1842.] 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL, 



CSS 



by one man in a pair of large engines, whereas by using the slide 

 itself four or sis would be required. This would, I should think, add 

 greatly to its efficacy by being so easily under control — more espe- 

 cially in the Thames and the Clyde, in the latter of which it is often 

 necessary to stop and reverse every two or three minutes. 



Mmj 17, 1842. Seyton. 



P.S.— Lap is not applicable to a valve to be worked by an eccentric 

 keyed on the shaft to work the engine both ways, for if it was set 

 right for going a-head, it would be too late to reverse. 



[We think very favourably of the plan for reversing engines com- 

 municated by our correspondent. In large engines its employment 

 might supersede the necessity of throwing the engine out of gear, and 

 the use of starting apparatus, and would confer a faci lity in starting 

 and reversing which does not at present exist. We do not under- 

 stand what our correspondent means by cutting oft' steam to tbe end 

 of the stroke. Tbe expression seems to us to involve a contradiction, 

 inasmuch as the act of cutting oft"nieans the prevention of more steam 

 entering the cylinder at some point of the piston's progress before the 

 stroke is completed. We may further observe that although it is 

 quite true that lap is inapplicable to a fixed eccentric intended to 

 move the engines both ways, it is not inapplicable to a fixed eccentric 

 in ordinary engines, for in ordinary engines no fixed eccentric can 

 move the engines both ways. Lap is only inapplicable in the case of 

 some double-ended levers, or in the mode of reversing our corre- 

 spondent recommends. — Editor.] 



THE VARIATION OF THE COMPASS. 



Observations made at the Royal Observatory, Greenwich, 

 G. B. Airy, Astronomer Royal. 



STEAm MAVIGATIOIV. 



THE "ATMOSPHERIC" STEAM-BOAT. 



This vessel is adapted for river navigation, the hull is entirely of iron, but 

 with deck and fittings of timber. Slie was built by Messrs. Ditchburn and 

 Mair, the eminent iron boat builders of Blackwall. 



The length between perpendiculars is 150 ft.; breadth of beam, 18 ft.; 

 depth of hold, 9 ft. 9 in. ; and drauglit of water, 4 ft. 8 iji. 



The engines were constructed by Messrs. John and Samuel Seaward and 

 Capel — they are upon the old atmospheric principle. There are three 

 cylinders placed immediately under the line of shafts, with the cranks set at 

 an angle of 120 degrees, so that in every portion there is the full power of one 

 cylinder in action, which produces an equable and uniform motion. There are 

 no cylinder covers or piston rods, as the connecting rods communicate at 

 once from the pistons to the cranks. The pistons are forced upwards by the 

 pressure of the steam, and then allowed to descend again by the production 

 of a vacuum underneath, the air constantly pressing upon the upper side. 

 There is one condenser and one air-pump connected with the three cylinders. 



The engines have been worked several times, and are represented to 

 have performed most admirably and efficiently, and the boat proved to be of 

 unrivalled speed. 



The following are the proportions of the engines : — the cylinders 47 in. dia- 

 meter, ar.d 3 ft. stroke ; the pistons make 34 strokes per minute ; tbe pressure 

 of the steam is represented to be 8 lb. on the square inch ; the paddle-wheels 

 are 16 ft. diameter, and have 24 hoards, 9 ft. long by 14 in. deep. 



The power of the engine may be calculated thus : — if we take the pres- 

 sure of steam in the upward stroke at 6 Ih., and the pressure of the atmo- 

 sphere occasioned by a vacuum underneath the piston, on the downward 

 stroke at 13 lb., allowing a trifle under the full weight of the atmosphere, 



we shall have a mean pressure of 



6 + 13 



= 9.5 lb. on the upward and down- 



ward stroke; and the velocity of the piston (3 ft. x 34 x 2) = 204 ft. per 

 minute ; and the area of each cylinder (47- x .785) = 1 735 in. ; we shall have 



for the power of the 3 cylinders ( 3 — — n^Huui ) ^ ^^^ horses nearly. 



And if we take the same proportion of nominal power to the effective power 

 as in the ordinary low pressure marine engine, which is generally half, we 

 shall have the nominal power of the three cylinders = 153 horses, which it 

 must be admitted is a tremendous power for so small a vessel. We cannot, 

 therefore, be surprised that the boat is one of the fastest, if not the fastest 

 on the river. 



We are at a loss to conceive what advantage Messrs. Seaward and Capel 

 expect to gain by the revival of the atmospheric engine for navigation pur- 

 poses, except it be to get a longer stroke in applying the direct action prin- 

 ciple. Against this we have double the capacity of steam cybnders, 

 consequently nearly if not quite double the weight and expense, 'besides the 

 very serious evil of exposing the interior of the cylinder at every downward 

 stroke to the cooling influence of the atmosphere. 



To show that the cylinders are double the capacity of those constructed 

 on the ordinary principle, let us lake the pressure of the steam as above at 

 6 lb., and the vacuum equal to 13 lb., which together is 19 Ih., just double, at 

 each upward and downward stroke ; at the mean pressure taken in the above 

 calculation, therefore, one half the capacity of cylinder will be sufficient. 

 Again, let us prove this more clearly, by calculating the power of two cylin- 

 ders working on the ordinary reciprocating principle, each 40-i-i diameter ; 

 the area of the two being 2600 inches, rather less than half the collective 

 area (5205 in.) of the atmospheric cylinders, tbe power will be, taking the 

 same stroke and velocity as before, and pressure (6 4- 13) = 19 lb, 



/19x2600x24\ ,„. ^ 



I 1 =30o horses. 



V 33,000 / 



The Slc<nn Association. — We must honestly confess that the Ster.-,n A.^soci- 

 ation has disappointed our hopes, in common with m<ist persons interested in 

 the advancement of the art of steam navigation. Not that «e have any 

 fault to find with anything the Associatiun has done — all of which is usefi'il 

 In its kind, and in our eyes has been wisely planned and ably prosecuted — 

 but that many things have been left unattempted of far greater moment than 

 any of those to which attention has been given, and which tbe Association 

 and it alone is competent to undertake. What, for example, can be of greater 

 imjiortance than the establishment of a system of registration, such as that 

 in I'se in Cornwall, of tbe consumption of fuel in different steam vessel.*:, in 

 relation to tbe power exerted; and who 1 ut the Steam Association could 

 establish such a system ? Tbe duty of the Cornish engines has by this simple 

 expedient been raised in the course of a tew years from 26 millions of pounds 

 raised one foot high, by a bushel or 84 lb. of coal to 120 millions — an increase 

 of mechanic. 1 virtue of nearly 5 times, simply in consequence of the care 

 and emulation the system of registration has excited. It would be too much 

 perhaps to expect an ec|ual increase of efficacy, or equal diminution of expense 

 in steam vessels, but that great improvement would be effected by registralion 

 is scarcely to be doubted ; and as it is the propelling power which constitutes 

 the great expense in the maintenance of steam vessels, we think the attention 

 of the yteam Association should first be given to that subject, and that tbe 

 discussion of pilotage laws, harbour duties, &c. — \^■hich, however settled, can 

 etlect steam interests little in comparison with the economizalion of fuel — 

 might be wisely postponed until a system of registration has first been organ- 

 ized. We refrain from going into any details relative to this question for 

 the present, and must be content with saying that in our apprehension tbe 

 details do not involve any practical difficulty or inconvenience. We trust 

 the Association will speedily give its attention to this subject, and will use 

 its best efiorfs to assist or facilitate the accumulation of all statistics of steam 

 machinery that appear calculated to advance steam navigation, and l>romote 

 the interests of those er gaged in it. Hitherto the Association has been the 

 representative, not of all the considerations proper to sleam enterprise, but 



01 those only which have a commercial complexion, and which any associ- 

 ation can do the least to ameliorate. It does not augur well to see our 

 engineers and ship-builders present only at the annual dinners ; but we trust 

 to see them soon taking a part in the Asiooialion's proceedings, and to see 

 tbe Association assume its proper position — not of the representative of the 

 counting-house — but of the representative of the science. 



The Locovwtive, No. I. — A boat with this cognomen is now running between 

 the Adelpbi Pier and Greenwich. She is fitted by Messrs. Braithwaile. Mil- 

 ner and (Vo. with a locomotive engine precisely tbe same as those working on 

 railways, with the exception that it is taken off its wheels, and set upon 

 bearers laid across the bottom of the boat, and instead of the railway wheels 

 there are fixed upon each end of the driving or cranked nxle a pinion wheel, 



2 ft. 4 in. diameter, which works into a cog wheel 7 ft. diameter, keyed ou 



10 the shaft of the paddle-v. heels ; the shaft is in one piece, and has a beuiiinff 

 in the centre upon the U p of the boiler, 'Ihc engine is what is generally termed 

 a 10-ton engine, fitted with two cylinders, 12 in. diameter, and 18 in. stroke ; 

 the pistons make 100 to 130 strokes per minute. The boiler is 3 ft. 2 in. 

 diameter, and contains 84 tubes 2 in. diameter and 8 ft. G in. long, and a cir- 

 culr.r fire box, 3 ft. 3 in. diameter. The pressure of the steam within the 

 boiler is from 43 lb. to 50 lo. on the square inch. The paddle wheels are 



1 1 ft. 4 in. diameter, with float board 10 in. deep, anil 5 ft. long. They have 

 a dip of about 18 in., and when the engine makes 100 strokes Jier minute 

 tbe padille-» heels make 33 revolutions. The length of the boat, which is 

 iron, is 103 It. on tbe water-line, with a beam of 11 ft. 6 in., and draws 2 ft. 

 2 in. when light. The tot.al weight of engine, boiler, paddle-wheels, and 

 coal boxes, is 12 tuns ; the engine room occupies 21 ft. in length of the boat. 



