keep tight throughout the life of the pipe, than is the form of joint used in wrought iron pipe, and the brittleness of cast iron, which renders it liable to break under shocks from extraordinary traffic on the street in which it is laid, or from strains thrown upon it through subsidences, due either to a disturbance of its foundation or to this foundation being originally insecure. The advantages and disadvantages of wrought iron pipe as compared with cast iron are of course the converse of the above.

In good soils cast iron will last indefinitely, and even under the worst conditions it will have a life of some years. Comparatively little wrought iron pipe has been used for gas mains, and none of this dates back far enough to furnish conclusive data as to the number of years that can be safely counted upon as the life, under ordinary gas distribution conditions, of such pipe when covered with a good protective coating of the kind described in the answer to a previous question. Experience with coated services shows, however, that when the conditions are at all bad, wrought iron corrodes much, faster than cast iron, and that even under comparatively favorable conditions a very perfect coating is necessary to make a life of twenty to thirty years reasonably certain.

The greater thickness of cast iron pipes permits the securing of sufficient depth of thread to make a good screw joint in the wall of the pipe itself, so that there is no necessity for using the service clips or saddles, without which a good joint cannot be obtained with wrought iron pipe. The use of these clips adds not only to the cost of the services, but also to the opportunity for leakage at the service connections.

In the smaller sizes, 3′′ and 4′′, however, wrought iron pipe can be laid, under normal conditions of price, more cheaply than cast iron pipe, and in small towns or the residential suburbs of large cities, where the soil is good, the services will never average less than 50' apart, and the probable consumption will never exceed the capacity of the above sizes, the advantages of wrought iron outweigh the disadvantage of possibly shorter life, and it is good practice to use it.

Another place where the use of wrought iron is advantageous is in the case of a line to be laid up a hill where the ditch

would have to be blasted out of rock, and where there is an opportunity to use a ditch already made for a sewer or a water pipe. A line of cast iron pipe could not be maintained in such a location, while the wrought iron pipe is strong enough not to be broken by any settling, and the line being laid on a slope there is little danger of its becoming trapped. (Trustees.)

Give a description, illustrated with sketches, of some form of recording pressure gauge used for taking street main pressures.

Ans. The form of recording pressure gauge that it is desirable to use for taking street main pressures depends somewhat upon whether these pressures are to be taken, at any one location, continuously or for a day or two only. Where the pressures are to be taken continuously, as at the gas works or the gas office, it is best to use a gauge which measures the pressure by the height to which a small bell sealed in water is raised by the gas admitted under it, since such gauges are the least subject to get out of order and give inaccurate records. Where pressures are to be taken only for a day or two a year, it is advisable, for the sake of its greater portability, to use a gauge based on the aneroid barometer principle, even though it is necessary to exercise greater care in using it.

A gauge of the class first mentioned, which may be called a "Float Gauge," is shown, partly in elevation and partly in section, on the left half of the accompanying cut. Outwardly it consists of a tinned iron cylindrical case, containing the bell and the sealing water, and surmounted by a half cylinder, also of tinned iron, the front of which is closed by a glazed door in the shape of half of a hexagonal prism, which contains the recording apparatus. At the top is a clock by which the recording mechanism is driven.

The bell, by the motion of which the pressure is measured, is made of tinned iron, and is provided with a cylindrical float, made concentric with the bell, with such an area that when submerged in water to the full depth of the bell, the weight of the water that it displaces is just equal to that of the bell and its appurtenances, so that when the bell is at the lowest point to which it can sink, it is just afloat when the pressure

inside of it is equal to that of the atmosphere, and therefore is ready to rise as soon as the interior pressure exceeds that of the atmosphere. This bell is guided by wheels fastened around

its lower edge and working on guides fastened to the case, and by a rod fastened to the centre of the top and passing up

through a small hole in the top of the lower part of the case into the space containing the recording mechanism. Gas from the street main, the pressure in which is to be recorded, is brought under the bell by means of a pipe entering through the base of the case and rising above the water line. A plug with a milled head is placed in the side of the case to drain off any water that may be run in above the water line and there is an outlet at the bottom for running off all the water. An opening, closed with a plug, is provided on the side of the case above the water line for putting in water when necessary. The stopcock on the gas pipe is usually made with a small side outlet, so that when the gas is shut off the interior of the bell is put into communication with the atmosphere through this side opening.

The height to which the bell will rise for each " gauge pressure possessed by the gas admitted under it, depends upon its weight. its height and the area of the annular space between the circumference of the bell and the float. It is evident that when the total pressure exerted by the gas on the crown of the bell is equal to the weight of the bell and its appurtenances the bell will rise to its full height, since the whole weight will be supported by the gas and therefore the float having no weight to carry will be forced up by the water until its bottom is on the level of the surface of the latter. The total pressure is equal to the gauge pressure of the gas. multiplied by the area of the portion of the crown on which the pressure is exerted, and the weight in pounds that will be supported by this pressure is equal to the gauge pressure, in inches of water, multiplied by the area, in square inches, of the portion of the crown on which the pressure is exerted multiplied by 0.036, the weight, in pounds, of a cubic inch of Conversely the pressure in inches required to fully raise a bell, the weight and effective area of which are known, is equal to the weight divided by the product of the area and 0.036. In any given case, therefore, knowing the dimensions of the bell and of the float and the total weight, the pressure required to raise the bell to its full height can be calculated and by dividing the height by the number of tenths of an inch in the pressure so determined the amount of rise for each 16′′

of pressure is obtained. Or, since this rise must be just sufficient to reduce the volume of water displaced by the float by an amount the weight of which is just equal to the total pressure exerted on the top of the bell by a pressure of 16" of water, knowing the diameter of the float the rise can be calculated directly by dividing the product of the area, in square inches, of the annular space of the bell and 0.0036 by the product of the area, in square inches, of the float and 0.036, or, what is the same thing, as can be seen by canceling, by dividing the area, in square inches, of the annular space in the bell by the area of the float multiplied by 10.

The recording mechanism consists of a cylindrical drum supported on a central pivot at the bottom and connected by a central rod at the top to a shaft coming down from the clock. This connection is made in such a way that it can be readily broken and the drum removed. The shaft is so geared to the clockwork that it makes one complete revolution in twentyfour hours, carrying the drum with it when the connection is made. A sheet of paper ruled with vertical lines to show the time and horizontal lines for the pressure is wrapped around this drum and fastened so as to be carried around with it as it revolves. A pencil mounted on top of the rod carried by the bell is pressed by a light spring against the paper and makes a continuous line recording the pressure throughout the twentyfour hours. The vertical ruling marking the time is made by dividing the space equal in length to the circumference of the drum into twenty-four equal parts, so that in each hour one of these spaces passes under the pencil point. The proper distance between the horizontal lines marking the pressure is calculated in the manner explained above and the paper ruled accordingly. Both the vertical and horizontal lines are suitably marked to enable the pressure and time corresponding to any point on the sheet to be read off at a glance.

In starting a record the paper must be so placed on the drum, that when the bell is open to atmospheric pressure the pencil point touches the line of zero pressure at the point corresponding to the time at which the start is made.

The Bristol gauge, one of the second class, is illustrated on the right half of the cut. The lower figure shows the pressure