Another method, which is certainly more novel than either of the others, consists in supporting a floor upon a bed of resin. The underlying earth was removed, and replaced with spent moulding sand, leaving trenches for the floor timbers, which were placed upon bricks laid without mortar. Melted resin was poured into the space alongside and underneath the timbers. The floor planks were then laid upon the timbers, the tops of which were about half an inch above the level of the sand. Holes were bored into the floor plank about four feet apart, and melted resin then poured into the holes, so as to interpose a layer of resin underneath the floor plank and beams. Upon this floor a top floor of hard wood was laid in the usual manner. This floor has been used for a number of years to support a large quantity of heavy machine tools, principally planers, without yielding or depreciation due to decay, and has proved to be most satisfactory.
In some instances asphaltum or coal tar concrete floors are not covered with wood, although it is much more agreeable for the help to stand upon wooden floors. It should be remembered that all these compounds are readily softened by means of oil, and they should be protected from oil by a coat of paint when not covered with wood; the preferable method being to first apply a priming containing very little oil, or a coat of shellac, and follow with some paint mixed up with boiled linseed oil.
(To be continued.)
The lecture was illustrated by about fifty views on the screen, which cannot be reproduced here, showing photographs of mills and mechanical drawings of the methods of construction alluded to in the lecture.
THE MECHANICAL EQUIVALENT OF HEAT.
By De Volson Wood, Professor of Engineering in Stevens Institute of Technology.
It is clearly intimated by Mr. Hanssen, in his determination of the mechanical equivalent of heat, published in the Scientific American Supplement, No. 642, April 21, 1888, that his object is to determine the absolute value of this constant. With his data he finds it to be 771.89 foot pounds. But the determination by direct experiment gives a larger value. Thus, the most reliable experiments—those of Joule and Rowland—give values exceeding by several units that found by Hanssen. A committee of the British Association, appointed for this purpose, reported in 1876 that sixty of the most reliable of Joule's experiments gave the mean value 774.1. The experiments were made with water at a temperature of about 60° F., according to the mercurial thermometer, and reduced to its value at the temperature of melting ice, according to the formula given by Regnault for the variation of the specific heat of water at varying temperature under the constant pressure of one atmosphere. According to this formula the specific heat of water increases with the temperature above the melting point of ice, so that the equivalent would be somewhat less at 32° F. than at 60° F. It will be found in Regnault's Relation des Experiences that he experimented on water at high temperatures, but more recently Professor Rowland has found that the specific heat of water is greater at 40° F. than at 60° F., thus reversing between these limits the law given by Regnault; the increase, as given by the most probable values, being, roughly, about 1/250 of its value at 60° F. The proper correction due to this cause would make the equivalent over 777 foot pounds, instead of 774.1. Professor Rowland's experiments, when reduced to the same thermometer, same temperature, and same latitude as Joule's, agreed very nearly with those of the latter, being about 1/1000 part larger; so that the chief difference in the ultimate values consists in the reductions for temperature and latitude. The force of gravity being less for the lower latitudes, the number representing the mechanical equivalent will be greater for the latter, since the unit pound mass must fall through a greater number of feet to equal the same work; so that the equivalent will be greater at Paris than at Manchester. Professor Rowland also found that the degrees on the air thermometer from 40° F. upward to above 60° F. exceeded those on the mercurial thermometer throughout the corresponding range, and that from 40° to 41° the degree was between 1/150 and 1/200 of a degree larger on the air thermometer than on the mercurial. Although this fraction is too small to be observed by ordinary means, yet, if it exists, it cannot be ignored if absolute values are sought. Regnault employed the air thermometer in his experiments, while Joule used the mercurial thermometer, and if Joule's value 774.1 be increased by 1/200 of itself in order to reduce it from the equivalent of the degree on the mercurial thermometer to that on the air thermometer, we get 778 foot pounds, nearly. Rowland found from his experiments that when reduced to the air thermometer and to the latitude of Baltimore, the equivalent was nearly 783, subject to small residual errors.
Nearly all writers upon this subject—except Rankine—have considered that the mechanical equivalent of heat, in British units, was the energy necessary to raise the temperature of one pound of water from 32° F. to 33° F., but Rankine defines it as the heat necessary to increase the temperature of one pound of water one degree Fahrenheit from that of maximum density, or from 39° F. to 40° F. For ordinary practice it is immaterial which of these definitions is used, for the errors resulting therefrom are much less than those resulting from ordinary observations. But when the value is to be determined by direct experiment at the standard temperature, Rankine's limits are much to be preferred; for it is so very difficult to determine exact values by observation when the substance is near the state bordering on a change of state of aggregation, as that of changing from water to ice. Observations made at about 60° F. were reduced by means of Regnault's law for the specific heat of water, as has been stated, which is expressed by the formula