"The mahogany base, shown in two halves by A and B ([Fig. 75]), is 3 feet 1 inch long, 33⁄4 inches at its greatest width, 2 inches at its least width, and 3⁄4 inch thick. Make a groove down the centre to admit the tube, and cut an opening 2 inches square right through the wood at the round end. Glue at the back of this a circular piece of pine or cedar, 3 inches in diameter and 1⁄2 inch thick, and screw a semicircular piece of the same thickness at the other end, with a ring for hanging.
"Fill the tube by degrees with pure mercury, boiling each portion, as introduced, by holding the tube in a nearly horizontal position over a spirit lamp, taking care not to crack it by too sudden heating. Half fill the wooden cistern with mercury, and when the tube is full, place a finger over the end, carefully raise it to a vertical position, and lower the open end below the surface of the mercury in the cistern. While some one holds the tube, glue on the two halves of the box lid and seal up the opening round the tube with wax or cement. Then fasten the tube to the base with brass clips and screws, and secure the cistern from shifting by gluing in wedges of wood. A thumb screw, with washer, for regulating the height of the mercury, is fixed at the bottom; this presses on a cork washer glued to the leather of the cistern.
"A hollowed hardwood boss is screwed over the top end of the tube, and a hollowed circular turned boss of mahogany, C, is glued over the bottom. The ivory or cardboard scale D, is of inches and tenths, from twenty-six and one half inches to thirty-one inches, the distance being measured approximately from the surface of the mercury in the cistern. A vernier having a scale of eleven-tenths of an inch, divided into ten parts, works in a slot on the scale and should be attached as shown at D.
"Before screwing on the scale, fix its correct position by comparison with the standard barometer. It is usual to place a small thermometer on the other side.
"With regard to the thermometers, it would be quite out of place here to discuss them at length, or to offer you a scientific explanation of the principles governing their construction. I may say however, that, as the barometer is intended to measure the different degrees of density of the atmosphere, so the thermometer is designed to mark the changes in its temperature, with regard to heat and cold. The first thermometers, so far as we know, were made less than three hundred years ago, and water, spirits of wine, or alcohol, and oil were used to fill the bulbs, in the order given. It was the great Halley, of 'Halley's Comet' fame, who first made use of mercury or quicksilver in these instruments, because of its being highly susceptible to expansion and contraction, and capable of showing a more extensive scale of heat. It is owing to this quality of expansion and contraction that the degrees of heat and cold can be measured. If you put your thumb on the bulb, you will notice the quicksilver in the little tube gradually rise until it reaches the limit of the thumb's heat. Thermometers, in this and nearly all English-speaking countries, make use of the Fahrenheit scale, which is different from those used in some other places; and this often causes trouble and annoyance.
"The scale of Reamur prevails in Germany. He divides the space between the freezing and boiling points into 80 degrees. France uses that of Celsius, who graduated his scale on the decimal system. The most peculiar scale of all, however, is that of Fahrenheit, the renowned German physicist, who, in 1714 or 1715, composed his scale, having ascertained that water could be cooled under the freezing point without congealing. He, therefore, did not take the congealing point of water, which is uncertain, but composed a mixture of equal parts of snow and sal-ammoniac, about fourteen degrees R. This scale is preferable to both those of Reamur and Celsius, or, as it is called, Centigrade, because: (1) The regular temperature of the moderate zone moves within its two zeros and can, therefore, be written without + or -. (2) The scale is divided so finely that it is not necessary to use fractions whenever careful observations are to be made. These advantages, although questioned by some, have been considered so weighty that both Great Britain and America have retained this scale, while nations on the Continent of Europe use the other two. The conversion of any one of these scales into another is very simple. (1) To change a Fahrenheit temperature into the same given by the Centigrade scale, subtract 32 degrees from Fahrenheit's degrees and multiply the remainder by 5⁄9. The product will be the temperature in Centigrade degree. (2) To change from Fahrenheit to Reamur's scale, subtract 32 degrees from Fahrenheit's degrees and multiply the remainder by 4⁄9. The product will be the temperature in Reamur's degrees. (3) To change a temperature given by the Centigrade scale into the same given by Fahrenheit, multiply the Centigrade degrees by 9⁄5 and add 32 degrees to the product. The sum will be the temperature by Fahrenheit's scale. (4) To change from Reamur's to Fahrenheit's scale, multiply the degree on Reamur's scale by 9⁄4 and add 32 degrees to the product. The sum will be the temperature by Fahrenheit's scale. A handy table can easily be figured out from the data given."
Mr. Gregg concluded his conversation for the night at this point, but promised to take it up again the first available evening.
Two or three nights afterward it was very wet and dreary. The boys and Jessie were called into the den by Mr. Gregg, where a brisk fire, made of limbs and branches gathered by the boys, was burning in the little fireplace, and the room looked bright and cheerful. The young folks all drew up around the fire to listen.
"I have so many things to talk to you about," said he, "that I scarcely know where to begin; however, I promised to tell you something concerning springs, so I will make these useful contrivances my theme to-night."