed to another operation.

Select a soft, sound, and accurately-cut cork, that will tightly fit a narrow-mouthed pint glass bottle, and with a 66 rat-tail” file or make a circular hole very neatly through the cork, so as to admit one end of a foot of common gas-tubing," either of pewter or copper.

If this does not happen to fit as accurately as you could wish, melt a little bees'-wax around it; the tube must now be bent into a shape something like the letter S turned thus, in. A little knack is required in thus bending the tube so as to make two regular and handsome bends ; for if you

do not mind what you are about, you will get a very ugly shape, flat in some parts, and round in others, on account of the unequal strain upon the metal; but if you stop up one end of the tube with a small cork, then fill it full of dry sand, and stop up the other end also with a

render the tube much more manageable, because as you bend it, the enclosed sand resists any unequal pressure, and supports all parts



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of the metal alike, and thus you obtain a very neatly bent tube; now remove the corks and shake out the sand. Put an ounce or two of granulated zinc into the bottle, and about half fill it with water ; and having arranged your pneumatic trough, and all its bottles and jars with cold water, as directed for oxygen, pour into what we will call the gas-bottle, that is, the glass bottle, some strong sulphuric acid (oil of vitriol), until you find a tolerably brisk effervescence ensue; then insert the curved pipe; wait a minute or so before you plunge its extremity under the water of the trough, in order that all the common air may be thoroughly expelled by the hydrogen which is evolving, for the effervescence is due to its liberation. Then proceed to collect it, by causing it to bubble through the water, as for oxygen or chlorine; you will find it come over very readily and very abundantly; but should the action fag after a time, empty out the liquid contents of the “gas-bottle,” leaving only the zinc, and insert a fresh charge of water and acid as before, and thus you may go on evolving hydrogen until all the zinc is entirely dissolved. Now you find that the hydrogen gas collected in your pneumatic apparatus is as perfectly invisible as common air or oxygen ; but perhaps during the operation of collecting it some has escaped, and you have observed a peculiar smell; this, however, is owing to impurities in the zinc employed in the experiment; perfectly-pure hydrogen is devoid of smell, but it is a most difficult matter to obtain it. However, what you have collected is quite pure enough for all your experiments; and now proceed to examine its properties.

Open a bottle of the gas, and immerse a lighted taper,—the hydrogen instantly takes fire, with a very pale flame, so pale, indeed, as scarcely to be visible in broad day-light; it is therefore an inflammable gas; but you will remark that the taper is extinguished,-so that hydrogen

- -although inflammable, and highly so, as subsequent experiments will show, yet will not support combustion. You will have a better opportunity of examining the flame of hydrogen if you make a hole through another cork to fit the large end of a long piece of tobacco-pipe, and placing this in the neck of the gas-bottle, after putting in a charge of zinc, water, and acid; allow the effervescence to proceed for about two minutes, and then apply a bit of lighted paper to the other end of the tobacco-pipe, the hydrogen will instantly take fire with a sharp pop, and burn for a very long time with a beautiful pale flame. This arrangement constitutes “Priestley's philosophical candle;” rather an unlucky term by the way, because, although a " philosophical,it is by no means a luminous candle, being the purest and palest form of flame hitherto known. Place it in dark room, and you will barely be able to distinguish the printed letters of a book even when held very close to it; but although thus non-luminous, it is most intensely hot; it will readily kindle a bit of paper or wood, and heat a bit of iron to a very high temperature, or if you happen to have a bit of thin platinum-wire amongst your “ chemicals," hold it in the pale flame and remark how intensely the wire is ignited, not burned, remember, and how much light is now suddenly evolved, by the introduction of this solid, and almost infusible and incombustible metal. The book actually becomes legible now; but remove the platinum-wire and it is no longer so.




All these facts regarding the increased light of pale flames by the introduction of solid bodies, will come before you on a future occasion; and therefore I do not enter upon them in detail at present.

Now, in the first experiment, with the bottle of hydrogen in the pneumatic trough, and also in the second, with the “philosophical candle," the gas took fire with only a very slight pop, scarcely meriting the title of an explosion, because it was tolerably pure and free from admixture with common air; but supposing this to have been present, what result would have taken place ? why, the hydrogen, instead of burning quietly, would have burned with explosion.

Let us take the “philosophical candle,” to illustrate this point. You find it burning quietly on, because the combustion is supported by the air around, which comes gradually around to answer the demand of the flame for oxygen, which, as I have frequently told you, is one of the constituents of air; but, supposing that we applied a light to the end of the tobacco-pipe, the instant after the acid was poured on the water and zinc, what would have been the consequence? why an explosion,- because the materials only occupying about one half of the bottle, the other half was filled with air; and when the first portions of hydrogen were evolved, mixing with this they had oxygen enough to support their combustion, not slowly, but rapidly, and the whole arrangement would have been blown to pieces by the explosion.

Hence, in experimenting with hydrogen, you should always be cautious to let the common air be completely expelled before you attempt either to collect or inflame the gas, otherwise a serious accident may happen to you, by the fragments of the glass apparatus being violently scattered around by the explosion.

You can safely make an experiment which will satisfy you regarding this matter, by taking a bladder rather more than half full of air, and then tying it on to the bent-tube of the gas-bottle, or the pipe of the “philosophical candle," until its inflation is completed with hydrogen; then, removing it from the tube, hold a lighted taper to its neck,-the mixture of air and hydrogen will explode with great violence, rending the bladder into threads.

Take a tin tube, about two inches in diameter and a foot long, closed at one end; fill it with water in the pneumatic trough, as you would any other vessel; then transfer into it six parts or small glasses full of air, and two of hydrogen,-apply a light to this, you get a loud explosion, and there is no danger of any accident happening. Do not attempt the experiment in a glass tube of similar size.

Supposing that you again take this tube and transfer into it one measure or part of oxygen, and two of hydrogen, and put a lighted taper to this mixture, you will obtain a yet more rapid and powerful explosion, because the oxygen being pure, and unmixed with nitrogen, as in atmospheric air, the hydrogen combines with it far more intensely and eagerly.

Next to the inflammability of hydrogen, its levity is the most remarkable character, and we will now proceed to examine this.

Take two bottles of hydrogen, place one of them on the table, with its mouth wpwards, and remove the stopper; hold the other with its


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