body in a climate such as is imposed upon us by the unalterable character of our British winters.

PRODUCTION AND DISSIPATION OF HEAT BY THE HUMAN BODY

From the standpoint of my subject, the human body may be regarded as a living organism which is unceasingly transforming the potential energy of its food into heat, with intermittent production of work, and which can only discharge its various functions properly, provided that its temperature is maintained constant within very narrow limits. The mean body-temperature, which varies only slightly with age or climate, is 37° C. (98.4° F.); in health and under ordinary conditions it is automatically maintained by various adjustments in the system itself, which is endowed with a remarkable power of rapidly responding to any changes in the physical conditions of its environment.

The heat generated in the body, amounting in the case of an adult man of a sedentary occupation to about 3000 or, in the case of a man engaged for 8 hours per diem in moderate work, to maybe as much as 5000 Kilogram-Centigrade Units per 24 hours, must be dissipated as fast as it is generated, or otherwise the body temperature would soon rise to a point incompatible with health or perhaps even life. This dissipation is effected to a small extent by the evaporation of water and the heating of air through the lungs, but for the most part by evaporation of water and radiation and conduction from the skin. According to estimates made independently by Helmholtz and by Dulong many years ago, the amount dissipated through the lungs is something of the order of 10 to 17 per cent., whilst that dissipated by radiation and evaporation from the skin amounts to between 80 and 90 per cent. of the whole.* These figures are doubtless only approximate, but they serve to show the very important part played by the skin in the process. In a climate such as ours, where the mean shade temperature is always below that of the body, the skin is always losing heat by radiation and conduction, at a rate dependent primarily upon the difference between its temperature and that of its environment, whilst the rate at which it dissipates heat by evaporation of water depends on a variety of circumstances. But it may be said that its power of evaporating water through the skin constitutes the body's principal safeguard against a disturbance of its normal temperature equilibrium when it is exposed to an unusually warm environment; it is the body's natural means of keeping cool when its environment gets too warm.

Vide Landois and Stirling, Text Book of Human Physiology, 4th Edition, vol. i. pp. 409, 410.

THE PHYSIOLOGICAL ASPECTS OF HEATING AND VENTILATION

The science of clothing, as well as that of indoor heating and ventilation, therefore, largely resolves itself into a question of helping the skin to maintain the normal body-temperature under the varying conditions of the day's work and life, without symptoms of discomfort and distress.

The following paragraph from a memoir published in the Philosophical Transactions of the Royal Society (B.207, 1916, p. 183) by Dr. Leonard Hill, F.R.S., and his collaborators, may be quoted here as giving perhaps the most recent and authoritative view of the physiological aspects of heating and ventilation:

It is not the actual temperature of the air but the rate of cooling which affects the cutaneous nerve-endings, by determining the difference between the temperature of the surface of the skin and the blood-temperature in the deeper layers. The amount of blood in the cutaneous vessels, the rate of evaporation of water from the skin, and the moistness of the skin surface influence our sensations of comfort or discomfort. The ceaseless variation in the rate of cooling, as in outdoor conditions, relieves us from monotony. A bracing wind cools the skin, tones up the muscles of the body, voluntary and involuntary, and impels us to take exercise to keep warm. Physiological research has shown that it is not the chemical purity, but the physical conditions of the atmosphere which act so potently upon us. It is the disaffection, or monotonous stimulation, of the vast field of cutaneous and nasal nerves on the outside of the body, not the absorption of poisonous inhaled products into the blood, which occasion the discomfort of badly, ventilated and crowded rooms. Monotony of the atmospheric conditions and the reduction of the body metabolism by diminished rate of cooling, with consequent loss of nervous tone and disordered digestion are, we believe, the chief causes of the ill-effects of confined sedentary occupations. The workers in such occupations are exposed to massive bacterial infection from the carriers of disease, coupled with a reduction of immunity resulting from their sedentary occupations in over-heated windless air, contaminated, as it may be, with products of imperfect combustion, and with a dust which predisposes to infection. This enfeebled state of health, together with massive infection, causes the epidemics of respiratory disorders, so prevalent in winter. Outdoor workers, exposed to cold and inclement weather, are singularly free from such affections.

IDEAL CONDITIONS OF COMFORT AND HEALTH

According to Dr. Hill, the natural conditions of comfort and health are those prevailing outside on a balmy summer day, namely, the warm ground for the feet, the radiant heat of the sun on one side of the body, a cooling breeze blowing on the other side and round the head. There is no monotony, but continual variation, in the conditions; whilst the cooling and drying breeze round the head means that more arterial blood has to pass through the respiratory membrane and air tissues to maintain the body temperature in these parts, and far more lymph has to pass through

the membrane to maintain the evaporation to saturate the drying

air.

Contrast these ideal conditions of health with the tropical and monotonous conditions of the warm humid air of crowded rooms heated by steam coils, and it becomes evident both what we ought to aim at, and what we ought to avoid, in our domestic heating and ventilation.

THE ENGLISHMAN'S PREFERENCE FOR OPEN FIRES

Bearing this in mind, let us now consider for a moment the characteristics of our trying British winters. What makes them so insupportable is not so much their severity, as measured by their coldness, but their dreariness, as caused by long monotonous spells of leaden skies, lack of direct sunshine, and enervating moisture-laden atmosphere, with frequent fogs or drizzling rain,

An examination of the Kew records for the 25 years period, 1881 to 1915 inclusive, shows that the average mean temperature in that locality for the three winter months (December to February) has been just a little below 40° Fahr. (39.75° more accurately), and the mean relative humidity a little over 80 per cent. And from the most recent charts published by the Meteorological Office I find that for the London area the mean maximum January day temperature is 40° Fahr., whilst the mean minimum night temperature is just a little under 34° Fahr. The daily ration of bright sunshine only averages 1 hours, whilst the average rainfall is at the rate of about 40 inches per annum.

These figures enable us to explain why it is that an Englishman objects to the Continental and American systems of closed stoves or central heating in his living rooms, and prefers to be warmed by the radiation from a bright fire. The Mid-European, whose metabolism is highly stimulated by the cold dry air and bright sunshine of his winter days, feels no particular discomfort, but possibly some relief, in the warm and relatively stagnant atmosphere of his living rooms, heated as they generally are by closed stoves, with consequent great economy in fuel. But an Englishman, oppressed as he is during the daytime by the humid air and dreary sunless skies of his winter environment, seeks relief in his home at nights by his radiant fire-side, and disregards with characteristic disrespect the vapourings of scientific cranks who condemn it as wasteful. And perhaps, after all, he is not far wrong.

If, then, the right conclusion of the matter is that, our British climate being what it is, we ought to aim at having warm floors and to avoid monotony in our physical environment, there is a good deal to be said, on scientific grounds, in favour of our open fireplaces. The real problem is how to combine their undoubted

hygienic advantages with greater economy in fuel and labour and less smoke.

DEFINITION AND MEASUREMENT OF COMFORT CONDITIONS

Before, however, considering the principles upon which a good open coal or gas fire should be constructed, let us see how comfort conditions' in a living room may be measured and defined, remembering (1) that the factors chiefly concerned in determining comfort' are the temperature, humidity, and movement of the air, and (2) that, in our climate at any rate, a large proportion of radiant heat from a glowing surface is conducive to health.

First of all, it may be said that the mere observation of the temperature of the air in a room by an ordinary thermometer is, by itself, of little value as an index of comfort,' and of no value. whatever as a measure of the amount of heat in it. For such purpose, the thermometer is a discredited instrument among scientific workers, and only the Coal Controller's Experts ever use it. Comfort is not determined by temperature merely, and a thermometer measures neither heat nor radiant energy. It only measures the degree of heat, not its quantity; whilst of radiation. it gives no measure at all, except its bulb be blackened, and even then its records are qualitative and relative only.

Although a room would not be comfortable unless the temperature of the air in it was maintained between certain limits, an equally important factor to be reckoned with is the relative humidity of the air, which may be measured by observing both its temperature and its dew-point by means of the dry' and 'wet' bulb thermometer respectively. For the amount of water vapour required to saturate' the air increases regularly with its temperature, and the relative humidity' of the air at any temperature is the degree of approach to saturation at that temperature. And the lesser that degree the greater the drying power' of

the air.

According to some experiments recently carried out in Chicago under the supervision of Dr. E. V. Hill, the comfort or otherwise of a person sitting in a room with a stagnant atmosphere may be defined in a chart (Fig. 1)5 on which the temperature of the air (as ordinate) is plotted against its relative humidity (as abscissa). Dr. Hill's experiments were made upon American subjects, who are accustomed to live in much warmer rooms than

5 An account of Dr. E. V. Hill's experiments appeared in the Journal of the American Society of Heating Engineers. An excellent résumé of the work was given in a paper read by Mr. A. H. Barker, of University College, London, before the Society of Architects on the 15th of November 1917, and subsequently published in their Journal, vol. xi (1918), pp. 3 to 32 inclusive, from which the two charts shown in this lecture are taken by kind permission.

we do; and were similar experiments to be made upon English subjects, possibly the zone lines' would come out differently, although a similar kind of chart would doubtless be obtainable. Therefore, it would seem that in a stagnant atmosphere comfort' may be fairly well defined in terms of its temperature and relative

humidity. Low humidity compensates for high temperature, and vice versa, or in other words, circumstances which increase evaporation from the skin compensate for a decrease in the rate at which heat is lost from the body by convection, and vice versa.