the lake is really due to a small moraine, which fills up a gap between the ridge just mentioned and the rock surfaces at the west side of the tarn. Rock in situ does not reach the bed of the brook till it has fallen over several feet vertically of moraine gravel, and boulders.

There are two other little tarns which I visited-those of Grisedale and Loughrigg—whose formation does not appear to be attributable to moraines, as they are completely surrounded by solid rock basins. In the case of Grisedale Tarn, from its situation near the head of a pass or watershed, and partly inclosed by steep and high cliffs, it may not improbably afford an instance of the "scooping" power of ice or snow when charged with fragments of rock, to which Professor Ramsay has referred the existence of some of the mountain lakes of Wales.

To recapitulate. The facts here detailed appear to show1. That there have been glaciers at different levels, descending as low as 500 or 600 feet after the retirement of the sea which deposited the Drift.

2. That the Drift-sea rose at least as high as 1000 feet, probably much more.

3. That many of the tarns owe their formation to embankments of moraine gravel.

4. I infer that (for reasons stated above) the glaciation of the larger and less elevated valleys, together with the hilly districts south of the mountainous tracts, is due to floating ice of the Drift-period, in the form of bergs and coast ice,

5. That there were at this period some agents, whether currents or winds, or both, impelling the icebergs southward, so that they always moved in that direction, unless prevented by barriers.

It is only proper to state in conclusion, that not being furnished with an instrument for measuring altitudes, some of the numbers in the above pages are only estimations. However, as there are many positions-as the summits of the mountains, the more marked prominences and passes, the surfaces of the lakes, &c.-the elevations of which are well known, the author had frequent data to guide him, and feels confident that the numbers stated are close approximations to the reality.

45

On the Application of certain Laws of Heat and Combustion to the Use and Economy of Fuel. Being the substance of a Paper read before the Aberdeen Philosophical Society, 11th February 1859. By ALEXANDER D. MILNE.

By Combustion is usually meant the chemical combination of two or more bodies with the evolution of light and heat. Our present inquiry refers to combustion effected for the production of heat by means of ordinary fuel.

There are two modes in which this subject may be investigated. We may begin with first principles and ascertained scientific data, and from these descend to general laws and rules for practical guidance; or we may set out by comparing those conditions under which good or bad results have been attained in practice, and thus arrive at general rules which shall be applicable wherever the same conditions or circumstances recur.

The former method we intend chiefly to follow, and shall endeavour to show that many valuable data have been attained by careful scientific research; that they admit of very wide application; and that the deductions are of the highest practical importance to all who employ fuel as a source of heat.

Both the modes of investigation referred to have been ably followed out by Drs Ronalds and Richardson. It is here intended to view the subject in a somewhat different aspect; reducing the more important points to a systematic form, and calling attention to some phases of the inquiry which are not generally understood, although they appear to be of great importance. To illustrate our meaning, we may refer to Table *III., in which is shown the inseparable connection between Draught, Initial Temperature, and Economy of Fuel.

The substances generally used as fuel are wood and coal in all their varieties. They owe their heating powers to one or both of two simple substances-to carbon, or to carbon and hydrogen. The heat is evolved as an accompaniment or result of the combination of the oxygen of the atmosphere

with the combustible. With carbon, the resulting compound is carbonic acid CO,, a transparent colourless gas; with hydrogen, the result is water HO, in a state of vapour. The nitrogen of the air passes along with these gases, not altered farther than by being heated to the same temperature as the carbonic acid and vapour of water.

Atmospheric air contains 23 per cent. oxygen and 77 per cent. nitrogen by weight.

The transformation that occurs in the combustion of carbon may be thus represented, atomically:

Carbon,

1 atom = 6 Products of Combustion. Atmos. air, Oxygen, 2 atoms = 16. J Carbonic Acid, containing Nitrogen,

Or, assuming carbon as unity,

Carbon,

1.

= 22.

53.6 Nitrogen,

53.6

}

Products.

Air, containing Oxygen, 2:667) Carbonic Acid, 3-667

Nitrogen, 8-933 Nitrogen, 8.933

In the case of hydrogen, the atom of which is represented

Hydrogen therefore requires, as compared with an equal weight of carbon, three times as much oxygen, and consequently three times as much air.

When we use the term "perfect combustion," we mean that these combinations have occurred.

But combustion may be imperfect. The carbon, or part, may be separated as soot; or if, while the carbon is at an intense heat, the supply of air be suddenly diminished, the oxygen may combine with two atoms carbon instead of one, and thus form carbonic oxide CO, instead of carbonic acid CO,. Or the hydrogen and carbon, where both are present,

may combine, forming hydrocarbons, such as tarry vapours, coal-gas, &c.; or the hydrogen may escape unconsumed. At present, however, we shall consider the laws which are connected with the perfect combustion of carbon and hydrogen separately.

The data relative to the calculations are given in Table I., which explains itself in so far. In columns 5 to 10, the temperature 60° is assumed as the mean; and 32° as that from which it is usual to calculate the expansion of gases by heat.

Columns 13 to 16, relating to specific heat, are of great importance. When we cool different gaseous, liquid, or solid bodies, they give out different quantities of heat; which, expressed numerically, are called specific heats. Two standards are used for comparison-air and water; to the latter we shall adhere in subsequent calculations. If water is assumed as 1.000, air is 2669; that is, water requires nearly four times as much caloric to raise its temperature any given number of degrees as is required by air.

The specific heats, by different observers, vary to some extent. We give those generally adopted. It yet remains to be determined whether specific heat continues constant at all temperatures: experiments by MM. Dulong and Petit would seem to indicate that at high temperatures, say 1000°, gases require more caloric to raise their temperature than at low temperatures.

The heating powers in column 17 are by Andrews, and are expressed in parts by weight of water heated 1° Fahr. by the caloric generated in the combustion of 1 part by weight of the combustible. The data may be relied on, as the results for carbon by Despretz, Grassi, and Andrews, vary only 2 per cent.; and for hydrogen, the results by Dulong, Hess, Crawford, Grassi, Fabre and Selbermann, and Andrews, vary only 8 per cent.

The results for compound combustibles, such as olefiant gas, containing both carbon and hydrogen, approach closely to the theoretical results as calculated from the separate heating powers of their components. Column 18 shows the approximation in the cases of olefiant and light carburetted

hydrogen gases. In most treatises on fuel, the subject is rendered unnecessarily intricate by assigning to each hydrocarbon a separate heating power. They should evidently be regarded as mixtures of so much hydrogen with so much gaseous carbon, because, in their ordinary combustion, they are previously decomposed by the influence of heat into their two elements, which burn in succession; as in the case of illuminating gas.

Carbon as a Source of Heat.

More or less pure, it is met with in various forms, as fuel. We will proceed to give the particulars of its "perfect combustion," in a tabular form, and also examine some of the phenomena which attend its application to heating purposes.

In Table II., the process of combustion, without reference to its application, is represented.

All the caloric generated is regarded as being present in the products of combustion-viz., the carbonic acid and nitrogen. The mean specific heat of this gaseous mixture is found in column 8; in order to ascertain the temperature to which it is raised, as in column 11; and the total amount of sensible heat it contains, as in column 10.

As regards column 10, experimenters having ascertained that the caloric generated by 1 part carbon heats 14,220 parts water (specific heat 1·000) 1° Fahr., it becomes a simple matter to find how many parts of the products of combustion. (specific heat 2596) will be raised 1° Fahr. 2596: 1.000 :: 14,220 54,776; which may either be regarded as parts heated 1°, or one part heated 54,776°, if such were possible.

To find the temperature, column 11, we have to considerIf the heat generated is such as will raise 1 lb. of the gaseous products 54,776°, what will be the temperature, seeing that the weight of these products is 12-6 lb. Thus,-54,776° 12.6 = 4347°, which is the intensity of temperature produced, supposing no loss by radiation or conduction, and the specific heats to be the same at high temperatures as at low.

Effect of Excess of Air.-We have seen that 1 lb. carbon in being burned requires 11.6 lb. air, and that the temperature