the top and at the bottom of the pipe? same, at which point will it be greater? answer. If the pressure is not the Ans. The only forces affecting the pressure in a pipe filled with gas and hermetically sealed, are the attraction of gravity and the pressure of the gas itself; that is, the force with which it presses against the sides of the pipe. According to the molecular theory of the composition of matter, this pressure which is exerted against the sides of the containing vessel by a gas is due to the incessant striking against these sides of molecules, each of which is trying to get as far away as possible from the others. In the absence of any disturbing force this pressure is exerted equally in all directions, since all the molecules move with the same velocity and have the same weight, and consequently possess the same energy, and as many travel up as travel down or sideways; and if the gas was imponderable, that is, had absolutely no weight, the pressure would be the same at all points in the height of the pipe. But each molecule has some weight and therefore tends, under the influence of the attraction of gravity, to move downward. This tendency to move downward decreases the velocity of the molecules moving upward, while it increases that of those moving downward, and since the force with which the molecules strike against the sides of the pipe or against each other depends upon the velocity with which they move, the molecules moving upward do not strike as hard, and therefore do not exert as much force or pressure as those moving downward. The difference thus produced between the upward and the downward pressure is in every case equal to the weight of the molecules. As, owing to its weight, each molecule from the top down transmits a greater downward pressure than it receives from the molecules above it, the effect is cumulative, and the total pressure at the bottom of the pipe will be greater than that at the top by an amount equal to the weight of the gas in the pipe, and the pressure per unit of area will be greater at the bottom than it is at the top by an amount equal to the weight of a column of the gas of unit area and of the height of the pipe. The same reasoning applies to the air in the earth's atmosphere, the pressure of which is greatest at the sea level and diminishes with the height above this level by an amount equal to the weight of a column of air of unit area and a height equal to the elevation of any given point above this level. (Trustees.) 6. What is meant by the atomic or combining weight of a subWhat use is made of this weight? stance? Ans. The atomic or combining weight of a substance is the ratio between the weight of an atom of the substance and that of an atom of hydrogen, the weight of the hydrogen atom being taken as unity, since it is the lightest of all known atoms. The atomic weights of the various elements do not, therefore, represent any absolute weights, but merely the number of times that the atoms of the respective elements are heavier than the atom of hydrogen. Atomic weights are useful in calculating the proportionate weights in which various substances enter into combination with each other, or into any chemical reaction. Since all chemical compounds are formed by the combination of atoms or multiples of atoms of the various elements, the weights of the different elements in any compound must always be proportional to their atomic weights, and therefore knowing the atomic or proportionate weights of the atoms of the various elements in any compound, the proportionate weights of each of the elements contained in the compound, and of the compound itself, can be determined as soon as the formula of the compound, that is the number of atoms of each element contained in it, is known. The same thing is true of the molecular weights of compound substances, since such substances always enter into chemical reactions in weights proportional to their molecular weights. = As an instance of the use of atomic weights, the formation of Carbonic Acid (CO) may be taken. This gas is formed in the combustion of Carbon (C) by the union of Carbon and Oxygen (0). The equation of the reaction between the two substances is C+O, CO, that is, one molecule of Carbon, consisting of one atom, unites with one molecule of Oxygen, consisting of two atoms, to form one molecule of Carbonic Acid, consisting of one atom of Carbon and two of Oxygen. The atomic weight of carbon is 12 and that of oxygen 16, so the proportion by weight in which they unite is 12 of carbon to 32 of oxygen. That is, 12 lbs. of carbon unite with 32 lbs. of oxygen to form 44 lbs. of carbonic acid, and, whatever the weight of carbon, the weight of oxygen re quired for its combustion and the weight of carbonic acid produced can be easily found from the above proportions. In the same way the proportional weights in which any substances enter into chemical combination are determined from their atomic or molecular weights. (Trustees.) 7. What is the unit used in Great Britain and America for measuring and expressing the illuminating value of gas, and what is the value of this unit? Ans. The unit used in Great Britain and America for measuring and expressing the illuminating value of gas is the "candle power." The literal meaning of "candle power," as a measure of illuminating value, is the amount of light given by the flame of a candle It is, however, evident that the flames of different candles give different amounts of light, and therefore, to obtain an amount of light that shall be as nearly as possible a definite and unvarying quantity and fit to be used as a unit of measurement, it is necessary that the candles employed should be made and burned in accordance with certain rules. Candles so made and used are called standard candles. It is also evident that after having obtained a standard candle, before any deductions can be made as to the comparative candle powers of different gases as obtained by separate observations, it is necessary that the rates at which and the burners in which the gas is consumed must be uniform, since the same gas consumed at different rates in one burner, or at the same rate in different burners, will give different amounts of light. Therefore the term "candle power" implies the use of a standard candle burning at a definite rate and of a standard burner and rate of consumption for the gas. The standard candle is a sperm candle of six to the pound and is burnt at a rate of 120 grains of sperm per hour. For a long time these general requirements were the only ones that had to be met, but wide variations in the amount of light given by different candles, all of which fulfilled the above requirements, led the London Gas Referees to adopt regulations for securing uniformity in standard candles, which prescribe the method and materials to be used in the manufacture of the wicks and the melting point of the spermaceti that are to be employed in making such candles. In the absence of any regulations on the subject in this country, it is well to see that the candles used fulfill as nearly as possible the requirements of the Gas Referees. These requirements are given in full in Butterfield's "Chemistry of Gas Manufacture," pages. 249 and 252. The standard rate of consumption for the gas is 5 cubic feet per hour. The form of burner used as the standard varies with the locality and the nature and quality of the gas to be tested. In England, when the gas to be tested is a coal gas of from 14 to 16 candle power, the burner generally prescribed by law for use as the standard burner is a special form of Argand burner known as Sugg's London Argand, No. 1. For cannel gas of from 20 to 26 candle power a flat flame burner is commonly prescribed as the standard. In this country the No. 7 Bray Slit Union burner has been very generally adopted as the standard burner, especially where the gas to be tested is either a carburetted water gas or a mixture of this gas with coal gas. The London Argand is also used in many instances for coal gas. It is always necessary to note what burner has been employed as a standard in each case before any comparison can be made between the reported candle powers of the gases supplied in different localities. From what precedes it is seen that when it is said that a gas is of a certain candle power the idea intended to be conveyed is that the gas will, when burned in a standard burner at the rate of 5 cubic feet per hour, produce a flame which gives a light equal to that given by a certain number of standard candles, each consuming 120 grains of sperm per hour. (Trustees.) 8. What considerations determine the depth at which street mains should be laid, there being no question of interference with any other pipe systems? Ans. The principal considerations that determine the depth at which street mains should be laid are their protection from damage by the traffic passing over them and from the influence of the changes in the temperature of the atmosphere. In Canada and the greater portion of the United States the depth at which mains are laid is governed by the latter consideration, since the depth required for protection from frost is greater than that required for protection from the traffic carried on along the roadway above the pipes. The customary rule is to keep the top of the mains just below the normal frost line. In the vicinity of New York City this requires the mains to be laid with three feet of cover, while in Canada and the Northern and Northwestern parts of the United States, in which the winters are very severe, the depth may reach six feet. In the Southern States, where winters are mild, the changes in temperature are of minor importance and the mains need not be laid any deeper than is necessary to protect them from the traffic passing over them. For this purpose a depth of 24 inches is sufficient, except in business streets where the traffic is apt to be heavy, in which case 2 feet 6 inches of cover should be given. When the temperature and traffic conditions are such as to permit of the mains being laid comparatively near the surface, the character of the pavement, either already existing or which will probably be laid in the future, must be taken into consideration and the mains laid deep enough to obviate any danger of the service pipes being interfered with during the laying or repairing of the pavement. Other pipe systems and subsoil structures also affect the problem at times, but, as stated in the question, these were to be left out of consideration for the purposes of this answer. Nothing is gained by going below the depth determined as above, and the deeper the mains the greater is the expense incurred both in their installation and their maintenance. It is therefore advisable not to go any lower than is necessary. (Trustees.) 9. Describe, with a sketch showing all the dimensions of drip and cover and the arrangement of the pumping pipe, a 6-inch line drip. Ans. A line drip is a cylindrical vessel made of cast iron. It has a solid bottom and is closed at the top by a removable cover which is usually fastened in place with a lead joint, the lead being run and caulked into a joint space between the cover and the drip. Hubs or bells of the proper size are cast on opposite each other near the top, and by means of these the drip is inserted into the pipe line at the low points and forms part of the line, the gas passing through its upper part while the condensation falls to the bottom. The condensation is pumped from the drip by a vertical wrought iron pipe, 3/4" or 1" in diameter, screwed through an 14" x 1" or an 14" x 3/4" bushing which is screwed into the cover. The lower end of the pipe is serrated and almost touches the bottom of the I |