found; for we know that the Romans used lead largely, and, as we have seen, pieces of the lead distribution pipes have been found.  It is possible, and even likely, that strong cords of hemp were wound round the pipes forming the siphons, as is related by Delorme in describing a similar Roman aqueduct siphon near Constantinople; Delorme also describes, in the aqueduct last mentioned, a pipe for the escape of air from the lowest part of the siphon carried up against a tower, which was higher than the aqueduct, and it is certain that there must have been some such contrivance on the siphons of the aqueduct constructed at Lyons.

Flacheron supposes that they consisted of small pipes carried from the lowest part of the siphons up along the side of the valley and above the reservoirs, or, in some instances, of taps fixed at the lowest part of the siphons.  The Romans have been blamed for not using inverted siphons in the aqueducts at Rome, and it has been said that this is a sufficient proof that they did not understand the simplest principles of hydraulics, but the remains of the aqueducts at Lyons negative this assumption altogether.  The Romans were not so foolish as to construct underground siphons, many miles long, for the supply of Rome; but where it was necessary to construct them for the purpose of crossing deep valleys, they did so.  The same emperor Claudius who built the aqueduct at Rome known by his name built the aqueduct of Mont Pila, at Lyons, and it is quite clear, therefore, that his engineers were practically well acquainted with the principles of hydraulics.  It is thus seen that the ancient Romans spared no pains to obtain a supply of pure water for their cities, and I think it is high time that we followed their example, and went to the trouble and expense of obtaining drinking water from unimpeachable sources, instead of, as is too often the case, taking water which we know perfectly well has been polluted, and then attempting to purify it for domestic purposes.

* * * * *

STEAM ENGINE ECONOMY.

By Chief Engineer JOHN LOWE, U.S.  Navy.

The purpose of this article is to point out an easy method whereby any intelligent engineer can determine the point at which it is most economical to cut off the admission of steam into his cylinder.

In the attack upon such a problem, it is useful to employ all the senses which can be brought to bear upon it; for this purpose, diagrams will be used, in order that the sense of sight may assist the brain in forming its conclusions.

[Illustration:  STEAM ENGINE ECONOMY.—­BY JOHN LOWE, CHIEF ENGINEER U.S.N.]

Fig.  XABCX is an ideal indicator card, taken from a cylinder, imagined to be 600 feet long, in which the piston, making one stroke per minute, has therefore a piston speed of 600 feet per minute.  Divide this card into any convenient number of ordinates, distant dx feet from each other, writing upon each the absolute pressure measured upon it from the zero line XX.