electrical discharge that has taken place on the earth; but, on the other hand, should the electricity not pass off to the earth, but be diffused through a mass of negative cloud, as soon as the transference takes place, this before comparatively-white cloud will assume the gloomy appearance of the primary thunder cloud, but never quite so dark to the eye, as a portion of the electricity was passed off to the earth with the rain. A thunder storm of the kind I have just detailed may be attended with another phenomenon which has been often set down as the echo or reverberation of the primary peals of thunder. This is caused by one of the most beautiful, yet no less wonderful, laws attending the movement or transference of this universal force. The idea of an echo or reverberation of sound is, when we take into view the materials (clouds) that are to reflect it, one that, on a little consideration, will appear to be quite untenable, and only tends to show the utter absence of the application of well known laws to explain these apparently enigmatical, but equally simple, occurrences.

To explain this, let us suppose that which often takes place according to the circumstances of the atmospherical currents, that another series of clouds are in the same stratum of atmosphere, and, at the same time, exactly opposite the series where the thunderstorm is taking place, and the direction of the discharge from north to south, as already predicated. The consequence of this state of things would be, that another series of actual discharges, accompanied with thunder, would take place in the other opposing series of clouds, but from south to north, and at the same time, much weaker in all the accompanying phenomena. This is caused by the law of electrical induction, which shows us, that no disturbance of electrical equilibrium can, by possibility, take place in one body, or a series of them, without causing a similar disturbance in any other conducting body in the vicinity. A series of clouds may be above or below the primary series, or they may be in almost any given position as relates to the primary, yet they will be necessarily disturbed by the first great discharges, and weaker peals of thunder will be the result, and at very opposite points of the compass.

All these various discharges will be attended with large dropped rain, and, if there is any echo or reverberation attending a thunderstorm, it must be in alpine districts, where the sound is re-echoed from hill to valley, and, even here, such must necessarily be of rare occurrence, as the peaks themselves serve as so many conductors that extract the electrical intensity from the clouds, while the water is descending in drizzling rain.

It is this attraction of the clouds to mountains that gives rise to many of the phenomena that influence the seasons in large districts, as on the Chilian and Peruvian side of the Andes we find a rainy season for many months of the year on the one side, while, at the

same time on the other side of the same ridge the land and vegetation is parched and dried for want of water.

With my present views, the explanation of this or similar phenomena becomes at once simple and easy. It may be said the Andes occupy the sea coast of one side of South America, with the vast Southern Pacific stretching out at their feet; while, on the other side, the immense plains of the Pampas extend for many hundred miles before they reach the Atlantic. In consequence of this relative position, as respects the ocean on the one hand and the land on the other, the mountain-peaks on the side next the Pacific attract the clouds that are formed by the water evaporated from its surface; but, from the height of the mountains themselves, they being much higher than that portion of the atmosphere usually occupied by clouds, rain can never get an opportunity to fall on the other side, because the clouds are really stopped in their progress and annihilated before they can by possibility reach it. This attraction of clouds by mountains has been long known and observed; but the true explanation is, not that the mountain attracts the cloud per se, but that the mountain, as a conductor of electricity, attracts the free electric fluid contained in the cloud to its vicinity. The cloud is, consequently, discharged of its electricity; then follows a precipitation of the water in the form of rain.

There are, occasionally, rare exceptions to this rule, where it is observed that the clouds are not always attracted by mountain peaks, and, when driven to their vicinity by a current, do not always discharge their rain. In such cases, the mountain is in the same electrical condition as the clouds, and, consequently, no action will take place in either body.

ROYAL VICTORIA GALLERY, MANCHESTER.
CONVERSAZIONE, JANUARY 6TH, 1842.

In Continuation of a Communication read on the 9th December, 1841.
By C. W. WILLIAMS, ESQ.

On the last occasion of my addressing this Institution, I explained the causes of the prevailing injuries to boilers, and proved that we were mistaken as to some of them. I shewed that the sediment in boilers assumes two different forms-namely, that of a chrystallised solid incrustation, and that of a loose mud-like body, held merely in suspension.

I proved that the first could not be the cause of injury, inasmuch as it was, of itself, a good conductor of heat; whereas the secondthe floating matter-became a positive non-conductor, after the

boiler had been suffered to remain some hours at rest, and this matter allowed to subside and become dry and hard.

[Mr. Williams then exhibited several specimens of this sediment, and which had all the appearance of a solid mass, and as hard as baked clay.]

Since our last meeting I have pursued the subject, and feel confirmed in the statements then made. I then stated, that in land engines no injury could arise to the plates of boilers from anything which might arise from the furnaces, beyond the ordinary wear and tear, and where no artificial blast was used, if due attention were paid to cleanliness in the interior, and maintaining the water at its proper level.

Marine boilers, however, had other inconveniences to contend with, as exemplified by the models then exhibited. These were formed of a number of narrow vertical passages, in which the steam, as fast as generated, became so mixed up with the water, that a free circulation of the latter, and its continuous access to the heated side plates of the furnaces, was much obstructed. The parts thus most exposed to the greatest heat became most liable to be injured by the impediments which these narrow passages interposed to the free ascent of the steam, as fast as generated, and to the free access of the water to the plates-this latter being the result of the rapid ascending current of steam in those narrow passages.

Again, when we consider that these narrow passages, usually but five or six inches wide, (and not unfrequently as narrow as three inches,) have to conduct the steam from two furnaces, one on each side, it may well be considered doubtful how a sufficient supply of the recipient, water, could be preserved in continual and adequate contact with the plates.

I also observed, that in marine boilers bulging or rupture was not known, except where there was a deficiency of water, or, what was equally injurious to the plates, when, from the peculiar construction of the flues, the free circulation of the water was impeded.

I thus shewed, that this obstruction of the circulation of the water, by which, in point of fact, steam, instead of water, was most in contact with the plates, was equally injurious, in marine boilers, to the water getting below the proper level; that, in either case, the result was, that the recipient, or absorber, being changed from water to steam, the heat could not pass as rapidly through the latter as it was received by the metal; that, as already observed, the conducting power of the metal was therefore obstructed, accumulation of heat took place in the plate, and the inevitable result was, softening, expanding, bulging, and rupture.

I will now, observed Mr. Williams, give an illustration of these facts. I have here (exhibiting two models) one a vessel containing water, and to which the heat is conducted by an iron pin, and the other by a pin formed of incrustation; each three inches long, and

three-quarters of an inch in thickness. On the last occasion, I stated I had not ascertained which was the best conductor; having now tested them accurately, I found that the advantage was in favour of the iron, as seen by the table I now exhibit. The water being at 46 degrees of temperature at the commencement, in both cases, that in the vessel with the iron pin conductor reached the boiling point (212 degrees) in 13 minutes, while that with the incrustation conductor required 17 minutes.

These conductor pins, we see, were passed into the flame, and the great heat was transferred, longitudinally, through them to the water; yet the heat transmitted through that great length of incrustation (three inches long) was very nearly equal to that of the iron. That I consider quite conclusive on the point, that no injury to the plate could occur from the want of an adequate conducting power in the incrustation; for if heat can be conveyed with sufficient rapidity through incrustation three inches in length, we can have nothing to apprehend from such thickness as is met with on boilers, and which rarely exceeds one half-inch on those parts of the boilers where the heat was sufficient to injure the plate.

When these boilers were removed from the heat, these conductors were found to be at so low a temperature as to bear the hand being placed on them.

Mr. Williams then exhibited some diagrams, representing plans and sections of the first boilers of the "Liverpool," as forcible illustrations of the injurious tendency of these narrow passages, or water spaces, to obstruct the contact of the water with the iron plates. In that vessel's boiler these water spaces were above five deep, and but five inches wide, by which injudicious arrangement the steam, generated by the hottest plates forming the sides of the furnaces, was necessarily so great, in quantity, that the recipient might be said to be steam instead of water, and consequently that those plates necessarily became overheated, bulged, and ruptured.

Here a large plate taken from the furnaces of the "Liverpool' was exhibited, satisfactorily illustrating the above reasoning. This plate was bulged and twisted, manifestly from having been red hot, and exhibiting some large patches and cracks, proving that the plate must have been overheated in consequence of the absence of water to absorb the heat.

Mr. Williams proceeded to state that this fact had been fully established by an experiment made by the engineer, who inserted a trial pipe into one of the boilers, the inner end of which opened into the water space between two of the furnaces, and on the line of the fuel on the bars; the outer end projecting into the engine room, and furnished with a cock. On trying this pipe, when the furnaces were in action, the engineer stated that he never could draw off any thing but steam, and from the spot of all others which demanded the

uninterrupted presence of water; and this, although the place was some feet below the general level of the water in the boiler.

This, then, clearly established the point, and accounted for the overheating, bulging, and cracking of the side plates, which continued every voyage to exhibit proofs of similar overheating, and to require constant repairs, while the roofs and other parts of the furnaces remained sound and perfect, with but a single exception.

That exception occurred on the occasion of the blow-off cocks having inadvertently been left open: the consequence was that that section of the boiler, containing two furnaces, was entirely emptiedthe roof and sides became red hot, and all collapsed. That section of the boiler had to be bricked up and thrown idle during the remainder of the voyage.

The point, therefore, to which I am desirous of directing attention (observed Mr. Williams) is this, that in seeking to protect the plates of boilers from injury, it is not to the fire, nor furnace, nor plates, our attention should be directed, but simply and solely to the nature of the recipient to which the heat is conveyed, as on this rests the whole question of injury.

Now this will naturally be cavilled at by those who have all their lives been talking and writing of the danger of hard firing and incrustation, and the want of due proportions in the fire and flue surface; yet I state the proposition broadly, that if we look to the recipient and its heat-absorbing properties, and attend to the interior of the boilers, we shall do all that is possible in the way of preventing injuries, except so far as regards over-pressure, but which we are not now considering.

Let us now inquire what are the several recipients of heat which present themselves in ordinary boilers; they are—

1-Water
2-Steam
3-Air

4-Incrustation deposit-crystallized
5-Loose deposit-uncrystallized

These are the recipients of heat which are to be met with in all boilers; but in marine engines there is no apprehension from the loose deposit on its subsidence, because it falls to that part of the bottom which is cool. Not so, however, in land engine boilers, where the lowest part, as in cylindrical boilers, is that which is exposed to the greatest heat, and hence the frequent cause of injury in the absence of internal cleanliness.

Thus we see, that so long as the water remains in contact with the plates, the latter can sustain no injury; since, being so excellent a recipient of it, the stream of heat (if we may so speak) is absorbed as rapidly as it is passed through the metal, and consequently the plate itself the conductor is unaffected beyond a certain point. What