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56 THE LOCOMOTIVE. [April, SI (it i|t# tmtti* * wa2l>s9 HARTFORD, APRIL 15, 1901. J. M. Allen, A.M., M.E., Editor. A. D. Risteen, Associate Editor. The Locomotive can fo obtained free by calling at any of the company's agencies. Subscription price 50 cents per year when mailed from this office. Bound volumes one dollar each. (Any volume can be supplied.) In our leading article last month, we inadvertently said that "all horizontal tubu- lar boilers should be set with the rear end some three inches lower than the front end." This is not correct. What we intended to say, was that such boilers should be set '• with the rear eud at least one inch lower than the front end." In the May issue of Insurance Engineering we find some statistics of boiler explosions in the United States, which are said to have been prepared from " records kept in the Fidelity & Casualty office." TTe appreciate the compliment that the Fidelity & Casualty company does us, in accepting our data as authentic, and in publishing statistics that we have collected, in preference to any that they may have collected themselves; but as it is a good deal of work to keep track of these things, we feel that it would be more graceful and courteous to give the Hartford Steem Boiler Inspection and Insurance Company credit for what is taken from it so openly. We have received from the publishers, Messrs. John Wiley
: 1901.] THE LOCOMOTIVE. 57 in degrees of such a size as to make the boiling point read 100 on the scale. In the original Fahrenheit scale, the temperature of the human body was taken as zero and the scale was so arranged that lower temperatures were denoted by larger numbers so that the numbers on the scale increased from the top towards the bottom. The present zero of the Fahrenheit scale was probably selected so as to avoid the use of " negative" temperatures so far as possible; and if this appears to be absurd, it should be remem- bered that Fahrenheit had never visited North Dakota in the winter, and that the only means he had of producing artificial cold was by the use of some such simple freezing mixture as pounded i«e and salt. Every "zero" of this sort, which is selected merely because it happens to correspond to the freezing point of water, or some other fluid, is purely arbitrary. We might have as many such zeros as we wanted to, and no one of them would have the least advantage over any other one, except on the score of con- venience. The absolute zei-o, or point from which we measure the absolute temperatures that we are discussing, was selected as a zero point for scientific purposes, because it differs from every other zero point that has ever been proposed in the one respect that it is not in the least arbitrary. The absolute zero, in fact, is the temperature of absolute cold. "We shall explain this point a little more fully presently, but just for the time being we desire to consider the absolute zero from a slightly different point of view. We wish to regard it, namely, as the zero of the " perfect gas" thermometer. Every reader of The Locomotive is doubtless aware that for accurate scientific purposes the ordinary mercury thermometer is no longer taken as the standard. Some form of gas thermometer is used in its place, in which the temperature is measured by the variation in pressure of a gas whose volume is kept constant. The gas used may be air, or hydrogen, or any other of the familiar ones that we used to call "permanent," before we knew better. Nitrogen is the particular gas that is used by the International Bureau of Weights and Measures, and the temperatures that such a thermometer gives are said to be given " on the nitrogen scale." There are slight differences between the readings of thermometers filled with different kinds of gases, and for this reason it is desirable to fix on one particular kind of gas as a standard. The choice of this standard gas is arbitrary, and is determined, not by any general fact of nature, but by the con- venience with which the selected gas may be procured and purified, and by certain other practical considerations that we do not need to dwell upon in this place. The ideal gas to use for thermometric purposes would be one in which the pressure is precisely doubled when we compress the gas,. at constant temperature, into half its original volume; and so on. In other words, one which precisely obeys Boyle's law, as explained in the issue of The Locomotive for January, 1900. Such a gas is called a "perfect gas." No perfect gas exists in nature, but those gases that can be liquified only with great difficulty approximate very closely to the state of a perfect gas, under ordinary conditions of temperature and pressure. Temperatures read from a thermometer filled with a perfect gas would be absolute temperatures. If we neglect, for the moment, the slight deviation of air from the ideal that we have in mind when we speak of a perfect gas, we may explain the existence of the absolute zero by the following quotation from Tyndall's " Heat a Mode of Motion " "We have seen," he says, "that the pressure of air is augmented by an increase of temperature. It has been shown that if the volume of the air is not allowed to change, we have, for every degree of temperature, a certain definite increase of pressure. Reck- oning from 0° Fahr. upwards, we find that every degree added to the temperature pro- duces an increase of pressure equal to 1/460 of that which the air possesses at 0° Fahr., and hence, that by raising the temperature to 460° Fahr. we double the pressure. An
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