1844.] 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



a crowd of small low biiildins;s with domes, pediments, &c. In 

 another, from the number of columns introduced, and from the point 

 of view chosen, not a single window or door was to be seen, and it 

 therefore looked like a tomb or some such building. Altiiough having 

 spoken so much in dispraise, I must acknowledge that all the drawings 

 exhibited great care in the drawing, and, I might almost say, beauty 

 of colouring; but the most of tliem departed from the printed direc- 

 tions of the Academy, namely, that no colour but sepia should be used, 

 for in some I noticed red, blue, yellow, brown, &c. And, again, in the 

 introduction in the perspective drawings of figures, such as soldiers, 

 horses, stalls, with their attendant old women, boys, &c., which, 

 in ray opinion, instead of adding to the beauty of the design, turned 

 it into a pretty picture, and no more. I fear from some little pieces 

 of bad perspective, also introduced, that some one had been employed 

 to put in that which certainly did not add to the beauty of the archi- 

 tectural design. 



The drawings for the silver medal, sent in by Mr. G. Perry, of the 

 West Wing of Greenwich Hospital, from actual admeasurement, ex- 

 liibited great care, both in the drawing and colouring. The Academy 

 reserving the power of withholding the medal, in a case like the [.re- 

 sent where only one competitor sent in, have signified that they con- 

 sidered the drawings of Mr. Perry worthy of the prize. Although these 

 drawings did not exhibit the artistic handling shown in the gold med.il 

 drawings, they give promise of superior abilities. A student informed 

 me that the probable cause of there being no other drawings in this 

 class was owing to the council's description of the part required to be 

 drawn, being very unintelligible, and that he bad no doubt the students 

 were afraid of commencing any drawings, as (hey might turn out after 

 all to be of a wrong part. But why did not they make inquiries? I 

 apprehend that either the Keeper of the Academy, or some other 

 member, would hare set the student right. 



Yours, truly, 



January 3, 1844. Q. 



A MARINE SALINOMETER. 



For the purpose of indicating the Density of Brine in the Boilers of Marine 



Steam- Engines. Invented by J. Scott Uussell,M.A.,F.R.S.E., F.R.S.S.A., 



Civil Engineer. 

 Bead before the Royal Scottish Society of Arts, 28th February 1842, 



the Honorary Silver Medal of the Society awarded, and reported in their 



Transactions. 



It was very early in the history of steam navigation that the inconvenience 

 of raising steam from salt water was experienced. When the Come/ descended 

 below Port Glasgow in 1812, the boiler was found to boil over, or prime, as 

 it is technically called by engineers, wlien part of the water of forced up so vio- 

 lently, along with the steam, as to pass over into the cylinder of the engine — 

 a circumstance always detrimental, and sometimes destructive to tbe engines. 

 This arises from the thickening of the water, its density being increased by 

 the retention of the soUd substances which compose sea water, and which re- 

 main and accumulate in the boiler, while the fresh portion of the water is 

 passing off in the shape of steam. This process of accumulation of solid 

 matter in the marhie boiler is by no means slow. The whole of the water 

 which a marine boiler usually contains is evaporated in three or four hours, 

 leaving the solid substances in the cubic content of boiler behind it, and 

 being replaced by salt water, with an equal quantity of depositary matter, 

 accumulating as rapidly as before; and since it is known the solid matter 

 amounts to as mucli as ^^ of the whole mass of water, it would follow, if the 

 process of ebullition could continue so long as 150 hours, there would be 

 deposited in the boiler a quantity of solid matter equal to the number of tons 

 of water in tbe whole content of the boiler. 



Long, however, before this degree of solidification can take place, evils of 

 a different description intervene to impair and put an end to the functions 

 of the boiler. The solid constituents of salt water which arc left behind do 

 not diffuse themselves uniformly over the whole liquid mass, so as to consti- 

 tute a homogeneous brine ; on the contrary, tbe new supplies of sea water, 

 as they enter the boiler, remain secluded from the former more saturated 

 brine, rise by their less specific gravity into an upper stratum, while the 

 denser lirine ferms a bed in the lower part of the boiler, and surrounds the 

 fire box and heater flues occupying the water spaces and legs, which are 

 usually at a high temperature, and which, in double tiered boilers, are gene- 

 rally the most intensely heated. The intense heat of the metal expels the 

 water from the brine in contact with it most rapidly in the hottest places, 

 and salt is deposited on the hottest parts of the furnaces and flues, extending 

 rapidly to those less heated, and so not only diminishing the evaporative 

 power of the boiler, but injuring its substance, and endangering its existence. 



Tbe remedy for these evils was very early invented. But I have not been 

 able to discover the inventor of the cleansing process commonly called 

 " blowing down," or " blowing off." It is almost universal, and is performed 

 in the following way : — There is forced into the boiler, at each stroke, rather 

 more water than is required for the supply of steam, so that the boiler be- 

 comes too full. Openings are then suddenly made at the bottom of the 

 boiler, and the brine at the bottom being violently ejected, carries with it 



any solid substances that may have accumulated near the bottom — the boiler is 

 thus cleansed ; and before the water has got too low, the openings are again 

 closed, and tlie boiler continues to be fed as formerly. Another remedy, 

 pretty generally adopted, is the brine pump, by which, for every portion of 

 water supplied to the boiler, about one-fourth part of that quantity of brine 

 is withdrawn from it. This process does not so tborougldy carry off all the 

 impurities as the former; but it is attended with a saving of fuel by a con- 

 trivance for giving to the feed-water entering the boiler a portion of the heat 

 of the discharged brine. Tne recent introduction of this process is due to 

 Messrs. Maudslay & Field of London. 



In whatever way the saturation of the water with solid matter may be 

 remedied, it is essential to the accomplishment of tliis object, that some simple 

 apparatus should be contrived for tlie purpose of showing when the cleansing 

 process is required, and whether it is successfully applied. If this be not ob- 

 tained, the usual consequence of acting on wrong data are sure to follow. 

 A contrivance was patented, whicli was thought promising, hut was found 

 liable to he mechanically out of order when most wanted : — a ball of greater 

 specific gravity than salt water was connected with an external index, bv 

 which there was indicated on the outside, the fact of the brine becoming 

 sufficiently saturated to float this ball. Another was to place in the glass 

 gauge of the boiler a glass hydrometer head, which would float when the 

 brine became saturated to a given point, and fall to the bottom in the ordi- 

 nary state of the boiler. But this fails entirely of accuracy, although very 

 elegant; for the brine of which we wish to indicate the density is in the 

 lower stratum, not the upper one, where the usual glass gauge is ]daced, and 

 irretrievable mischief might be done before the indication would show any 

 change. 



I have lately employed, in some large ships destined for transatlantic 

 voyages, a species of brine gauge, or index of saturation, which is found to 

 possess every advantage, and which I therefore desire to communicate to the 

 public through this society. The drawings sent are such as may enable any 

 engineer to construct them for himself. The details of the arrangement of 

 the apparatus were made under the direction of Mr. James Laurie, formerly 

 one of my assistants; and he also has obliged me by writing out the annexed 

 description of the operation of using the index. 



The principle I have used is the well-known law, " that the heights of 

 equiponderant columns of liquids vary inversely as the densities of those 

 liquids." 



If I take open glass tubes bent in the form of the letter U, and pour one 

 fluid into one of the sides, and another fluid into the opposite side (taking 

 care to use the heavier liquid before the other); the one being mercury, and 

 the other water, they will stand at tbe height of 1 in. and 13 in. respectively. 

 If I use alcohol and water, they will stand at the height of 10 in. and 8 in. 

 respectively, tbe height of the one fluid being always greater than that of the 

 other, in the proportion in which its weight, density, or specific gravity is 

 less. In like manner fresh water and salt water will stand at heights of 

 40 in. and 41 in., showing a ditt'erence of 1 in. 



Tlie use which I make of this principle is as follows : — I reckon the best 

 scale of saltness of a boiler to be that which takes the common sea water as 

 a standard. Sea water contains -^ of saline matter. When the water has 

 been evaporated, so as to leave only half tbe quantity of distilled water to 

 the same quantity of saline matter, I call that two degrees of salt, or brine 

 of the strength of two, and such brine would show, in fig. 3. the columns 40 

 and 42, or double the saltness of sea water, indicated by a difference of 2 in. 

 A farther saturation would be indicated by a difference of 3, 4, 5, and 6 in. 

 between tlie columns, and so indicate 3, 4, 5, 6, and any further degrees of 

 saltness — a range which may be made to any degree of minuteness by the 

 subdivision of the scale of inches. This scale is that which appears to mc 

 most simply applicable here — and it is that which I adopt for marine boilers. 



The mechanical apparatus which I liave employed to give this indication 

 is perfectly simple, and has the advantage of being such as the engineer 

 already perfectly understands. To the marine boiler I apply two water 

 gauges of glass, instead of one as at present used; they both serve the pur- 

 pose of the present glass gauges, and the pair would be valuable for this, if 

 for no other reason, that there would always be a duplicate when one is 

 broken, an accident not unfrequcnt. To these gauges I simply attach small 

 copper pipes, so that one of them may be placed in communication only with 

 the salt brine in the lower part of the boiler, and tbe other with tbe feed- 

 water which is entering the boiler ; the one then holds a column of brine, and 

 the other of pure sea water, and each inch of difference shows the degree of 

 saturation. 



Without the use of any attached scale, the engineer, by a little practice, 

 comes to know in bis particular vessel, what diflFerence in inches can be ad- 

 mitted without danger, and at what difference of lieight it is imperative to 

 blow off. But it is convenient to have an attached scale. It may be satis- 

 factory to state, that the practical range of scale in an ordinary boiler in the 

 ordinary working, is G in. to 10 in., a difference sufficiently great to be easily 

 oliserved. The rule of working them is nearly this : — Continue the operation 

 of blowing off until, if possible, tbe difference of the columns is less than an 

 inch; it will be unnecessary to blow ofi' again until the dift'erence is at least 

 in. As a practical rule, I find that it is necessary to blow off when ths 

 brine at the bottom has about three degrees of saltness. But this will vary 

 exceedingly, according as the construction of the boilers is more or less judi- 

 cious. When the heat is greatest in the lowest portion of the boiler, and the 

 flues return above, they will be most liable to salt, and require the most fre- 

 quent cleansing. 



