Examination of this Report—Mr. Brunel’s Evidence before the Select Committee on Atmospheric Railways, 1845
In this report Mr. Brunel rested his recommendation principally on two assumptions, which he held to be indisputable—(1) That stationary power, if freed from incumbrances such as the friction and dead weight of a rope, was superior to locomotive power; and (2) That the Atmospheric System of traction was theoretically a good and economical method of applying stationary power, and that it was also a practical and working system, as had been shown in its first and somewhat crude application at Dalkey.
The superiority of stationary as compared with locomotive power depends on two principles—(a) That a given amount of power may be supplied by a stationary engine at a less cost than if supplied by a locomotive. (b) That the dead weight of a locomotive forms a large proportion of the whole travelling load, and thus inherently involves a proportionate waste of power—a waste which is enhanced by the steepness of the gradients and the speed of the trains.
A detailed examination of these principles is given in the note to this chapter.
It is there shown that at the time referred to stationary power could be obtained for one farthing per horse-power per hour, while locomotive power cost more than one penny per indicated horse-power per hour, the cost of the locomotive power being more than four times that of the stationary power. On a level line at a speed of 60 miles per hour, for each horse-power usefully employed, a locomotive, in consequence of its own dead weight and the friction of its machinery, is obliged to expend more than one and a half horse-power; in this case, therefore, the useful work done costs nearly seven times as much as if it had been performed by a stationary engine. Again, on so moderate an ascent as one in 75, for each horse-power usefully employed, the locomotive has to expend at 40 miles per hour more than two horse-power, and at 60 miles per hour three horse-power; so that the useful work done costs in the one case nearly ten times, in the other thirteen times, as much as if it had been performed by a stationary engine.
This great advantage of stationary over locomotive power was a sufficient justification for introducing a system which promised to realise it to any considerable extent; although many difficulties might have to be encountered.
As has been already stated, Mr. Brunel had satisfied himself that the Atmospheric System was theoretically economical, and that its trial at Dalkey had shown that it was practically free from mechanical objections. Mr. Stephenson, indeed, had admitted that the mechanical details had been brought to a remarkable degree of perfection; but this admission was not any qualification of the radical difference of opinion which existed between him and Mr. Brunel, to which Mr. Brunel drew attention in his report of August 1844.
The subsequent abandonment of the Atmospheric System led many to believe that Mr. Brunel had been rash in rejecting the detailed investigations and conclusions of Mr. Stephenson, and that the adverse conclusions which Mr. Brunel had refused to entertain were subsequently established.
But, in fact, the failure of the Atmospheric System was due to failure in some of its mechanical details, and was not due to those inherent defects in its principle which Mr. Stephenson in his report considered that his experimental data had established.
Mr. Brunel’s opinions were again brought prominently into notice in the evidence given by him before a Select Committee of the House of Commons, which was appointed in the Session of 1845, in consequence of the number of projected lines which it was proposed to work by the Atmospheric System.
The following letter, written by Mr. Brunel to one of the members of the Committee, explains what he considered to be his position at this time:—
April 3, 1845.
I am summoned to attend your Committee on Friday, and as it is known that I have expressed opinions favourable to the Atmospheric System, and that I am actually applying it upon a line of some length, it would be considered an absurd affectation, and would, moreover, be useless to attempt, to avoid giving evidence when so called upon; but I am a most unwilling witness. I think it rather hard upon a professional man, who wishes to be cautious and prudent, that he should be called upon to express general opinions, which, if written even in the most studied and careful language, cannot be so worded as to be applicable to every case that may hereafter arise, or to be proof against the unfair and unscrupulous attack of the paid writers on these controversies. I mention these difficulties, which I feel that you may make some allowances for, if my feelings should appear in my evidence, or if that evidence should appear to fall very short of the opinions I am known to entertain, and which I must entertain to induce me to apply the system extensively, as I am doing. I find it difficult to define the points upon which it would be desirable to examine engineering witnesses, and I really believe that, entering freshly upon the subject and feeling as one of the public, you are more likely to elicit the useful points than one who, like myself, has been turning his whole attention (lately, at least) solely to the mechanical construction. However, I enclose a copy of a letter I addressed to a party interested in the patent, which refers to my opinions on the several points—opinions, however, expressed without that caution to which I referred as so necessary.
March 31, 1845.
I object very much to giving evidence upon the abstract point of the applicability of a particular system, and thus furnishing general opinions which others are to apply as they may choose to particular cases; and if I could, I would refuse to give evidence at all before the present Committee, whatever might be the consequence to the promoters of the Atmospheric System. Circumstances, however, render such a refusal impossible; but I am equally anxious not to be drawn into becoming an advocate of a system.
When I gave evidence last year,  although it was then very much against my inclination, it was in support of a particular case; and it was only incidentally that my opinion was advanced as to a system. The evidence is now avowedly sought in support of a system, and I do not, as I before stated, intend to become an advocate of this or any other system. I mention these my views to prevent disappointment. If the following facts and opinions are likely to be of use to the Committee, I can give evidence on them.
I made experiments upon the portion of railway laid on Wormwood Scrubs. These experiments were made for my own private satisfaction, and not made public in any way.
They satisfied me of the mechanical practicability of the system.
In 1843 and 1844 I made several experiments upon the Dalkey railway.
The result of my observations and of those experiments is an opinion that the mechanical difficulties attending the application of this system may be overcome, and the whole as a machine made to work in a very perfect manner; that is, as a mechanical power for locomotion it will generally, but not in every case, be more economical than what is strictly called locomotive power, that high speeds may be more easily attained, that, from the absence of the locomotive engine, the rails may be constructed and maintained in more perfect order, and as a consequence the carriages may be constructed and worked in a more perfect manner, and so as to run more smoothly, and that in all respects the travelling may be rendered more rapid, more luxurious, and more safe. As regards the last, viz. safety, collisions may be rendered altogether impossible, or most remotely possible; while all other sources of danger, now very small, may be almost entirely removed by the increased perfection of the rails.
As regards first cost, a single line may be made to answer all the purposes of a double locomotive line for most railways, except main lines in immediate connection with the metropolis, or forming trunk lines for others with important branches not under the same control; and a single Atmospheric line will generally cost as little, often less, including the working power, than a double locomotive line without the engines. I am now constructing a line of 52 miles in length entirely for the Atmospheric System. I already see many advantages to be attained in the setting out and constructing of a railway, if originally designed for this system; the principal advantages can only be attained, and, above all, the principal difficulties in the system can only be properly provided against, where the line is originally designed for the system; the choice of gradients and curves and levels, the position, and, above all, the arrangements of stations, will generally be totally different in the two systems, and the difficulties to be avoided will equally differ.
It is unnecessary to give any extracts from Mr. Brunel’s evidence,  as it only repeats the opinions and calculations embodied in his report of August 1844.
The Committee, while they allowed that experience could alone determine under what circumstances of traffic or of country the preference to the Atmospheric or the Locomotive System should be given, reported very strongly in favour of the general merits of the Atmospheric System.
 I.e. before the Croydon and Epsom Committee. See above, p. 138.  It will be found on pp. 35-52 of the Minutes of Evidence taken before the Atmospheric Committee (ordered to be printed April 24, 1845).