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From the positive pole./ -(- of the self-exciting induction machine the circuit goes over F, G, H, to the inner coating of the jar L. The potential can be regulated by the position of the plate A. The negative pole J—is connected with the balance beam C of the other part of the potential regulator. As soon as the potential is high enough, C, which is negatively excited, will be drawn to the positively excited plate A, which causes the other end of the beam, which has a wire loop, to dip into the quicksilver reservoirs N, and is then held in that position by the electro-magnet M. By this a connection is made with a battery which is in the drawing indicated by E. The current from E draws an iron core into the coil R, thereby lifting with the other end of the lever (to which the core is attached) the rod GH to the position KN. This interrupts the current between the induction machine and the jar, but shunts it into the circuit which leads to the spark-producing terminals BB. At the same time a current leading to another battery indicated by E, is interrupted by lifting the electro-magnetic current interrupter out of the reservoir p. The interruption of this current, which, in connection with M, kept the camera closed, allows a spring to open it, and at the same time strikes a gong at the firing place. The firing and closing of the camera (by hand*) follow immediately. In this manner a proper charge of the jar was assured—an important matter, for had it been too small, the shot might not have caused discharge, and had it been too great, a premature discharge would have taken place.

Everything worked with great accuracy, and the whole transaction occupied less than a second.t The apparatus never failed.

[graphic]

Fig- 5

The entire arrangement of the installment can be seen in fig. 5. The whole apparatus was enclosed and the light deadened by a wooden hut (14 m. long, 2.5 m. wide), with two openings in the line of flight of the projectile (indicated by the arrow). From the induction machine J near the store O the circuit leads over the potential regulator P, the jar E and the current interrupter £7 to A, where the shot closes the circuit, thence to the place of the spark B, and further to the outer coating of the jar, and back to the other pole of the induction machine. The batteries E and E, of 8 Bunsen elements each, were outside of the hut.

The optical parts of the apparatus, namely, the illuminating apparatus B, with the achromatic lens / (5 cm. aperture, 30 cm. focal length), the head of the Schlieren apparatus L (21 cm. aperture, 3 cm. focal length), and the cam

* This closing could of course have been done automatically, and would be recommended were the apparatus to be a permanent one.

t The man firing the gun was so expert that it was hardly possible to notice a time interval between the sounding of the gong. We considered that we had to abstain from electric firing on grounds of personal safety, and would advise against it in similar cases.

era A' are of course in a straight line. The distances are Bl— 25 cm., //. = 4.5 m.. LJCzz.6 m. The light of the spark B, passing through /, fills the aperture L completely, so that it, L, looks brightly illuminated as seen from K.

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The illuminating apparatus is represented in fig. 6. It consists of a parallel opiped of hard rubber, which can be moved horizontally, vertically,and also around the line of sight as axis. A small shallow groove contains the electrodes of magnesium wire, and is covered with a glass plate after the hard rubber has been painted with petroleum. This arrangement obliges the spark to extend in a straight line, and can therefore be made parallel to the edge of the opaque screen * which is placed in front of K. This causes great increase of light, and makes it possible to use a very small spark of very short duration. The capacity of our jar was barely 1000 cm.

The adjustment of the optical parts is made as follows: The different parts are placed approximately at the above mentioned distances, and the center of B, I and L and the objective of A"are placed approximately at the same height. The lenses /, L are then removed and the frame of L so placed that its image will be in the center of the ground glass plate of the camera K; the frames of / and B are then so placed that their images will be concentric with that of L. If then the lens / is replaced and a candle-flame is held in front of A', its reflection on / will show if the axis of the lens coincides with IK. The axis of the lens /. is adjusted in the same manner.

In order to place B in the proper position, so that the image of the spark will be projected on the screen in front of A", and preferably so that it will be bisected by the screen in the direction of its length, a candle-flame, or, better still, a platinum wire made incandescent in a Bunsen flame is used. This flame or wire is placed immediately in front of the screen and opposite to the center of K, and at the same parallel to the edge of the screen at the place where the image is to appear. If the spark-score B is then placed so as to appear in the well defined part of image of the candle-flame or wire, then reciprocally the image of the spark B will coincide with the candle-flame or wire.f A small after-adjustment of the screen or small movement of A" completes the installation. During our experiments the screen was placed vertically and generally to the left, looking in the direction of L.

The muzzle of the gun was distant 12 m. from the place where it caused discharge, i. e. A. The projectile here formed the connecting bridge between two parallel, vertical wires of 0.5 mm. thickness ; these were covered with rub

•See XIV, 1, 1888.

t By application of the optical law of reciprocity, the problem is, as in case of a geometrical construction, considered solved ; but it becomes necessary to seek for the conditions of this solution. ber and were about 3 cm. apart. The projectiles used were blunt, caliber 4 cm., length 6, and others pointed at both ends so as to leave at each end a circle of 16 mm. diameter, caliber 4 cm., length 9.8 cm. The velocity in front of the lens L waa about 670 sec m.

The fear that the pressure of air would cause disturbances was found, if not without foundation, at least greatly exaggerated. The shock of the discharge had no appreciable effect on the independently placed parts of small diameter of the apparatus. The illuminating apparatus only, which besides being placed in the sand had to be fastened to the wall of the hut, had to be readjusted. The shock, however, produced no movement in the wall of the hut until too late to disturb the experiment*.

The picturea which were taken were on the whole similar to those obtained by firing rifle projectiles, and proved the correctness of the assumption formerly expressed, that the experiments with small models were sufficient provided the velocity was tbe same.' There were, however, some new phenomena; the wave in front of tbe blunt projectile showed double ; the part closest to its head seemed to be apparently an isolated, flat disk of air which seemed to disperse sideways.

l'h« double-pointed projectile snowed, besides a single head-wave, three isolated waves which emanated from the sides of the projectile at different

places. . , , ,

We will not give an exacter description or analysis of the pictures, as we have vnv», during many varied laboratory experiments, obtained still better pictures, nth we will treat later.*

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BOOK NOTICES.

Practical Marine Surveying. By Ensign Henry Phelps, U. S.N.

While this book was written to fill the need of a suitable text-book on the subject at the Naval Academy, it thoroughly bears out its title and is eminently practical, and will be found useful by all who wish to acquire a knowledge of marine surveying. There have been a number of books written on this subject, but this is the first one that is really practical and by means of which any one with sufficient education to handle the instruments and make the calculations can learn to make a good marine survey. It contains many valuable "wrinkles," some of which have been passed along for years in the Coast Survey, and others that are the result of experience in the survey of the Western Coast under the Hydrographic Office, Navy Department.

American Railroad Bridges. By Theodore Cooper, M. Am. Soc. C. E.

In this work Mr. Cooper gives a history of American bridge-building, together with a description of the existing and accepted types and the methods in use to-day, and it cannot fail to be of interest to all who design, manufacture or use railroad bridges. In the historical portion, under the head of wooden bridges he commences with the "Great Bridge" built across the Charles river, between Old Cambridge and Brighton, in 1660, and carries it along to 1844, when the Pratt truss was patented, in which the tension members were of iron. Under the head of iron bridges he commences with Thomas Paine's letter, written in 1803, and ends with the iron lattice bridge built 1865-66 over the Connecticut river. He then takes up the history of long-span bridges, commencing with the bridge built 1863-64 over the Ohio river at Steubenville, and continuing the history up to the present day. He then gives the theory and practice of designing and proportion, a description of the manufacture of bridges, of the typical American railroad bridges with a discussion of their relative merits, and concludes with a discussion of the failure of bridges. There are a number of plates and some tables giving the result of physical tests of full-size bridge members. K. W.

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