The cooling by respiration in moist air is therefore about one eighth of that in dry air at the same temperature. But this is not all the heat lost by evaporation, nor the greater part; the loss by the skin is nearly twice that by the lungs under the same conditions. Here also the same law holds, the greater the relative moisture the less evaporation and consequently the less cooling.

According to Lavoisier and Seguin, 900 grams of fluid per day are discharged by perspiration, and 500 grams from the lungs, making 1400 grams of fluid lost in twenty-four hours. The evaporation of this quantity of water will consume 750 units of heat, or about one fifth of all the heat produced in the body in twenty-four hours.

The production of heat in the animal body, and its maintenance at a normal standard, are two of the most important processes in the living organism. The two chief means for regulating the temperature of the body are the skin and the lungs. Of these the most direct and simplest is that by the cutaneous perspiration. The relations of these organs to the atmosphere, therefore, are of great importance in the question now under consideration.

But the rate of evaporation and consequent cooling depends in great measure on the aqueous vapor already in the atmosphere. That this relative amount has a material influence on our individual comfort there is no doubt. It is certain that on those days when the proportion of humidity is greatest, even the healthiest feel an oppression and languor, and that on other days when the humidity is less there is an exhilaration of spirits and an increase of muscular energy.

It is worth while, then, to recall the laws governing this aqueous vapor, for it pervades the atmosphere, is one of the main causes of its movements, and the only fluctuating ingredient in its composition.

The evaporating power of air raised to a higher temperature is increased. A quantity of air absolutely humid at 59° F. holds an amount of vapor equal to of its weight; at 86°, 4; at 113°, & ; at 140°, so that while the temperature advances in an arithmetical progression, the vapor-diffusing power of the atmosphere rises with the accelerating rapidity of a geometrical series having a ratio of two; with the same ratio, evaporation increases, and consequently the cooling

process.

It is upon this play of forces in the aqueous vapor and the air, and the movements they bring about, that we must rely for the comfort of our patients in the heats of summer. It is not a question of changing the temperature of the air; practically, we cannot alter that nor its

humidity, in the volumes required for ventilation. It is a question of the rate of evaporation from a perspiring surface, which again is governed in great measure by the velocity of the air; and this by the improvements in the arts we can control.

If, on the other hand, we attempt to attain our object by cooling the air before it enters the ward, we are met with this fact. If air absolutely humid comes in contact with warmer air also saturated, the latter will be cooled, it will approach the dew-point, and, if its moisture is condensed into visible vapor, will give out heat. Evaporation consumes heat, condensation liberates heat.

In our first experiment the previous cooling of the air did not bring it to the point of condensation, but its relative humidity was increased; the rate of evaporation was therefore diminished, and to that degree it was a disadvantage.

The quantity of air required for our purpose cannot, as we have already said, be determined by instruments of precision alone; it must be learned by experiment and the declared sensations of the sick.

The movement of the air around us, and it is never still, — the natural ventilation as it is called, is much greater than is generally supposed. Repeated experiments have shown that at two feet a second we first feel the air as a moving body; less than that we consider a perfect calm. And yet at this velocity air would move from end to end of our ward of 60 feet in 30 seconds, and across it in half that time, quite unnoticed by us.

To give comfort during the excessive heats of summer the sick require three or four times the air needed for satisfactory ventilation in winter. It required 400,000 cubic feet an hour for our sixteen patients, and yet while this large quantity was passing through the ward it was only known, except at the registers, by the accompanying sense of freshness and pleasant coolness; it was never felt as a draught.

"The great regulator of the heat of the body is undoubtedly the skin." Physiology teaches that perspiration is a secretion, in a sensible or insensible form, constantly going on. Increased heat increases perspiration, and the evaporation of this increased quantity consumes in work a large portion of the heat derived from the atmosphere, and thus prevents an undue rise of the temperature of the bodily organs. The very intensity, therefore, of the peripheral circulation, under the action of heat, leads the way to relief.

Experiments made more than a hundred years ago prove that, if the skin perspires freely and the perspiration be readily evaporated,

the temperature of the body may remain nearly normal in an excessively hot atmosphere, - even more than 200° F.

In the present atmosphere of mixed air and aqueous vapor. with which it is never saturated, evaporation and convection must coexist. So long as the expired air is loaded with moisture, and the skin performs its perspiratory function, and the movement of the surrounding air is under our own control, if, so to speak, we own a breeze, we may confidently rely on our ability to dispense its comforting and refreshing influences to the patients in our hospital.

The following observations with the wet and dry bulb thermometer may serve to illustrate the cooling of a moist surface. June 17, 1894, a thermometer in a still room was at 78° F.; after covering the bulb with a piece of thin cotton cloth moistened with water, and fanning it for five minutes with a common fan, it fell to 72°, - a difference of 6°. The same thermometer on the same day at 99°, treated in the same way, fell to 77°, —a difference of 12°. A thermometer in the open air in the shade, July 13, 1894, with a gentle breeze, was at 95°; with a moistened bulb, at 73°, a difference of 12°. The relative humidity at the same time was 53%.

But the air must be in motion. A perspiring patient in still air is surrounded by an atmosphere permeated by much aqueous vapor; this must be diffused and carried away from the neighborhood by the continued arrival of fresh drier air, to get the full cooling effect due to evaporation.

It is in this way that simple agitation of the air in a warm still room brings relief, as with a common fan, or the rotary fan of the shops, or the Indian punkah. So it is with a ride in the open electric car on a hot day; the relief is immediate. There is no atmospheric change either in temperature or in moisture; it makes no difference whether we move through the air, or the air moves by us, the sense of cooling is the same. In both, we are surrounded by air constantly renewed, bringing with it the pleasurable sensations and invigorating influences belonging to a freely moving atmosphere.

What these influences are to those in health we know; what they are to those languishing on beds of sickness, those only who have experienced them can fully appreciate. That the patients in our hospital have derived much comfort from them, their repeated declarations fully prove. Besides the physical comfort they give, like the suggestions of flowers and music, with which the sufferings of the sick are now so often soothed, these large volumes of air fresh from the fields seem to hold up to the mind of the convalescents suggestions of other

scenes, which displace, for the time at least, present surroundings, and encourage the hope, so helpful to the sick, of a speedy return to their former enjoyments.

The experience of the Cambridge Hospital leads to these two conclusions: first, that fresh air directly from the open, in the quantity and manner there supplied, can be made to give great comfort to the sick during the heats of summer; and, secondly, that previous cooling of the air so supplied is difficult and practically useless.

To this may be added, what is of much importance to charity hospitals, that the method here adopted is the least expensive of the cooling processes hitherto made generally known.

INVESTIGATIONS ON LIGHT AND HEAT, MADE AND PUBLISHED WHOLLY OR IN PART WITH
APPROPRIATION FROM THE RUMFORD FUND.

XXII.

CONTRIBUTIONS FROM THE PHYSICAL LABORATORY OF THE MASSACHUSETTS INSTITUTE OF TECHNOLOGY.

XLIII. - EXPERIMENTS ON THE RELATION of
HYSTERESIS TO TEMPERATURE.

BY FRANK A. LAWS AND HENRY E. WARREN.

Presented by Charles R. Cross, October 10, 1894.

THIS paper gives an account of some experiments to determine the effect of temperatures much above the normal on the dissipation of energy by hysteresis in a specimen of steel.

At the time of the beginning of this research, in February, 1894, there were no complete studies of this subject known to us. A casual reference is to be found in the Proceedings of the American Institute of Electrical Engineers, Vol. VII. p. 325, 1890, by Prof. Harris J. Ryan. The tests there referred to were made on a cast-iron ring. The maximum temperature employed was 360°. The details of the measurements are not given. A short paper by Dr. Wilhelm Kuntz appeared in the Electrotechnische Zeitschrift, Vol. XIII., May 6, 1892. In this Dr. Kuntz showed that the hysteresis loss decreased with rise of temperature. A second paper by the same author appeared in the Electrotechnische Zeitschrift, Vol. XV., April 5, 1894. The magnetometer method was used by Dr. Kuntz in this work. In this paper tests of several ferrous materials are given, as well as some on a specimen of nickel.

At the outset of this research it was decided that alternating currents should be used, and that the losses should be determined by a Wattmeter, thus reducing the time required for observations to a minimum. The instrument which we designed and used is shown in Figure 1. We have decided to call the arrangement a Watt-balance. Mr. A. E. Kennelly has given in the Electrical Engineer, December 21,