Mr. R. Stephenson, in a most able and impartial report on the Atmospheric railway, addressed to the directors of the Chester and Holyhead railway, comes to conclusions extremely unfavourable to the invention. Mr. Stephenson's views are founded entirely upon experiments made by himself and his assistants; which are so fully and clearly detailed in the report, and their bearing so clearly but temperately stated, that we regret that our limits will not admit of our giving more than a summary of this valuable document.
The first subject treated of in the report is the amount of leakage. To determine this amount for each part of the apparatus separately when at rest, the engine was stopped after a certain amount of exhaustion had been effected, and the rate at which the mercury in the barometer fell was noted, repeating the process with various lengths of the vacuum tube, and with the pump and connecting pipe alone. These experiments show that the amount of leakage per minute may be taken as uniform, and the average of a considerable number of them gives the leakage, at the density of the external air, as 219 cubic feet per minute for the connecting pipe and air pump, and 252 feet per minute for the vacuum tube, or 186 cubic feet per minute for a mile in length. The report then goes on to observe,- "but it is evident that all leakage must be introduced into the vacuum tube, not at the density of the external atmosphere, but expanded according to the degree of rarefaction of the air in the tube; and hence the effect of this constant amount of leakage upon the velocity of the piston in the vacuum tube will be various at the different heights of the barometer.
For example, at Kingstown the leakage of the connecting pipe and pump is 219 cubic feet per minute, and that of the vacuum tube 252 cubic feet per minute; or, 471 cubit feet of air at the density of the atmosphere is introduced into the vacuum tube in each minute: but if the height of the barometer in the tube be 15 inches, or the air twice rarefied, the effect of this leakage will be doubled, and the quantity of air to be extracted from the tube in each minute will be increased by 942 cubic feet; and if the air in the tube be five times rarefied, or the barometer stand at 24 inches, it will be increased by 2,355 cubic feet, instead of 471 cubic feet, in each case. As the degree of exhaustion advances, the retarding influence of the leakage upon the speed becomes more and more serious; for while the velocity of the air pump piston remains constant, or very nearly so, and the cubic extent of each stroke is the same whatever the density of air, the effect of the leakage is increased with the rarefaction, and the maximum velocity attainable by the train is proportion ably lowered."
Previously to calculating the effect of these conditions upon the velocity, the report details, in a table of great extent, the experiments made to ascertain the practical velocities under different circumstances. We have arranged Mr. Stephenson's summary of the results in Table A.
Mr. Stephenson then proceeds to calculate the effect of the leakage (as ascertained with the apparatus in a state of rest)upon the theoretic velocity, computed from the velocity of the air pump piston, and the ratio between its area and the area of the piston in the vacuum tube; and the calculation shows a loss upon the theoretic velocity of from 7 to 30 per cent. in the apparatus at Kingstown, and from 12 to 48 per cent. with a vacuum tube of 4 miles in length; the loss varying as the pressure is increased from 3 to 12. 2 lbs per inch. But a considerable difference is also found between the practical velocity and this last calculated, showing a further loss of velocity, increasing from 26 to 41 per cent. between the pressures of 9 and 12. 2 lbs per inch, making a total loss on the theoretical velocity between these pressures varying from39to 71 per cent.
No. of Train.
Weight of Train.
Vacuum at Starting.
Miles per hour.
5 . 2
* This high vacuum at starting was obtained by holding on the brakes.
+ This Train started with this vacuum owing to a portion of the train standing on the downward incline.
The weight of the ten following trains increased gradually to C4.7 tons, and the height of the barometer to 24.4 inches.
This difference between the calculated velocity and that obtained in practice shows that the amount of leakage is greater when the apparatus is in motion; part of which Mr. Stephenson supposes to arise at the air pump and at the tube piston.
Upon the subject of the weight drawn, friction, maximum velocity, etc, the author gives the following table, (B.)
In this table those trains are selected from the twenty experiments before mentioned which present the most uniform and valuable results: the data for calculation are given in the first 7 columns; the 8th column gives the total power of the air pump during the whole time the engine was in motion. In each experiment the mean resistance to the air pump piston is multiplied into the velocity of the air pump piston, and is increased in the proportion between the total time the air piston was in motion, and the time required for a train over the entire distance, at its maximum uniform velocity. The power indicated by the air pump during the motion of the train, as shown in the 9th coiumn, is found by multiplying the total resistance to the air pump piston due to the maximum uniform vacuum into the velocity of the piston, and subtracting the product from the total power of working the air pump (column 8), the remainder shows the power absorbed in attaining the vacuum, as given in column 10. Column 11 gives the amount of power absorbed by the train at its maximum uniform velocity on the ascent of 1 in 115; and column 13 gives the power indicated by the friction and gravity of the train, multiplied into the maximum uniform velocity.
It will be observed that this power is in every case less than that indicated by the maximum velocity of the train; and the difference between the two, enormous as it is at the higher velocities, Mr. Stephenson considers must be ascribed to the resistance of the atmosphere; and it is accordingly placed under that head in column 14.
In connexion with this part of the subject Mr. Stephenson observes, "In referring to this column representing the loss of power from the resistance of the atmosphere, it will be observed there is a very rapid reduction in the loss as the speed is diminished; indicating most satisfactorily the excessive expenditure of power, and consequent augmentation of expense, in working at high velocities upon railways." This remark is of course equally applicable to all railways, whatever may be the motive power employed, and it is here introduced only for the purpose of showing, that the attainment of speed exceeding that which is now reached upon some of the existing lines of railway, is a matter of extreme difficulty, and that the atmospheric system is not exempt from that