This has, no doubt, many advantages, but again adds to the cost of construction, on account of the necessary protection against frost and deterioration during the winter months; the ordinary protection during this time being the filling of the tank with water, which of course has to be pumped out previous to the construction of the holder.

When properly constructed and carefully executed, it is reasonably to be expected that iron tanks will have the desired results as to perfectness; but this cannot be asserted of the brick tank, as often its defects are not developed until the same is being filled with water, when, in many instances, apparently good and sound tanks show a large amount of leakage.

There are many causes which may render a brick tank defective before the holder is completed. Where much water is encountered and pumping has to be continued after the tank is completed, to admit of the erection of the iron-work, it is possible that the back-filling on the exterior of the tank wall near its base is undermined, thus forming a receptacle for the water that penetrates the brickwork at this point, and is, no doubt, in instances, the cause of leaks that appear to be unaccountable. As the exterior of the tank is not accessible, the most careful examination of the interior of same would not detect the defect. These conditions do not exist in the construction of iron tanks; and should any defects be developed in the latter, they are at once observable and easily remedied.

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The claim that the erection of an iron tank does not deteriorate the ground is, of course, only of importance under peculiar conditions. In our large and rapidly growing citities, where ground in favorable locations increases in value with the growth of the population, a lot of ground occupied by a brick gasholder tank remains comparatively valueless, as it is not applicable for anything else but a gasholder; but iron tanks are susceptible of being taken apart and reconstructed in different locations, thus admitting of the disposal of the original site upon advantageous conditions.

The gradual development and progress in the construction of wrought-iron tanks is clearly shown by the exhibited plans and photographs. The appearance of the guide-frame shown upon these drawings, especially for three-lift holders, will probably raise

the question, at the first sight, if the latter is sufficiently strong in all its parts.

Fortunately, the engineer is not forced to construct from personal perception, but from measure of numerical values; which are applicable, with entire safety, for the resisting capacity of the materials that enter into the construction of the guide-framing.

My first construction was a tank 22 ft. 6 in. in diameter by 11 ft. o in. deep, erected in 1885 for the Equitable Gas Light Company, of New York city. The standards of the guide-frame were supported directly upon the concrete foundations below

tank. This idea was abandoned in the construction of the second tank at Lynn, Mass., in 1886, where the entire weight of guide-framing is supported by the side plating of tank. In all later constructions the idea of supporting the entire weight of guide-framing upon side plating of tank was maintained.

The bottoms of tanks are all perfectly horizontal, and must be solidly embedded upon the foundations with cement grouting. The latter is poured under the bottom, after the same is in position, through hand-holes constructed for this purpose on the inner surface of bottom.

The strains imposed upon the iron work of tank by internal pressure, are, for the shell, directly proportional to the diameter of same, and in the construction shown do not exceed in any part of the riveted joints 10,700 lbs. per square inch, and the strength of riveting, as compared with the plating, is, for the lower vertical triple riveted seams, 81 per cent., for the upper double riveted seams, 76 per cent. of the plating.

The required strength at the upper edge of tank is obtained through a horizontal continuous circular girder of plate iron.

The supports for standards of guide-frame consist of large brackets extending through the entire depth of tank, constructed of angle and bar iron of suitable dimensions.

After the standards have been attached to the brackets they form a part of them, and through them the entire weight of guide-frame is uniformly distributed through the side plating of tank and its foundation. By these means the pressure from the weight of guide-framing upon the foundation is not only uniformly distributed, but is also reduced to the minimum per unit of surface.

The overflow near the upper edge of tank is provided with two outlets at different levels, so that the water-line can be lowered when necessary to admit painting the inner and upper surfaces of tank to prevent oxidation at this point. In large tanks it is very necessary to make provision for expansion and contraction.

The guide-framing consists of wrought iron trussed standards. composed of shape and bar iron, solidly riveted together, and are connected at each other and to the tank by wrought iron latticed girders provided with polygonal braces of plate iron, and diagonal braces of bar iron. Through the polygonal and diagonal bracing

the entire structure is so secured that the strains imposed upon the same by wind pressure are transferred to the continuous circular girder surrounding the upper edge of tank and to the side plating of same.

A careful static calculation of the strains in the guide-framing has resulted in a construction light in appearance as compared with the usual arrangement of guide-framing, and yet one possessing greater resistance to the heaviest wind pressures than the ordinary construction, especially where the columns are connected to the masonry of brick tanks.

I received a practical proof of the correctness of this construction from a test of a triple-lift holder and iron tank recently erected in Memphis, dimensions 100 ft. 0 in. in diameter by 100 ft. o in. high. When the holder was being tested and was inflated to its full height, it was caught in a tornado, in which the wind reached a velocity of seventy miles per hour, passing through the ordeal without the slightest damage, although buildings in the immediate neighborhood were demolished, trees uprooted, etc.

With the construction of iron tanks it becomes an important question how to prevent the formation of ice. If sufficient steam can be spared from an already existing boiler, this system of heating the water should be applied; but where an additional apparatus is required for this purpose, the system of heating by hot water has the preference.

I have no practical data at present as to the heating surface required in proportion to the volume of tank, but shall be glad to lay before you during our next meeting the results of my experience in this direction during the coming season.

A system of heating the tank water by hot water circulation deserves probably the preference, on account of the little attention necessary, as only the fuel supply to the boiler demands the latter. With either the low pressure steam or hot water systems it is required that the heating surface is uniformly distributed around the inner circumference of tank. A circulating pipe near the inner circumference of tank, entering the same immediately above the bottom and leaving the same at an adjacent point, is all that is required for tanks of ordinary dimensions. For larger tanks, two or three circulating coils must be applied;

and if each of them is provided with proper valves, the amount of heating surface and consumption of fuel can be proportioned to the temperature of the atmosphere.

There have been erected under my supervision single, double and triple-lift gasholders with wrought-iron tanks above ground, varying in capacity from 3,000 to 500,000 cubic feet.

The two triple-lift holders constructed at Memphis and New York are probably of special interest on account of their large dimensions, which are as follows:

Memphis-Erected in 1887; inner section 93 ft. 6 in. diameter by 24 ft. 4 in. high; middle section, 95 ft. o in. in diameter by 24 ft. 4 in. high; outer section, 96 ft. 6 in. in diameter by 23 4 in. high; tank, 99 ft. o in. in diameter by 25 ft. 4 in. deep. (See illustration.)

ft.

New York-Erected during the present season and about to be completed; inner section, 85 ft. 9 in. in diameter by 28 ft. 4 in. high; middle section, 87 ft. 3 in. in diameter by 28 ft. 4 in. high; outer section, 88 ft. 9 in. in diameter by 27 ft. 4 in. high; tank, 91 ft. 6 in. in diameter by 29 ft. 4 in. deep.

In the construction of these holders particular attention was paid to the arrangement of guide rollers and carriages. For the rollers the combined systems of radial and tangential rollers were applied. All the carriages are constructed entirely of wroughtiron under recently obtained patents. The carriages are so arranged that by their application all strains imposed upon the channel bars of the cups by the weight of the different sections are eliminated and transferred directly to the vertical stiffeners of the same.

The numerous recent executions of gasholders with wroughtiron tanks above ground apparently prove the correctness of the often heard assertion, "that the majority of brick tanks are defective."

During the present year the excavation for a brick gasholder tank in the city of New York, for the Standard Gas Light Company, had progressed to nearly one-third of its intended depth, when it became exceedingly doubtful if the continuation of the excavation and erection of the brick tank would warrant the desired results.

After carefully considering the condition, the Gas Company