The earliest reference we have of William Murdoch's model steam carraiges comes in the correspondence between James Watt, Thomas Wilson (Watt's agent in Cornwall), and Mathew Boulton, (senior partner in the firm of Boulton & Watt - Murdoch's employers). 

DOCUMENTED EVIDENCE :

1784, March 7th  - Thomas Wilson, Watt's agent in Cornwall, in a letter to James Watt

The Earliest mention of Murdoch's model steam-chariots : 

"He (William Murdoch) has an amazing genius and I am almost afraid will lead him too far, he has mentioned to me a new scheme which you may be assured he is very intent upon, but which he is afraid of mentioning to you for fear of your laughing at him, it is no less than drawing carraiges upon the road with steam engines, for my part I think there is too many difficulties ever to answer, or going up and down hills, and I c I do not mention this from him, viz by his desire, but as it has taken such hold of him, I think it is right that you should be acquainted with it, and either consult with him, on the means he proposes to make it answer if you think it practicle, or endeavour to convince him of it's impossibility, he says that what he proposes, is different from anything you ever thought of, and that he is positively certain of it's answering and that there is a great deal of money to be made by it, I shall inform him that I have wrote to you on the subject, which I have a notion is what he wishes but did not know how to do it himself"

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"In relation to Wheel carraiges, I do not consider that there (will) be much difficulty in moving them by steam, but that the (amount) of Boiler fuel and water necessary would make a very considerable weight, which at present I see no way of lessening - I should in (any) case use no condenser yet the water necessary for a machine which exacted the power of one horse would be two cubic feet for (each) hour and as water is not everywhere to be got, at least not without the inconvience of stopping, provision should be laid in for 4 hours besides the quantity always necessary contained within the Boiler. So that supposing the vehicle to be a post chaise with two persons their common luggage and driver I compute that the weight would be at least three thousand pounds. In other respects I esteem that thing certainly possible and practical, whether profitable or not I am doubtful considering the great expense attending the (setting) new schemes a going and the (rubbs) which attend them. Mr Boulton will converse with William about it when he comes down to Cornwall, till then it had better sleep." 

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"Wm has your letter respecting the Carraiges, but has he did not read it, then, I have not seen him since have not heard what he thinks of your objections, which with me has great weight, however, when Mr Boulton comes they will talk over this and I hope he will be induced to drop the sanguine opinion of it, he has held for I cannot think it will answer the expectations he had formed, and if pursued would take him from his present business, which can ill spare him."

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"I hope Mr Boulton will not be long before he arrives, as he begins to be really wanted and principally on William Murdoch's account who is discontented and chiefly owing to the carraige scheme, in which I believe he must be humoured, as he seems bent upon it, it will be best to take no notice of it tho till Mr Boulton comes as he (Wm) is remarkably captious, and requires to be delicately treated, however I do not doubt but he will be managed, (we must not lose him), but I am sorry to find it requires more than I expected."

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"I am yours of y 15th, and am more sorry to hear what you tell me of W.M. .... I am sorry I wrote anything to you on the subject of the carraiges but I mentioned only a small part of the arguments which have hitherto deterred me from meddling with them, anything we can do in reason to bring his mind to a proper bias you may readily suppose we will do."

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"William seems more contented lately, and I hope when Mr Boulton arrives he will be set right, but he is still bent upon his carraige scheme, which I wish had never entered his brains"

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"I shall do all I can to make a pleasant Murdoch but I fear it is not of my power to do what you wish. I have talked to him a little about Chariots which I find is a family madness. His father and him were about one with a corn engine some years ago. He seems fearful lest somebody else should go upon the same scheme."

"I fear he will not hire or agree term of years upon any term unless we could accomplish it by a side wind. If we could enter into an agreement with him to make or try to make wheel carraiges in partnership for a term of years but at the same time oblige him to do our Cornish business and no other except wheel carraiges. .........I know no way of bribing him so effectively as to talk to him about his hobby horse......"

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"I have had a little conversation with Murdoch about Wheel cargs. He proposes to catch most of the condensed Steam by making it strike against broad Copper plates and the condensed part trickling down may be caught in a reservoir. This may do some good in rain or frosty weather & he proposes to have different sized revolvers to apply at every hill & vale according to their angle with ye horizon. What he wishes to do may be right by ye means he proposes will not do. I find his Father hath a W1 Carraige to go without horses which I believe Logan Malcomb lately saw in Scotland as this was one of the first mechanical amusement wch Murdoch ever turned his attention to in his youth it serves to account for his furur he is seized with : I verily believe he would sooner give up all his Cornish business & interests than be deprived of carrying the thing into execution. When a Man is mad in any way it is vain to reason with him about his disorder."

"He doth not directly say he hath a right to work pistons by Steam but he says he thinks or fears that the Trumpeters or somebody else will then take out a patent for W1 Carrgs wch is the thing he says that makes him so sollicitous to have secured. Now as you are going to specify sundry new applications of Steam Engines Qy if it may be prudent to specify the application of it to W1 Carggs without making any drawing. I propose this by way of taking posssession & saveing the expense of a patent." 

"I believe a Steam Engine carg may be made lighter than a common post Chaise if you include the weight of the horses for in going up hill animal power must be furnished by them to raise the weight of their own bodies wch I consider only as other kinds of Steam Engines. I think a very light and cheap modell might be made takeing one of our drawn tubes of an Inch diamr to make the Cylinder of." 

"I am sensible 'tis wrong to crowd you with new schemes at this time but my only reasons were the two following viz - 1st to seize the present moment of putting it into our specification & 2nd to ask you whether I shall make any agreement with Murdoch or not to acquiesse in any thing wch tends to keep peace & that doth not interrupt or retard our present real business. "

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"You will please come to some determination as soon as possible about Wm, he seems to be a little more reconciled to his situation than before, but far from being easy, and before Mr Boulton leaves us, he ought to come to some eclairissment with him, to enable him to do which, I would recommend you to state to Mr Boulton your sentiments fully on the Carraige Scheme and what part you would be willing for Wm to take in it, without this I am fully persuaded after Mr Boulton leaves us, that Wm will be more uneasy both to himself and others than he already has been." 

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"I have given such descriptions of engines for wheel carraiges as I could in the time and space I could allow myself; but it is very defective and can only serve to keep other people from similar patents."

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"I think Murdoch is getting better into tune but I must say something more to him before I leave upon Carraige subject or he will relapse. We must make the best of mankind as we find them."

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Extracts from the copy of  patent No 1432 filed 28th August 1784; and enrolled 24 August 1784 - Seventh Improvement :

Watt skips through boiler design in a couple of lines, as he clearly has no intention of using strong steam except occassionally

He then states "as usual in other steam engines I employ the elastic force of steam to give motion to the pistons and after it has performed it's office, 
I discharge it into the atmosphere by a proper regulating valve"

or

"I discharge it into a condensing vessel made airtight and formed of thin plates or pipes of metal having their outsides exposed to the wind or to an artificial current of air."

Authors note : The first alternative repeats the fourth article of the original 1769 patent, the second alternative, of surface cooling, is William's idea as described in the letter between Boulton & Watt of August 6th. 

Watt then describes a rather complicated system of gears for transmitting the rotative motion from the engine to the drive axle of the carraige. He postulates a two person chaise requiring an engine with a cylinder seven inches in diameter and going at sixty strokes a minute with a double acting engine, and there the patent rests. 

Letter entilted "Some of my idea's on the wheel carraige scheme". 

Briefly, Watt deals with the idea of saving water by recondensing it, and of the gears, which he pen sketches, before embarking on the total weight to be moved. He then goes further into the power to weight ratio problem and the need for smooth road surfaces, before concluding that the existing design would only produce a speed of one and half miles an hour. 

He suggests that 4mph is what is required and poses a number of questions which must be answered to determine the probability of success or failure. The second, ways of reducing the size and weight of the boiler and water, he answers positively (incorporating Murdoch's surface cooling idea). 

Watt then states "Thirdly, whether it will require steam of more than one and a half atmospheric density to exert power equal to six pounds on the inch, (otherwise a longer, heavier cylinder will be required or the engine will have to work at more than sixty strokes a minute which he thinks impracticle), "principally because the rotative motion already turns too fast for the axle of the chaise".

He then recites a number of other problems which he thinks can probably be overcome - gears, brakes, soot and smoke, and the shaking of the coach causing both greater fuel consumption and greater stress on the engine parts "but perhaps some remedy may be devised for this in constuction"

Watt continues in his letter to Boulton : 

"From what I have said, and from much more which a little reflection will suggest to you, you will see that without several circumstances turn out more favourable than has been stated, the machine will be clumsy and defective, and that it will cost much time to bring it to any tolerable degree of perfection, and that for me to interrupt the career of our business to bestow my attention on it would be imprudent. I even grudge the time I have taken to write these comments on it."

"I now come to the most difficult part of this subject which is the proper conduct to be observed with regard to Murdoch. I fear that if we enter into the scheme with him that it will take up his whole attention, to the neglect of more material business both to himself and us and that in case of not immediately succeeding, which is to be expected, you will be led from one experiment to another hoping for an amendment, and then it may succeed only ambiguously which is the most dangerous.... if he succeeds tolerably he will naturally attach himself totally to it and we shall lose the benefit of his knowledge and experience in our Cornish business (unless the result of the experiments prove flat denial to the scheme.)

"I shall  prove no obstacle to that scheme my only reasons against it were that I feared it would deprive us of a valuable man, and it would if we were to be concerned in it divert us from more valuable business and perhaps prove to be a sinking fund and lastly that I did not like that a scheme which I had revolved (sic) in my mind for years and hoped should be able at some favoutable time to bring perfection if capable of it, should be rested from me or that I should be compelled to go into it as a secondary person. But now I have had the latter objection give way. And as to the first I think it will take place at any rate so we must make the best of it. Nevertheless let us proceed with caution..."

"I will reduce what I have to say into two propositions which are the only ones I can think of making."

"The first is that the whole concern shall be carried on with our money and that he shall have a certain share of the profits after deduction of interest, say the fourth or less if less will satisfy him for superintending the business, and We bind ourselves to lay out £100 on experiments solely under his direction, (for I assure you I will not meddle with it in any degree), and bargain to be for a term, ten or more years, provided the business be carried on and also provided that the agreement shall be null and void in so far as respects us, unless he shall within one year make a machine or engine which will drive or move a post chaise, carrying two ordinary persons and a driver with two hundred pounds of luggage besides the engine the boiler and stocks of fuel for four hours and of water for two hours moving at the rate of four miles per hour on the average on a common Cornish Turnpike road and at the same time the machine to be manageable by a man of ordinary capacity. And also that he shall not neglect the business entrusted by us to his care in Cornwall or unreasonably absent hmself from the same without our leave."

"The second proposition is for us to resign the whole machine entirely to him, on condition of his paying us a certain yearly premium on each machine proportionate to the number of horses such machine has ordinarily been drawn by..... and bind ourselves to grant such licence to him only provided he brings the machine to success in three years, and supplies the demand which may thus arise so as not to bring us into any scrape with the public, but I believe this part of the bargain must not be expressed in any legal document lest it militate against. I have done nothing in my last patent on that head but collect inventions which were secured to me before and I am sorry I have included the article (on steam carraiges) lest he should think I did it in opposition to him whereas I put it in to stop the gap against wolves and wasps and other venemous creatures. This last proposition meets my ideas the most fully as I do not wish to be diverted from our business and I do not chuse to risk money at the directions of a man over whom I may not have a proper control and in a scheme I do not entirely approve of..."

"If the first proposition is gone upon I would not choose to have anything to do with it if he has more than one quarter profits but in such case would refer the premiums for machines to you."

"The thing I mean to inculcate strongly in this letter are : 
Firstly, to attempt to dissuade him from the scheme on prudential motives without saying the thing will not do, because that may not be the truth and may militate against us in the bargain for permission."

"Secondly, if that cannot be done, to sacrifce such a sum as a hundred pounds on experiments rather than lose a valuable man and I would consent to swim in the case of our being to have a premium only."

"Thirdly, in no case to make him a partner, only to give him a share of the profits."

"Fourthly, to limit the sum to be spent on experiments and the time allotted for it in which time if he does not accomplish it we are to be free from bargain."

"Fifthly, to determine accurately what the machine is to do it's properties to be such as would make it useful."

"Sixthly, to make it the means of tying Murdoch to the business if it does not succeed."

"Seventhly, in the latter case to fix his wages and emoluments clearly and distinctly but nb the man must always be our servant and not his... as he has been a very usefull servant to us, though his conduct lately has been very disagreeable I think we ought not to impose too harsh terms upon him, nor any futher prevent him from going on with his scheme, and if he will leave us, to part with him on the most friendly terms, and if he rather chooses it, to give him the premium on the first six engines in a present rather than give the hundred pounds, or if you think it necessary, to give him both the one and the other. And this to be considered more in the light of a reward for past services, than any hope of them in the future....."

"You have now my full opinion on the subject which use as you please but I beg this letter may be kept secret even from you own family, such things should not in fact be trusted to writing."

Watt's "pre-emptive" steam carraige

The letter was accompanied by a rough sketch of Watts "pre-emptive" steam carraige, a crude, unformed and impractical sketch in comparison to the working model, which Murdoch had by this time built and tested without Watts knowledge. 


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A postscript to the letter, added by James Watt Jr, in reference reads : 

"...that this must be a mistake as Mr R. Boulton (Mathew's son) said he saw it work in 1784."

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M.Arago published an historical eloge of Watt translated by Watt's biographer Muirhead. In referring to Murdoch, Arago also refers to a friend who "saw the model steam carraige run round Murdoch's living room in Redruth in 1784".

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"At the time Mr Murdoch was making his experiments with his Locomotive Engine, he greatly alarmed the clergyman of the Parish of Redruth. One night, after returning from his duties at the mine, he wished to put to test the power of his engine, and as railroads were then unknown, he had recourse to the walk leading to the church situated about a mile from the town. This was rather narrow, but kept rolled like a garden walk, and bounded on each side by high hedges. The night was dark, and he alone sallied out with his engine, lighted the fire or lamp under the boiler, and started the Locomotive with the Inventor in full chase after it. Shortly after he heard a distant despair - like shouting; it was too dark to perceive objects, but he soon found that the cries for assistance proceeded from the worthy pastor, who, going into the town on business, was met on the lonely road by the fiery monster, who he subsequently declared he took to be the Evil One in persona. Whoever has been on one of our modern railroads on a dark night, and seen an apporaoching train - now no novelty - may easily imagine what effect the awful sight would have on the nerves of an elderly gentleman of the last century; and although the Demon was of small dimensions, yet it was a total stranger and quite unlooked for, in such a locality."

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In all the correspondence between Watt, Boulton and Wilson, there is no specific mention of the existence of a model locomotive. Nor is there any surviving correspondence from Murdoch to the three previously mentioned correspondents, therefore we can only guess as to Williams response to the "offer" Watt outlines to his senior partner, Mathew Boulton. Williams' opinion on steam-carraige matters, if recorded in any correspondence, does not survive. 

The model in the science museum is Model No1 (the only survivor), built by trial and error between 1781 & 1784 and although unseen by Watt, Boulton & Wilson, it was seen by others to whom Murdoch had demonstrated it to at his home in Redruth, Cornwall & witnessed by R.Boulton Jr, M.Arago, the elderly Parson and his daughter.