376 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



[November, 



sort of estinialion is not very diflicult, at all events we can arrive at an 

 approximation siiHiciently near fur practical pnrposes. 



For, if we assume tliat all the moving or nibbing surfaces through- 

 out the engine are equally smooth, that all the packings and bearings 

 are unifiirnily secured down, that all parts are well lubricated ; then 

 the comparative quantity of friction in the several parts will be, as the 

 area of one of the rubbing surfaces, multiplied into the distance which 

 it moves up on the other rubbing surface. 



We obtain tluis the following rules ;— 



1. For the relative quantity of friction due to the piston, multiply 

 the circumference of the piston by the depth of the packing, and by 

 the distance which the piston moves up and down in the cylinder. 



2. For the friction of the main shaft bearings, multiply the square 

 of the circumference by the length of the bearing. 



3. For the friction of those bearings which do not revolve entirely 

 round, but oscillate backwards and forwards, as the beam, gudgeons, 

 &c., multiply the area of the bearing into the angular distance moved 

 backwards and forwards during one revolution of the engine, Sec. 



4. It should be observed, however, that when one of the two rubbing 

 surfaces is hemp packing, the amount of friction will be at least double 

 what it will be when botli surfaces are metal. 



5. Furthermore, there are certain bearings which receive the direct 

 strain of the engine, while others do not. The following receive the 

 direct strain, viz. ; the crank pin, the fork head gudgeons, the main 

 gudgeons, the upper and lower bearings of the side rods ; now the 

 quantity of friction upon these several bearings will be considerably 

 more than that which is simply due to the tightening down of the 

 bearings, as before assumed ; it is difficult to say what may be the in- 

 crease of the friction from this cause, but it will be safe to assume that 

 the friction on these bearings will be three times greater than what is 

 due to the other bearings. 



Upon the foregoing principles therefore, is calculated the following 

 table of the comparative friction of the different parts of an engine, 

 having a 40-inch cylinder, a 3i-feet stroke, and furnished with the 

 common D slide. 



Table of Comparative Friction of the moving parts of a Steam Engine. 

 Piston, with lieinp packing 4 in. deep, 



84-33G 



niovinc; a distance of 84 in. 



„ „,„ J Pis'"" i"''- hemp packing 44 in. deep, 

 ■^ moving 84 in. 



,,..nfn\ Air pump bucket, hemp packing 3 in. 

 •1 deep, and moving 42 in. 



o.-tro\ Bucket rod, henlp packed 3a in. deep. 



■\ 



moving 42 in 



P „,„ J Two plunger poles, with 

 I 3 in. deep, moving 42 i 



th hemp packing 



10-059 



Flat face 

 1-680 



The slide face metal and 



metal liack hemp packed. 



Back, hemp j- G in. at top, G in. at 



8-064 



Slide rod 

 315 



bottom, moving 7 in. 

 each wav. 



U-250|^'-^ 



main shaft bearings moving en- 

 round metal and metal. 



18 in. circiuii. 

 9 in. deep 

 18 in. dist. 



:■. (rule 5) 



2,208.) The hearing at outer end of paddle shaft 



lo.V in. circum. 1 ff^ i ■ ■ i- i i 



^ . , I i.nni Crank pin, moving entirely round 



C in. long > i-62i< .K ' ,, ,. ". , . •^., 



fi ■ d" t I I recemng the direct strain of the en 



and 

 gine. 



■i-2— 



18 in. circmu. 



7 in. long 



9 in. dist. 



{The two fork liead joints moving at an 

 angle of 45" each way, but receiving 

 the direct stain of the engine. 



side rods same as 



fi-804 J The two main gudgeousreceiving the strain 

 I of the engines and moving 90° each way. 



45 in. circum. 



13} in. deep ^ 3-543 <j Eccentric ring moving quite round. 



45 in. dist. 



1000 Sundry small joints. 



163 123 



Therefore, if it be assumed that the total quantity of friction in a 

 steam engine is as 103-123, then will the relative quantity of friction 

 in the several parts be nearly as is represented by the numbers in the 

 preceding table. 



ON THE THEORY OF TOLLS UPON CANALS AND 

 RAILWAYS. 



.Sir — As I am aware that Mr. Ellett's remarks on Canal and Railway 

 Tolls, extracted in your Journal for September, have attracted some 

 attention, and have been received as sound and judicious principles by 

 some persons, who are in a position which enables them to carry out 

 these principles into practical operation, I beg to offer a few observa- 

 tions, with the view of pointing out what I conceive to be erroneous in 

 Mr. Ellett's statement. 



Mr. Ellett's object is, so to regulate the charge of toll upon a canal 

 or railway, as that every part of the country through which the line 

 passes, near or remote, may derive from the improved mode of con- 

 veyance the same advantage, an equal share ,of trade. And he con- 

 tends that this cannot be effected by the system of tolls that generally 

 prevails, namely, a fixed mileage, or a certain rate per ton per mile ; 

 and he therefore recommends the adoption of the directly opposite 

 method, viz., that the lowest charge should be levied on the trade that 

 is brought from the greatest distance, and increasing grathially as we 

 approach nearer to the mart or place of consumption, tTiat the heaviest 

 toll should be charged on that which comes the shortest distance. And 

 Mr. Ellett then proceeds to show that this plan would produce the 

 largest trade, (that is, would command the largest extent of country,) 

 and the greatest amount of revenue. 



Now all Mr. Ellett's argument depends upon one little i\ssumption,- 

 whicli he quietly introduces, without remark or explanation, quite un- 

 conscious that it contains the grossest fallacy. The market price of 

 any commodity at the pla^e of consumption may be said to be fixed, 

 (for our present purpose,) and, in order to obtain a sale for this com- 

 modity brought by the canal or railway, the cost of production and the 

 expense of conveyance must not exceed the fixed market price. Mr. 

 Ellett takes for granted that the cost of production is fixeil also, and on 

 this rests the whole theory of tolls. " Let us also assume that the cost 

 of producing this article (lumber) is G dollars per ton," and the market 

 price being fixed (10 dollars,) he consequently assumes that the ex- 

 treme cost of carriage which the article can bear, so as to be sold in . 

 the market, is fixed too, that it must not exceed 4 dollars, in the in- 

 stance given. But lie assumes also, and it follows in like manner from . 

 the preceding assumption, that the cost of production is fxed, that the 

 article can always bear this fixed charge of 4 dollars, that whether the 

 commodity be brought from near or far, whether it is carried 100 or 

 400 miles,' it can always bear the full charge of 4 dollars for carriage, 

 •and cannot, in any case, afford more. And on this assumption Mr. 



