The possession of a self-registering Clinical Thermometer in every household, in a tropical or malarious climate is a necessity, more especially when at a distance from medical aid. It is a means for assisting to ascertain most accurately, in a very few minutes, whether a child or patient is really ill, and the necessity there might be for the administration of remedies; for calling in other and more proficient assistance; or to allay anxiety that might have accrued from a nervous fear, when possibly no actual illness existed. From one single observation of an _abnormal_ temperature, we learn: "1. That the patient is really bodily ill. "2. When there is considerable elevation of temperature, we know that there is fever. "3. When there are extremes of temperature, we know that there is great danger. "The mean normal temperature of the human body in health is 98·6° Fahrenheit's scale."[5] This may vary in health, in exceptional cases from 97·5° to 99·5° Fahrenheit. The accompanying diagrams represent (_A_) a thermometer that has been employed, we will say, in a case of Intermittent fever, the Index showing the temperature to have been 104°; (_B_) a thermometer with the Index at 95°. The Index in a Clinical Thermometer is a small quantity of mercury separated by a bubble of air from the rest of the mercury; or, owing to the peculiar construction of the glass tube, without any air being present, a portion of the mercury separates itself from the bulk, and remains separated in the tube, as an Index. Upon placing the Thermometer in a favourable situation, in or against the body of a patient, owing to the amount of heat with which it there comes in contact, the mercury in the bulb expands, and the Index is pushed up to the highest point in the Thermometer that the heat of the body at that time is capable of causing, if the Thermometer is kept in contact with the body for the requisite time; and when the Thermometer is removed from its contact with the body, the mass of mercury suddenly cooling down, contracts, and returns into the bulb, the Index being left behind at the point to which the heat of the body had forced the mercury in the bulb to raise it; thus the Thermometer in Diagram A shows that in that instance the Index had been raised to 104° Fahrenheit and that that was the temperature of the body at that observation. In commencing to take the temperature of a patient, first of all see that the Index is in the position as seen in Diagram _B_, not necessarily always as low as 95°, but at all events well under the arrow →, which marks off the mean normal temperature of any one in healthy viz., 98·4° Fahrenheit, as given in most English thermometers (98·6° as used on the Continent). The temperature of an infant or young child is frequently found to be 99° whilst in health.