- 100

116. The temperature of the air is ascertained by the Thermometer, fig. 12, which consists of a small closed glass

tube, having a bulb at one end, and partially filled with mercury or spirit of wine. Of these fluids mercury is the best, owing to its uniform expansion by heat; the quickness with which, from its low specific heat, it indicates changes of temperature; and the great range of its fluidity. A spirit thermometer must be used when the temperature falls below --37o.9, the point at which mercury freezes, as determined by Dr Balfour Stewart. Spirit thermometers are also of great use for registering the greatest cold.

117. The following points in the construction of thermometers must be attended to. The mercury should be

pure and dry, and boiled so as to expel the air; the bore of the tube should be equal throughout; the bulb should be large in proportion to the bore, so that the degrees may be large and easily read; and no air should be left in the top of the tube. It is also most desirable that thermometers should be filled and hermetically sealed for at least a year before the scale is en

graven on them. The reason is, that the fibres Fig. 12.

of the glass take some time to assume their permanent position; and since, in this transition state, the atmosphere pressing on the exterior surface of the bulb constantly tends to push the mercury further up the vacuum above the column, the result is that the bulb becomes permanently contracted in size, and the column stands higher on the scale. Nothing is more common than to find such thermometers, after being some time in use, to read 0°.5 or 10.0 higher than they did when previously compared with the standard. Owing to the changes to which they are subject, thermometers should occasionally be compared with a standard thermometer, or have their freezing-point tested by plunging them in melting ice.

118. Division of Thermometer Scales.—Before the indications of different thermometers can be compared, it is necessary that there be two fixed points on their scales, each of which indicates precisely the same temperature. The points which have been chosen are the temperature at which water freezes, and the temperature at which it boils when the barometric pressure is 29.905 inches reduced to 32°. In both cases distilled water must be used; for if the water contain salts or other impurities, it will freeze and boil at different temperatures. If the pressure exceed 29.905 inches, the temperature of the boiling point will be higher, but if less it will be lower, the proportion being one degree of Fahrenheit for every 0.589 inch of pressure at moderate heights. Hence, when the barometer is 28.000, water will boil at 208o.6 instead of 212o. At Santa Fé de Bogota, which is 8727 feet high, and atmospheric pressure is 22.061 inches, water boils at 1970.46; in Mexico, at 7000 feet high, it boils at 200°; and at Quito, 9000 feet, it boils at 194°.

119. Advantage is taken of this circumstance to measure roughly the heights of mountains. The temperature at which water boils is observed, from which the pressure of the air is deduced, and compared with the pressure observed at the same time at some neighbouring place, the height of which is known. From the difference of the two pressures the height is ascertained. An observation should be made at a lower level where the height is known before ascending the moun

tain whose height is to be measured, and another observation in descending, so that errors arising from fluctuation in the pressure may be compensated for. The exact hour of the day of each observation should also be noted, so that the correction for daily variation may be applied.

120. The space on the scale between these two points has been divided in different ways. FAHRENHEIT, a native of Dantzic, fixed the zero-point at the greatest cold then known to have occurred, in Iceland, supposing that lower temperatures would seldom require examination. The space from freezing to boiling he divided into 180 equal parts; and since his zero-point is 32 of these parts below freezing, the freezing-point of water is 32°, and the boiling-point 212°. This is the scale in common use in England and America, and its practical advantages over other scales are these: (1) The degrees are smaller than in the other scales, and hence greater exactness in observing is attained; and (2) as the temperature rarely falls below zero, the minus sign is seldom required, -an advantage of some value in summing up and printing tables of temperature.

121. Celsius, a professor at Upsal, divided the scale between the two fixed points into 100 parts, the freezing-point being zero. This thermometer is generally called Centigrade, from the division of its scale into 100 parts. It is used in France and some other Continental countries, and is extremely convenient for purposes of scientific inquiry.

122. In Reaumur's thermometer the same space is divided into 80 parts, the freezing-point being the zero of the scale. It is in use in Germany and Russia.

123. It is often required to convert temperatures expressed in the Centigrade or Reaumur's scale into Fahrenheit's scale, and vice versa. Since the space between the two standard points is divided in Fahrenheit's thermometer into 180 parts, in Centigrade into 100 parts, and in Reaumur's into 80, the proportions are

F:C::9:5 F:R::9:4, &c. Hence to convert Centigrade degrees to Fahrenheit, we multiply them by 9 and divide by 5, and add 32° to the result, because 32° on Fahrenheit's scale is the zero of the Centigrade scale ; and to convert Fahrenheit into Centigrade, first subtract 32°, multiply by 5, and divide by 9. In Table V. the Centigrade thermometer is compared with Reaumur's and Fahrenheit's from 140° to -40° F.

124. Self-Registering Thermometers. The most important temperatures in their relation to climate and most other inquiries are the highest which occur during the day and the lowest which occur during the night; and to record these, various thermometers have been devised, well known as maximum and minimum thermometers.

125. Maximum Thermometers. The maximum thermometers generally used are Rutherford's, Phillips's, and Negretti and Zambra's. Rutherford's Maximum Thermometer has a movable steel index on the top of the mercurial column. When the instrument is in use, it is hung horizontally, and as the temperature rises, the mercury pushes the index before it, but as it falls the index is left, thus registering the highest temperature. It is set by bringing the steel index to the surface of the mercury by means of a magnet ; or by simply holding the instrument upright, and thus permitting the index to fall gently down on the mercury, shaking or tapping it slightly if required. The objection to this thermometer consists in its liability to get out of order by the index oxidising, and then getting plunged into the mercury; and so certain is this to take place in a few years at most, that the instrument cannot be recommended. The end of the index next the mercury is sometimes covered with a coating of glass, by which its tendency to oxidisation is prevented. A maximum thermometer thus constructed forms perhaps the best that has yet been invented, because the index is not easily shaken out of its position, and it may be set with a magnet without requiring to be removed from the hook to which it is attached, thus lessening the risk of breakage.

126. In Phillips's Maximum Thermometer, fig. 13, a portion of the mercurial column is detached and kept separate from the other part by a minute air-bubble. When in position it is hung horizontally; then as the temperature rises the whole column moves along the scale, but when it begins

[graphic][merged small]

to fall the detached portion is left behind at the point to which it had been pushed, thus registering the greatest heat. This is an excellent thermometer. Care, however, should be taken to select one in which the detached portion is little more than 1} inch in length ; for if it is longer, its weight in the tube makes it easily shaken out of its place; and if shorter, it is so troublesome to set, that in striking it so as to send down the detached part to the mercury, the risk of breaking it is considerable.

127. In Negretti and Zambra's Maximum Thermometer, the tube is bent at the part of the tube near the bulb, and the bore of the tube is contracted at the angle. It is hung horizontally; with a rising temperature the column is pushed along the scale, but when the temperature begins to fall, the column of mercury breaks at the angle where the bore is narrowed, thus leaving the mercury in the tube at the highest point to which it has been driven. It is also an excellent thermometer; but since the detached portion of mercury in the tube is always of very considerable length, and therefore of some weight, it is liable to be shifted out of its place when shaken by accident, or by the wind during storms.

128. In selecting a Phillips's, or a Negretti and Zambra's thermometer for the registration of very high temperatures, such as those recorded by thermometers exposed to the sun's rays, care ought to be taken to see that the thermometer registers sufficiently high. For these thermometers only register properly up to a certain point; the reason being, that there is almost always a portion of air, however small, in the top of the column, and hence when the mercury has risen to near the

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