He dwelt on its advantage to mechanics, in enabling them to have a common standard where there is now so much confusion and arbitrary method of measurement, as in screws, taps, dies, etc., both in government and private shops..

Mr. J. P. Putnam continued the subject by reading a com munication on the advantages of the system, and the means that had been taken to introduce it into various countries. That decimal division of weights, measures and money, is of the first importance in computation is shown by the praise generally bestowed on the federal currency; and that engineers share in this opinion may be seen by their using decimals of a foot where they can, although, by so doing, they are at variance with the general practice of the community.

The real difficulty is not with the proposed system, but with making the change. The cost of such a revolution, affecting so large a proportion of the transactions of life, must be enormous; but it is equally true, if not quite so obvious, that it is costing enormously to keep up the present confusion. It is already used in the mint and on the coast survey; men of science have used it for many years; it is used by analytical chemists, and will soon be used by many architects and constructors all over the country. To accomplish its introduction concerted action is necessary.

In the metric system, the weight and dimensions of every material thing, whether solid, liquid, or gaseous, whether on land or on water, whether in the earth or in the heavens, -and whether determined by the scale, plummet, balance, barometer, or thermometer, are ascertained by a method absolutely uniform, entirely simple and equally suitable to the use of all mankind, - resting upon a single invariable standard of linear measure, with multiples and submultiples, like those of our monetary system, exclusively decimal, with appropriate names, similar in all languages — and itself secure against the possibility of change, or loss through carelessness, or accident, or design, by being constructed on scientific principles, and copied for distribution among the different nations of the

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world. The standard unit of length, the metre, is the ten millionth part of the quadrant of the meridian, equal to very nearly 39.37 English inches.

NEW METHODS OF TOPOGRAPHICAL SURVEYING.

January 13, 1876. Prof. E. C. Pickering made a communication on some new methods of topographical surveying among mountains, devised by himself, as distinguished from the old method of trigonometrical survey, which is very expensive as well as inaccurate, and from the plane table, which answers very well for the determination of a few points.

He exhibited a new instrument, with various modifications, for use among the mountains. His object was to dispense with the graduated circle, and its bulky and heavy stand, and to be able to make accurate observations without the necessity of ascending the mountains, thus saving much time, labor and expense. He brought to the notice of the meeting three different methods, each having its special applications.

The first was a new stadium, as a substitute for the one commonly used in the coast survey and elsewhere, which requires the services of two persons, and which is also inaccurate. He uses an ordinary small and portable telescope, lightly but firmly mounted, having a plane glass mirror in front of the object glass; by placing this mirror at the proper angle, the two images of an object are made to coincide. Two positions being taken on the same line, at right angles to the object seen, the measurement of the different angles from these different points enables the observer to measure the distance with a great degree of accuracy.

This instrument, used for measuring horizontal distances, being made to turn 90° on its longitudinal axis, may then be employed for measuring heights. He uses a good level firmly attached to it, and a spider-line micrometer, or eye-piece, containing a divided scale. It is practically, therefore, a zenith telescope, without vertical circle or heavy mounting, but with a

rigid connection between the level and the instrument. Its liability to error is very small, and its use easy and satisfactory.

The third form of apparatus was for working out details, after certain main features had been determined by other means; it was specially valuable for making contour maps, which by ordinary processes are difficult and expensive. His device was a modification of the camera obscura, easily transported and managed. He explained how, by this simple apparatus, the height and velocity of clouds may be determined, the distance of a thunder-cloud, and the velocity of sound.

Attention was drawn to the simplicity and efficacy of these instruments, which would so diminish the cost, and increase the accuracy of topographical surveys, that no State need be deterred from authorizing such survey on the ground of the great expense of the topographical work.

NEW WIND VANE.

January 27. Prof. Pickering described a new form of wind vane, an invention of Mr. W. H. Pickering, now a student at the Institute.

It consisted of an ordinary vane, which had attached to the base of its pivot-socket two arms inclined downwards, one on each side of the vane. To the left hand arm was attached a ball, and to the right a piece of ground glass, of such a size that when viewed from the front it would present the same amount of surface as the ball, both about as large as the letters denoting the points of the compass. The object is to show the direction of the wind under all circumstances, especially when a person in the line of the wind might be unable to tell whether it was north or south, east or west; when the ball appears to the right of the pivot the vane is pointing toward you, and when to the left, away from you. The ball is the object judged by, as the glass would be invisible at a short distance, the use of the last being to counterbalance the weight of the ball and to counteract the action of the wind on it. The improvement is a very cheap one, and can be easily applied to any vane now in use.

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INSTRUMENT FOR DRAWING CURVES.

Mr. John M. Batchelder, of Cambridge, sent for exhibition a new instrument, devised by himself, for drawing curved lines, such as are ordinarily made by the French curves.

A diagram of the various and graceful curves which be may drawn by this instrument was exhibited. The opinion was expressed that the flexibility of the steel bands was such that extreme care would be necessary to insure accuracy.

Mr. Moore spoke of the importance of preserving in some way the diagrams made in illustration of the papers read before the Society, in many cases indispensable for the permanent value of communications.

DIFFERENTIAL COMPASS.

February 10. Mr. S. W. Holman, a student of the Institute, in the Department of Physics, presented in behalf of the inventor, Mr. George Iles of Montreal, a new form of mariner's compass, called a "Differential Compass."

The instrument was designed to detect variations of the compass needle, due to the disturbing action of iron ships, and to give the mariner a means of correcting this error at any time, thus obtaining the magnetic north.

After explaining the effect of the iron mass of a vessel on the compass, illustrating the action by magnets, he described the model.

It consists of two compound needles pivoted one above the other. The upper needle is made up of a series of bundles of small steel magnets, held between two long strips of aluminum; the bundles are arranged with their like poles pointing in the same directions, the north pole of one bundle being placed toward the south pole of another. As each small bar magnet can be much more intensely magnetized in proportion to its bulk than a larger one, we can obtain by this arrangement a much more intensely magnetized needle than with an ordinary steel bar. The lower compass needle is an ingenious arrange

ment of similar small steel magnets across a bar of aluminum, like poles being all upon the same side of the bar. In this way is produced a compound needle, which sets with its length east and west; when pivoted one above the other, the needles set at right angles.

If a piece of soft iron, or an attractive magnetic pole, be introduced into any quadrant near the needles, the quadrant is diminished to an acute angle. Upon this fact is based the method of determination of the deviation of the compass and its compensation. The conclusion of the inventor was that any disturbing mass at any distance would cause the needles to deviate toward each other in some quadrant, forming an acute angle there; this acute angle was to be rendered a right angle again by the repellant pole of a compensating magnet under certain prescribed conditions.

Mr. Holman showed that, from experiments performed by him on this compass in the laboratory, the action of disturbing forces at a distance was not as Mr. Iles had expected, that an acute angle was produced only when such force was at a small distance from the needle as compared with the length of the small magnets composing these needles, and that the compass failed to show any variation from a right angle between its needles, when a disturbing magnet had moved them both through a very large angle from a true position.

He then showed, upon theoretical grounds, the reason for these two actions at different distances, making evident in this way the fallacy of the instrument, which had acquired a wide reputation.

NEW SYSTEM OF RIFLING CANNON.

March 9. Prof. E. L. Zalinski, U. S. A., read a paper, illustrated by numerous diagrams and models, on a new system of rifling heavy guns, devised by Mr. S. P. Ruggles, a member of the Society.

The following extracts embrace the principal points in the paper.

He sketched briefly the object of rifling guns, the various