From the Preface. The observations set forth in the following pages are part of an inquiry commenced ten years ago into the heat-producing power of the heart. Some new and interesting points have emerged during the investigation, and through the kindness of Professor Starling, of University College, a paper on the "Effects of the Movement of Air and of Water upon the Pulse-rate in Man" was communicated to the Physiological Society in December, 1911. This subject is dealt with more fully in the latter part of this little book. My notes contain in all more than twenty-five thousand observations of the pulse-rate, the blood-pressure, and the superficial and deep temperature of the body, under various conditions. For recording the temperature the Fahrenheit degree, which being smaller than the Centigrade lends itself more conveniently to a minuter division, has been used throughout, and when the rectal temperature is being watched continuously with a delicate thermometer a change so small as 0.1 F may be recorded with confidence. The influence of exposure of the body to cold sufficiently intense to lower the deep temperature by overpowering the effect of the reactionary processes has not been studied; nor have I attempted to trace the practical application of the results described or their bearing on the subject of acclimatization.