The graduation, as marked on the stem of the thermometer, is usually to single degrees, but anyone can learn to read to tenths of a degree by a little practice. Care must be taken to have the eye opposite the top of the mercury column. Suppose it to be between 50 and 51, the exact number of tenths above 50 is to be estimated thus: If the mercury is just visible above the degree mark it is 50°.1, if distinctly above the mark 50°.2, if nearly one-third of the way to the next mark 50°.3, if almost half-way 50°.4, exactly half-way 50°.5, a little more than half-way 50°.6, about two-thirds of the way 50°.7, if nearly up to the next mark 50°.8, and if just lower than the mark of 51° it is 50°.9. The eye soon becomes accustomed to estimating these distances. [Illustration: FIG. 1.—_Reading Thermometer Scale above and below Zero._] In using a thermometer below zero, the observer must pay attention to the change in the direction of reading the scale, the fractions of a degree counting downward from the degree mark instead of upward from it, as in readings above zero. Readings below the zero of the scale are distinguished in recording them by prefixing the minus sign. The annexed figure shows the reading of two thermometers graduated to fifths of a degree, one showing a temperature of 1°.4, the other of -1°.4. The British Meteorological Office now recommends the use of the Centigrade thermometer graduated from the absolute zero, _i.e._, the freezing point is shown by 273°, the boiling point as 373°. _Care of Thermometers._—Mercurial thermometers will always be employed for ordinary purposes in places where the temperature is not likely to fall to -40°: _i.e._, everywhere except in the polar regions and the interior of continents north of 50° N. These thermometers are very strong and are not easily broken except by violence. The one vulnerable part is the bulb, which is of thin glass and filled with heavy mercury. Hence, in carrying thermometers, care has to be taken to protect the bulb from coming in contact with any hard object. The best way to carry an unmounted thermometer is in a closed brass or vulcanite tube with a screw top, the inside of the tube being lined with india-rubber and provided with a cushion of cotton-wool for the bulb to rest on. If the thermometer is mounted in a wooden frame it should be secured in a box so that the frame is firmly held and the bulb projects into a vacant part of the box, which may be lightly filled with cotton-wool or provided with a deep and well-padded recess. Every thermometer which is not graduated above 120° should have an expansion at the top of the tube which the mercury that may be driven beyond the scale by over-heating will not fill; otherwise any accidental over-heating will break the bulb.