estimated at about fifteen pounds per horse power; and taking the ratio just mentioned, it will follow that the average hourly consumption, under marine boilers, would be about ten pounds per horse power. We have thought it necessary, however, to seek other and more direct means of determining the value of this element, the most important in the investigation of the question before us; and we have accordingly obtained from the log-books of nine of his Majesty's steamers, in the service of the Admiralty -from one steamer, reputed to be the most efficient in the service of the Post Office, and one of a private company-considered to be of great power and efficiency-the actual quantity of fuel consumed in long intervals of time in which these vessels made numerous voyages. These results are in near accordance with the estimate which we have just given. The following table exhibits, in the fourth column, the average hourly consumption of fuel in pounds' weight per horse power, in the steamers whose names are stated in the first column, computed from the total number of hours, steaming at full power, expressed in the second column, and the total consumption of fuel in lbs. during that time, expressed in the third column.

The first nine of the above vessels are Admiralty steamers. The Dolphin is a packet in the service of the Post Office between Liverpool and Dublin; and her consumption of fuel is taken from 446 trips between these ports.

If the average of all these results are taken, it will be found to be very nearly equivalent to the estimate obtained from the experiments made at Soho.

On comparing the hourly consumption of fuel in the Admiralty steamers, a considerable superiority in economy is apparent in the

VOL. LXV. NO. CXXXI.

I

This superiority

performances of the Medea and the Dee. must be ascribed mainly to the greater efficiency and power of the machinery with which these fine frigates are supplied.* Something, however, must also be attributed to the superior magnitude and power of the engines, it being generally found that engines of greater power are attended with a greater relative economy of fuel.

The consumption of fuel will depend in some degree on the quality of the fuel itself-some coals burn quickly, but develop heat with a proportionate rapidity, so that, though their hourly consumption would be greater, they might give a proportionately increased speed to the vessel. But we shall presently consider the question of speed combined with power; meanwhile we shall perhaps be justified in assuming that, with all attainable care in the management of the furnaces, and boilers (on which much depends), and with a proper selection of fuel, the average practical hourly consumption, under marine boilers, cannot be less than about eight and a half lbs. per horse power.

The voyages of all the steamers, to which we have referred, have been made since the middle of 1834; and are therefore free from the objection brought by Mr Field against the reasoning of Dr Lardner. They lead nevertheless to nearly the same estimate of the hourly consumption of fuel as that given by Dr Lardner at Bristol.

The major limit of a probable steam voyage will depend not only on the average rate at which the fuel is consumed, but also upon the average at which the vessel shall be thereby propelled, The extent of misrepresentation and misconception which prevails respecting the rate of steamers is difficult to be credited. The disposition of captains and owners to magnify their performance has led them naturally, in reporting their rate, to state their best performances, and not their average speed. A further source of misconception and error will be found in the confusion of British statute and nautical miles. We wish in this article to be understood uniformly to estimate rates and distance by knots or nautical miles,+ without allowing for casual deviation from the proper course, and not including delays and stoppages

The engines in both these vessels were built by Maudsley and Company.

A nautical mile or geographical mile is the one sixtieth part of a degree, but the measure of a degree in British statute miles is 69 1-15th miles. The proportion, therefore, of a nautical mile to a British statute mile is nearly seven to six-the rates of vessels expressed, therefore, in

at intermediate ports; and we shall, as before, limit ourselves to the performances of steamers since the summer of 1834.

We have before us the particulars of twenty-six passages performed in 1835, between Falmouth and Malta, by the following steamers:-viz. Firefly, Tartarus, African, and Blazer. The total time of these voyages was 8682 hours. Taking the distance between Falmouth and Malta as 2000 miles, the average rate obtained from these data would be six miles an hour.

As this average, however, is derived from the performances of vessels which are not all the most powerful and efficient, we shall avail ourselves of the data supplied by the logs of the steamers already referred to, giving their hourly rate. The following table exhibits, in the third column, the average hourly rate for each of the vessels severally, whose names are in the first column, deduced from the total number of hours' steaming at full power, expressed in the second column.

Having thus obtained the average rate, and the hourly consumption of fuel, the quantity of the fuel which will be consumed in a given distance-or what is the same-the distance over which a given quantity of fuel per horse power will propel a vessel, is a matter of easy calculation. For brevity we shall adopt a term which has been introduced in the statement of Dr Lardner, at the Bristol meeting; and shall call the distance over which a ton of coals per horse power is capable of propelling a vessel the locomotive duty of the fuel. This locomotive duty,

British statute miles, will be numerically greater than in nautical miles in the proportion of seven to six.

computed by a comparison of the hourly rate of the vessel with its hourly consumption of fuel, is exhibited in the following table.

The following table exhibits such particulars of the magnitude and proportions of the vessels, and the construction and power of the machinery, as may further illustrate the results obtained in the preceding tables:- ས

It appears, therefore, that of these vessels the Dolphin has the greatest average speed; the Medea and the Dee the greatest economy of fuel; and the Medea the greatest locomotive duty. The Dolphin is a packet, built* expressly with a view to speed,

*

Built by Mr George Graham, Navy Yard, Harwich, Essex..

supplied with the largest possible proportion of power,—having machinery of the very best description, constructed by Bolton and Watt. The extent of her power, compared with hertonnage, renders her incapable of taking more coals than are sufficient for about 250 miles of one continued run; and, therefore, no conclusion can be drawn from her performance which would be applicable to long sea voyages. It will be observed, however, that the great speed of this vessel is obtained by the expenditure of a more than proportionate quantity of fuel. While she has an average speed greater than that of the Medea, in the ratio of 97 to 78, she consumes fuel in the higher ratio of 120 to 83. The necessary consequence is that she has an inferior locomotive duty.

The Medea and the Dee are vessels of the same class, but the Medea is of more recent construction.* The splendid machinery in both of these vessels was supplied by Maudsley and Field; and from the circumstance of the equal economy of fuel, exhibited in the preceding tables, and obtained in each case from long continued performances, the engines may perhaps be regarded as of equal efficiency. The Dee has engines more powerful, in proportion to hertonnage, than the Medea; the proportion of power to tonnage in the Dee being ten to thirty-three; and in the Medea ten to thirty-six. The chief cause, however, of the superior speed of the Medea is, no doubt, owing to the circumstance of her being propelled by feathering paddle-wheels, while the Dee is furnished with common ones. The ratio of the rates of speed of these vessels being as seventy-eight to sixty-five, is very nearly that which has been generally found to be the effect of substituting the feathering for the common paddle-wheels.

Besides the performances of the steam-ships just adverted to, we have before us the particulars of a vast number of voyages of the Dundee and Perth steam-ships plying between London and Dundee. The average speed of these vessels exceeds even that of the Dolphin, being 999 miles, or very nearly 10 miles an hour. We have not been able to ascertain with sufficient certainty or precision the consumption of fuel by which this speed is attained. We have, however, reason to believe, that it is above ten pounds per horse power per hour; from which it would follow that their locomotive duty is not higher than that of the Medea.

The circumstances of weather and water, to which these vessels were exposed, may perhaps be taken as fair average weather in

*The Dee was built by Seppings, and the Medea by Lang. The date of the construction of the Medea was, we believe, 1838.