cends to the same height, under the same atmospheric pressure. Fig. 3. 212 72 32 ... Boiling point. 39. These invariable positions, which are termed the freezing and the boiling points, are marked upon the scale to which the tube is affixed. In Fahrenheit's thermometer, figure 3., the interval between them is divided into 180 parts, each of which is called a degree (10) and as the freezing point is marked 32°, the boiling is therefore 212°. The divisions are extended downwards from 32° to 0, or the zero point, and when extreme degrees of cold are to be measured, the range is continued to 20°, 40°, and even 60° below zero. If the air is colder than 40° below zero, a spirit thermometer must be used, since mercury becomes solid at this temperature. When Simpson, a late northern traveller, wintered, in 1838, at Fort Confidence, 67° N. lat., he cast a bullet of mercury, the temperature being 49° below zero. Upon firing the ball, it passed through an inch plank, at the distance of ten paces; but flattened and broke against the wall, three or four paces beyond. In addition to the mode of graduation adopted by Fahrenheit, several others prevail (C. 570), which it is not necessary here to discuss. 0 20 FAHRENHEIT'S Freezing point 40. The thermometer employed for meteorological purposes, should be made as accurate as possible, and in Into how many intervals is the space between them divided in Fahren heit's scale? What are the intervals called? How many degrees is the freezing point? How many the boiling point? What is the zero point? When must a spirit thermometer be used? order to ensure its perfection, many niceties must be observed in its construction. 41. FIRST. The tube should be of equal size throughout the whole stem; else the same increase of temperature will not produce the same increase in the height of the mercury, throughout every part of the tube; and so of the decrease. SECONDLY. The bulb should be large in proportion to the tube; for then slight changes in temperature will be rendered perceptible, and the delicacy of the instrument increased. THIRDLY. The mercury should be pure, dry, and recently boiled, in order to free it from air; and, when in the tube, should there again be boiled, to drive off any air or moisture collected within. LASTLY. When the mercury is at the summit of the tube, and every thing else has been expelled, the top of the tube must be perfectly closed by the fusion of the glass, leaving, when the mercury has cooled, a void space or vacuum above. 42. When a thermometer has been exposed to great changes in temperature during the course of a year, the position of the freezing point upon the scale is found to be somewhat altered; for, if the instrument is then placed in melting snow, the mercury is usually seen to stand a little higher than 32°, and less than 33°. This change would occasion a constant error in the observations, and meteorologists therefore verify their thermometers at stated intervals, in the way just mentioned. 66 SELF-REGISTERING THERMOMETER. 43. The object for which this instrument is constructed, is to obtain, in the absence of the observer, the highest and lowest temperature of the day, or of any other interval of time. What precautions must be taken to construct an accurate thermometer} What change occurs in the position of the freezing point? How are thermometers verified? For what purpose is a self-registering thermometer used? One of the most correct thermometers of this kind, now in use, is' that invented by Mr. James Six, of Colchester, which is represented in Fig. 4. It consists of a long glass bulb, G, narrowing into a fine tube, which is first bent downward, forming the arm a b, and then upwards, forming the arm cd, which terminates in a small cavity, L. The two arms contain mercury, which extends down from a on one side, and up to con the other the bulb and the rest of the tube are filled with alcohol, except the upper part of the cavity L. Upon the top of the mercury in each arm rests an index (which is more perfectly seen at A), consisting of a piece of iron wire capped with enamel, and loosely twined with a fine glass thread; when the mercury descends, the index would fall, were it not for the glass thread, which, pressing like a spring against the sides of the tube, supports the index, in any position. SIX'S SELF-REGISTERING 44. The action of the instrument is as follows: When an increase of temperature expands the spirit, the mercury is depressed in the arm a b, and elevated in c d, carrying the index up with it. If the temperature now falls, the spirit contracts, and the mercury descends in c d; but the index remains in its last position, from the pressure of the glass spring against the tube; and, as it does not fit tightly to the latter, the alcohol above it flows readily by. As the cold augments, the mercury rises in a b, bearing up the index of this arm, until an increase of temperature occurs, when the mercury here falls, and the Describe Six's, from fig. 4. index continues stationary. Thus, the highest point to which the index rises in the arm, a b, indicates the least temperature, and that in cd the greatest, that happens in any interval of time, as a day, or a year; and the scale, as is evident from the figure, is graduated accordingly. 45. After every observation, each index requires to be adjusted; this is done by means of a magnet, which, being moved down the side of the arm, draws the index after it. Another instrument of this kind was invented by Rutherford, (C. 575.) MEAN DAILY TEMPERATURE. 46. The mean or average temperature of the day, would be accurately found by observing the thermometer at intervals of an hour during the whole twentyfour, and dividing the sum of the temperatures by the number of observations, viz., twenty-four. This method is however too laborious, and meteorologists have endeavored to arrive at the same result from two or three daily observations. 47. According to Kaemtz, a celebrated German meteorologist, if, in Germany, the thermometer is noted at 6, A. M., 2, P. M., and 10, P. M., and the sum of the temperatures divided by three, the quotient will differ but little from the true mean. The rule adopted in the State of New York, under the direction of the Regents of the University, is as follows: Mark the temperature, first, between daylight and sunrise; secondly, between 2 and 4, P. M.; thirdly, an hour after sunset: add together the first observation, twice the second and third, and the first of the next day, and divide the sum by six; the result will be the mean. The mean daily temperature at Philadelphia has been found, from the hourly observations of Capt. Mor What is understood by the mean daily temperature? decai, to be one degree less than the temperature at 9, A. M. 48. By taking the average of all the mean daily temperatures throughout the year, the mean annual temperature is ascertained. It is also obtained by the aid of the self-registering thermometer, the average of the two extreme temperatures being regarded as the mean of each day. 49. VARIATIONS OF TEMPERATURE IN Latitude. By comparing situations differing widely in latitude, it is found that the average annual temperature of the atmosphere diminishes from the equator towards either pole. This will be seen from the annexed table, which presents the results at the sea level, for nine places. 50. From this table it is also evident, that places having the same latitudes, in the two hemispheres, do not necessarily possess the same average temperature. This is owing to a great variety of local causes, the effect of which cannot always be accurately estimated. 51. VARIATIONS IN ALTITUDE. The temperature of the air diminishes with the altitude, but the law of decrease is very irregular, being. affected by the latitude, seasons, hours of the day, and a diversity of local circumstances. It may however be assumed, as a general rule, that a loss of heat occurs to the extent of one degree Fah. for every 343 feet of elevation. This is an How is the mean annual temperature found? How does the temperature of the atmosphere vary in respect to latitude? Give examples. Do like latitudes in different hemispheres have the same temperature? How is the temperature affected by altitude? What is the general law of decrease? |