CIRCUITS.

The complete arrangement of the electrical circuits used with the different pieces of apparatus is shown in the diagram, in which D is the dynamo, T the transmitter tube, S a switch which completed the circuit of the transmitter just a moment before firing the gun to prevent heating the coils, L, L2 the line wires leading to the proving ground; Q a bank of resistance lamps; X,X,X, etc., the screen wires shunted across between the line wires; Y, Y,Y, etc., the devices for restoring the current successively between the screens; and L and L' two 50-volt arc lamps in series which for convenience were lighted by the same dynamo. The electrical tuning fork is controlled by the cells E; M is the motor for running the camera plate; and at 0 are represented four storage cells for energizing the same. G is the gravity-switch for exposing the camera and firing the gun, and C the camera whose slide is operated electrically by the cells I from the gravity-switch at VV. The firing circuit contains the electric primer P at the gun, the line wires P、P., the dry cells S', and the gravity-switch terminals UU.

The principal ballistic result obtained from these experiments may be said to be the locating of a maximum point in the velocity curve outside of the gun. This maximum point is, in the case of the gun and conditions of loading described, at six or seven feet from the muzzle of the gun-certainly more than five feet and less than ten-or about 25 calibers in front of the muzzle.

A few preliminary experiments were tried with a view of determining the value of this instrument for the measurement of velocities inside the bore, but any mention of them is withheld until further experiments can be made.

The preceding is in nearly the words of the authors, but is much condensed. In connection with this instrument we would say that extension of its use into ordinary practice seems to lie in the elimination of the gravity switch and the substitution of a sensitized sheet of paper or gelatine film for the circular photographic plate used. We suggest winding the sensitized sheet on the cylinder of a Schultz chronoscope, and see no reason why it should not then be as susceptible to practical use as is the Schultz. The traversing gear of the latter would obviate the present necessity of the gravity switch and allow of the use of this chronoscope as a megagraph without affecting its value in its present distinctive field, that is, as a micrograph.

The instrument is so accurate that the cutting of a screen wire by the side in place of the point of the ogival head of the projectile causes an irregularity in the record. So we may fully expect with its further development to see rewritten the literature on the resistance of the air to projectiles, as well as possibly the thermo-dynamics of the gun itself.

THE LIMIT OF HUMAN ENDURANCE OF HIGH AIR PRESSURES.

[ENGINEERING NEWS.]

The dangers to men working under high pressures of compressed air, due to the pressure itself, and especially to the change from the high to the normal pressure on leaving the air-lock, have long been recog

nized and studied by engineers and physicians, and it is now well understood that only comparatively young men, of sound constitution, should be employed, that the hours of labor under pressure should be short, that the pressure should be reduced very gradually in the air-lock, that the men should rest after coming out, and that a hospital air-lock should be provided, in which men may be placed and treated with heated compressed air if chilled, afflicted by the bends' or "caisson disease," or otherwise injured by a too sudden reduction of pressure. A hospital of this kind, as used at the Hudson River tunnel works, was illustrated in our issue of June 14, 1890.

A series of interesting experiments as to human endurance of higher pressures than are usually employed in compressed air work has recently been made by Mr. Hersent, the engineer in charge of the new harbor works at Bordeaux, France, where the quay foundations are being constructed by the compressed air system, and we take the following particulars of these tests from Engineering, of London. As the sponge divers descend from 160 to 200 ft. without injury, it was considered that workmen should be able to endure corresponding pressures under the better conditions of an air chamber, and Mr. Hersent therefore formed a commission of doctors to work with him in ascertaining if men could safely sustain a pressure of 70 lbs. per sq. in. The test chamber was fitted with windows, a telephone, electric light and a steam coil, by which any desired temperature could be maintained. Three men volunteered for the tests; one being a regular compressedair workman, the second an occasional workman, and the third a man who had only entered the working chamber on a few occasions. These men were subjected to pressures for a length of time, usually about one hour. The tests were commenced with a pressure of about 28.4 lbs. per sq. in., and the pressure increased very gradually, by about 4.27 lbs. per day, to 76.8 lbs. per sq. in., while the time for the pressure reduction was increased about 10 minutes for each 1.42 lbs. increase in pressure. The period of compression was also increased, but to a smaller degree, this being of less importance. All three men sustained without difficulty a pressure of 46.9 lbs., with a reduction period of 56 minutes. One of the men, being indisposed from an independent cause, was then withdrawn. At 58.3 lbs. pressure the man who was used to working in the chamber felt some temporary inconvenience, and at 65.4 lbs. his companion, who was not accustomed to compressed-air work, had to be withdrawn, as he suffered from pains in the side. There was no trace of paralysis, but it was not considered safe for him to continue the test, which was finished by the first man alone, who sustained a pressure of 71.1 lbs. for one hour, the pressure being then reduced in 2 h. 25 mins. When released from the chamber this man took some sulphurous baths, which had cured the pains of his companion, and then underwent the final test, in which the pressure was raised to 76.8 lbs. in 45 minutes, continued for an hour, and then reduced to normal pressure in 3 h. 3 mins. The temperature was increased from 56° F. to 68° F. during the compression, maintained at 68° during the test, and then gradually increased to 86° F. during the reduction of the pressure. The man suffered no inconvenience, with the exception of a tingling sensation, which passed away after a short time. It is considered that, if certain precautions are taken, men in good health can sustain a pressure of 76.8 lbs. per sq. in., that means should be provided for heat

ing the chamber at will, and that good ventilation should be maintained during the reduction of the pressure. As it has been proved that the workmen should rest after leaving the air-lock, especially after working under high pressures, elevators should be provided to bring the men to the surface. These experiments go to show the practicability of men working under compressed air at greater depths than have yet been attempted.

The greatest pressure thus far used in compressed-air work was 52 lbs., corresponding to a head of 120 ft., in the East River Gas Co.'s tunnel, described in our issue of July 11. This was the extreme reached on this work. The ordinary pressure was about 45 lbs., corresponding to a head of 104 ft. At the Limfjord Bridge, in Denmark, men worked for some time at a depth of 113 ft.

THE IMPROVED GATLING GUN.

[IRON AGE.]

The Naval Rapid Fire Gun Board have submitted to Captain Sampson, chief of the Naval Bureau of Ordnance, a report on the trial of the improved gatling gun. In its general features the gun is the same as the 30-caliber gun tested by the board last year. It differs in important details, however. The caliber of the improved invention is .236 and the length of the barrels 26 inches. A change in the firing mechanism has been made, and now the operating is done from the right-hand barrel instead of the lower one. The change allows more time for the operation of the extractor, and reduces the danger of disabling the gun by hang fires. A novel electrical attachment is used with the gun, which permits of about 1800 shots a minute being fired, a remarkable showing. The improved gatling underwent tests in competition with the Browning automatic gun. Later it will have to meet several foreign inventions. The experiments with the gatling gun began with the firing of 100 rounds deliberately. This was followed by the discharging of 100 rounds rapidly. No time was taken for the first test, but in the second the record was 9 seconds. There were no interruptions to the firing. Then 20 rounds were discharged; time 3 seconds. One miss fire occurred. Forty rounds were fired in 5 seconds, 50 rounds in 6 seconds, 100 rounds in 7 seconds, 200 rounds in 13 seconds and 400 rounds in 37 seconds. One man operated the crank, and two men were at the feed. Later 400 rounds were fired in 30 seconds. Without any difficulty 460 shots were fired in one minute.

In the course of the last test one case containing ten cartridges was split laterally. Then followed the firing of 100 rounds deliberately, every fifth cartridge being a dummy. The experiment was satisfactory to the board. Two hundred shots with the extreme elevation and 200 shots with the extreme depression were then discharged, the time for the first being 24 seconds and for the latter 21 seconds. With the barrel moving in train and elevation, 200 rounds were fired in 30 seconds without any interruption. In these experiments two men successfully operated the cranks.

To determine how quickly damaged parts of the mechanism could be replaced, a test ensued of taking out an old and putting in a new lock; the time was 28 seconds. The board fired the gun 20 times with one

lock removed. No interruption occurred, but two unfired cartridges were thrown out. Some ill luck attended the attempt to fire the gun continuously for five minutes. A bullet became jammed in one of the barrels, and in one chamber two cartridge cases were found one against the other, the heads of both having been pulled off. The jams were but temporary. The record of the five minutes' firing was 1980 shots, 40 of which were satisfactory hits. The range was 500 yards. With the range at 1000 yards the number of good hits was ten.

The board then made experiments with the gun having the electric motor fitted on. The motor is attached to the breech of the gun casing by an interrupted screw. The electric current is regulated by a switch, and the connection with the firing mechanism made and broken by a push-button held in the gunner's hand. The weight of the motor and the casing is 106 pounds; their length, 21% inches. The additional length of the gun when the motor is attached is 20 inches. The projection of the central shaft from the breech when the motor is unshipped is 2 inches. As stated, with the motor in operation, the gun can fire 1800 shots a minute. The board says that several hundred shots were fired, the only interruption being due to the temporary jams in the gun or feed strips, and that the action of the motor was smooth and regular throughout.

PAPER SAILS.

[MARINE RECORD.]

An innovation in yachting circles is now being talked of, nothing less than sails made of compressed paper, the sheets being cemented and riveted together in such a way as to form a smooth and strong seam. It appears that the first process of manufacturing consists in preparing the pulp in the regular way, to a ton of which is added 1 pound of bichromate of potash, 25 pounds of glue, 32 pounds of alum, 11⁄2 pounds of soluble glass, and 40 pounds of prime tallow, these ingredients being thoroughly mixed with the pulp. Next the pulp is made into sheets by regular paper-making machinery, and two sheets are pressed together with a glutinous compound between, so as to retain the pieces firmly, making the whole practically homogeneous.

The next operation is quite important, and requires a specially built machine of great power, which is used in compressing the paper from a thick, sticky sheet to a very thin, tough one. The now solid sheet is run through a bath of sulphuric acid, to which ten per cent. of distilled water has been added, from which it emerges to pass between glass rollers, then through a bath of ammonia, then clear water, and finally through felt rollers, after which it is dried and polished between heated metal cylinders.. The paper resulting from this process is in sheets of ordinary width and thickness of cotton duck; it is elastic, airtight, durable, light, and possessed of other needed qualifications to make it available for light sailmaking.

The mode of putting the sheets together is by having a split on the edges of the sheet, or cloth, so as to admit the edge of the other sheet. When the split is closed, cemented and riveted or sewed, it closes completely and firmly.