Pneumatic Bridge Foundations. 387 inches. The exactness with a telescope magnifying twenty times would be twice as great. The striking advantage of the instruments then, for exactness is unquestionable; indeed, as the exactness might be still further increased by measuring a number of miles in one lines an d by painting the rod in black and white (the best colors for distinct visibility~ and twice as easily seen as the red marks have been reckoned), the instrument is well fitted even for quite

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... xact geodesical measurements. The ease with which the measurements can be checked, so that large errors of reading are avoided, should also be borne in mind. The general ease and quickness of telescopic measuring has always been recognized. The delays of chaining are saved; at the expense, however, of an addition to the telescopic work. Not only is the great trouble of training chain men avoided, but instead of two chain men with a rod, only one rodman is needed, whose duties are so simple (merely to hold the rod plumb in a good place) that he can also serve, at times, as axe-man. _As the observations at the instrument are more numerous, it is the more desirabl% though not absolutely necessary, in topographical work, to have an assistant to make all the readings to be noted by the head of the party. The adjustableness s then, of the cross-hairs, and the easily legible marking of hundredths of a foot on the rod, by means of angular figures, or otherwise, make even greater exactness attainable than by Porro's improvements, without the excessive complications of his instrument, or its niceties in workmanship; and, at the same time, permits both leveling and measuring with only one rod. The rod lighted inside makes telescopic measuring and leveling easy underground, where chaining is peculiarly disagreeable. Such measuring is far more exact than chaining~ as well as quicker and easier.