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in a very short time, a shoal will be found occupying the same position as the one removed. If they are the result of any geological obstruction, that obstruction must be removed. If produced by any causes arising out of the condition of the river, the cause must be removed by altering that condition that gave rise to them.

The causes of the existence of shoals in the bed of a river, are the extension of the cross-section of the stream beyond its normal breadth, and for this reason we find also generally shoals where the current passes diagonally across the bed from one concave shore to another, or where the river is divided by a sand-bank or island. If the breadth of the current be increased, its depth must be diminished, in order to equalize the sectional area, which the river is able to keep open. In either of the above cases, as in all cases of shoals formed by incidental causes, the cause being removed, the river removes the effect, and relieves itself. The water of a river, if kept in a regular, uniform channel, of a normal breadth, will always keep it clear of shoals. To assert that the same volume and velocity that forced their way, and dug a channel for themselves from the mountains to the sea, (provided the one be not dissipated, the other diminished, and the action of both partially neutralized,) could not remove a few feet of sand that obstruct their channel, would be an absurdity, and a contradiction of the principles of Hydrodynamics.

A shoal in the channel of a river is sometimes merely temporary and accidental: the last flood produced it, and the next may sweep it away. Where, however, it continues permanently fixed, and subject to very little change from the action of high or low water, it is the result of some regularly and steadily-acting cause, or in some instances, to speak more correctly, it owes its existence to the diversion and dissipation of the power that could have removed it.

The bar at the mouth of the Mississippi is of this last character. It is an immense accumulation of mud and sand in the mouth of the river, 100 or 120 feet higher than the bottom of the river at NewOrleans. It has a depth of 12 feet of water on it, which has continued for years without any material change. This accumulation of sedimentary matter is the result of certain causes which, for as many years back as we have any information, must have combined to form it and keep it there. It is a remarkable fact, however, that, no matter how the river may swell with floods, or the gulf be agitated by storms, the average depth of water on the bar remains the same. This is a positive proof that there is a fixed cause, or combination of causes, which has produced it; and that, while the storms from the Gulf do not add to it, neither can the floods of the river remove it, in the present condition of its channel. It is a proof also, if any were wanted, that any temporary removal of that bar, by dredging operations, or otherwise, while those causes still remained to reform it, would be a mere waste of time, labor and money.

The bar owes its existence to the fact, that the scouring power of the river is nearly exhausted before it reaches the gulf, by the divisions of its volume into so many passes and innumerable bayous, while it is still further checked by the resistance offered to it by the

dead-water of the Gulf, and by its too great and unnecessary expansion near the mouth.

The volume of water in the river which by its weight and velocity at New-Orleans makes a channel of upwards of 130 feet deep, is drained of a portion of its power before it reaches the passes, where it is divided between the several channels that run towards the Gulf. Yet the quantity that enters the passes is still further divided by the bayous, or lateral outlets, which run off on either side, dissipating its strength, and almost entirely destroying the velocity and power of the current, until enough only is left to keep 12 feet of water on the bar. Were the same volume of water that at New-Orleans makes a channel of 130 feet deep, kept undivided until it reached the Gulf, so that its whole power might remain concentrated, the bar would be swept away far into the Gulf; to harmless distance. No bar could stand before the volume and velocity of the Mississippi river, if the first were not divided, the latter diminished, and the power of both broken before it reaches the Gulf.

The resistance of the dead-water of the Gulf is another agent in keeping the bar at the mouth of the Mississippi. This, when opposed to the divided volume, and almost exhausted velocity of the current, checks it still further, and causes a sufficient deposit of sedimentary matter to maintain the bar in its present condition, which, were it not for that resistance, even the present diminished volume and velocity of the water in the passes would partially wash away. The fact that the resistance of the water in the Gulf, passive or active, has not been sufficient to increase the bar, and entirely close up the mouth of any of the principal passes, shows clearly, that the water of the river that flows through the mouths of the passes, even with its divided volume and diminished velocity, still retains power enough to neutralize that resistance. The conclusion is therefore irresistible, that the power of the entire volume of water in the river, if kept united, and its velocity undiminished, would so far exceed that resistance, as not only to sweep away the bar, but to render its reformation impossible.

The expanse at the mouth is another agent in causing the bar. This expanse is due to the want of proper protection to the banks or sides of the channel, which are subject to the action of the upland waters, and the tides caused by the winds, whose successive abrasion waste the banks and enlarge the mouth.

The waters of the Gulf, when agitated by southerly winds, which propel them into the mouth of the pass, will force their way along the banks, where they meet with less resistance than in the mid-channel. This will produce the middle ground or sand-bank, and the two lateral channels, which are designated in the mouth of the South-West Pass by the pilots as the East and West Pass. Between these two lateral channels eddies are formed, which cause here, by the opposition of the two currents, the sand banks; while the dashing of the waves, agitated by the high winds, washes away the projecting edges of the low banks that extend into the Gulf. The expanse thus caused still further weakens the power of the current of

the river, by permitting the water to spread out too widely, decreasing its depth, checking its velocity, and increasing its tendency to deposit.

It must be observed that the shoal is situated within the mouth of the pass; that it is formed by the deposit of the sedimentary matter brought down by it, and that the only agency the water of the Gulf has in its formation is, the passive resistance with which it checks the velocity of the current of the river. Hence it is, that the water of the Gulf, even when driven into the river by southerly winds, produces no material change, if any at all, on the bar, and adds nothing to the accumulation of sedimentary matter that has contributed to form it. Whatever tendency to add to it the influx of the Gulf water, whether caused by tides or high winds, may have, is more than counterbalanced by the reflux which must necessarily follow.

This must be obvious to every one. When an elevation of the Gulf water takes place near the shore, either from the action of periodical tides or high winds, it necessarily forces its way into the mouth of the river. Having forced its way in and raised the surface of the water inside by the resistance which it offers to its passage out, and by the addition of its own volume, the tide turns and ebbs, or the winds subside, and the water forced into the river turns to seek its level in the Gulf. But as the Gulf water, acted upon by the tide or wind, has to meet in its passage up the river the force of the current, neither its velocity nor its power can be very great. When, however, it turns to seek its level in the Gulf, having no longer the resistance of the current of the river to contend against, the increased fall which its accumulation in the river gives it towards the Gulf, together with the additional impulse which the current of the river, now pressing it from behind, must give it, renders the velocity and power of the reflux, of course, much greater than that of the influx. The reflux, aided by the fall, and the volume of water, must certainly carry out more than the influx opposed by both can bring in.

The power of the current of the river is greater than the passive resistance of the Gulf water. This is evident, from the fact that it is able to maintain a depth of 12 feet over the bar. That the sedimentary matter, brought down by the river, is mostly, if not all, washed into the Gulf by the current, and carried away, is also evident from the fact, that the river is not adding to the bar. When the current of the river meets the water of the Gulf, it does not lose all its velocity at once. Its velocity is checked and somewhat diminished, but the current continues to force its way through, or rather over the dead water, partly from its original impulse, and partly from the pressure from behind, until at length the velocity, regularly diminishing as it progresses, ceases altogether, and the river water mingles with, and is lost in the vast expanse of the Gulf; while the immense mass of sedimentary matter that passes out over the bar, no longer supported by the velocity of the river, sinks in the deep water outside the bar, and becomes subject to the action of the Gulf currents, which soon removes it to other and more distant places.

Were the passive resistance of the water in the Gulf increased

until it became equal to the velocity of the river current, or the velocity of the river current diminished until it no longer exceeded the resistance of the Gulf water, the river current would, of course, be stopped at the mouth, and being no longer able to carry out into the Gulf the sedimentary matter it brought from above, that matter would be deposited there, and would increase the bar, until it rose to the surface of the water and completely shut up the river. If, on the other hand, the volume that now passes over the bar were enlarged, and the velocity increased until the power of the current was doubled, is it not evident that the inevitable result would be a decrease in the height of the bar, and an increase in the depth of the water over it? Close up the lateral outlets, confine the water of the river in its main channel, and contract the present expanse at the mouth, and a depth of twenty, thirty, or forty feet is just as possible and as practicable as the depth of twelve feet in the present condition of the channel.

To one familiar with the principles of Hydrodynamics, the removal of a shoal or bar in a river containing the volume of water that the Mississippi does, appears a matter so plain and palpable, that he can scarcely realize the fact that its practicability is doubted. The very fact, that the present defective condition of the channel of the river has caused that immense deposit of sedimentary matter, is a sufficient proof that, were it improved, the power that deposited it there could remove it. And, once removed, there is no danger that the Gulf will form one beyond where the present is situated. The water from the Gulf did not make the present bar, but, on the contrary, maintains a great depth of water immediately outside the mouth of the pass; and if the volume and velocity of the river current be increased sufficiently to remove the bar or deepen the water over it, there is no probability of a fresh one being formed; for, if the Gulf water cannot increase the present bar or form another outside of it, in the face of only twelve feet of water, how could it do so, were that depth increased to twenty, thirty, or forty feet? In fact, the more the depth of the water on the bar is increased, which can only be done by first increasing the force of the outward motion of the water, the further you remove the possibility of the formation of a new one. The depth of water is entirely and exclusively dependent upon the preponderating effect derived by the discharge of the river waters, or the force of the outward motion of the water.

From what I have said it will appear evident, that my object is to show, that the bar at the mouth of the Mississippi is caused by the diminution of the volume of water, by means of the great number of bayous or lateral outlets, which, by dividing it, check the velocity and partially destroy the power of the current; as, also, by the too great and unnecessary expanse of the channel of the passes near the Gulf. I have also endeavored to show, that the only way to remove the bar or deepen the water over it, is to remove the causes which have created it. If, therefore, lateral outlets diminish the power of a river, by diminishing the volume of its water and checking its velocity, closing them up is the best remedy, which, by keeping the volume of water that comes from above together and maintaining its velocity,

will preserve to the river the same power to deepen its channel at the mouth, that it has and does exercise at New-Orleans, or at the upper entrance of the passes. And, also, if the expanse at the mouth of the river has a tendency to weaken the power of the current that flows over the bar, and diminish its velocity, by permitting it to spread out over a wide bed, contracting it, so as to make it uniform with the average width of the channel, must be the best remedy; as the same volume of water, when concentrated, meeting with less resistance, and having a greater velocity, must certainly exercise a greater power over the bed of its channel and the dead waters of the Gulf.

ART. V.-MANUFACTURE OF SUGAR.*

THEORETIC INDICATIONS TO BE FOLLOWED OUT IN THE EXTRACTION OF SUGAR FROM RAW SUGAR-CONTAINING JUICES; AND VIOLATION OF THESE INDICATIONS, IN THE PRESENT PROCESS OF SUGAR MANUFACTURE. THE VACUUM PAN.

A CONSIDERATION of the deductions arrived at in the previous chapters, leaves no doubt existing as to the proper indications to be followed out, in the extraction of sugar from raw sugar-containing juices. I might have said, indication,—for every subsidiary matter tends to the one great end, of reducing the complex saccharine juice, with all possible haste, to the condition of a solution of sugar in water.

Although, in practice, the sugar producer will never attain this theoretical summit of perfection, yet he should always regard the various stages of his manufacture from that assumed point of view:which, if never permitted to be varied,-never allowed to be overcast with vague doubts, each succeeding well-directed experimental ef fort will assuredly lead nearer, and nearer, to the truth. If once departed from, however-if once the sugar-extracting operation be viewed from other directions, though apparently nearer to the mark,then the whole perspective of the theory is gone ;-confusion takes the place of order, doubt of precision, fallacy of facts;-the reasoning process breaks down, and all attempts to emerge from the mental chaos are in vain.

The great aim to be kept in view during the process of sugar extraction, being the removal with all due haste of every thing except sugar and water, the subsidiary indications are, to evaporate the latter at the lowest temperature, consistent with practical necessities,— and to effect crystallization in accordance with the rules laid down before.

In carrying out the first or grand indication, it is evident that we should seek for some defecating or purifying agent which is either capable of being totally removed from the sugar solution,-or which, if allowed to remain, should be productive of no injurious tendency.

* Continued from December number.

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