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We find in the " Annual of Scientific Discovery," for this year, the description of an instrument of the above name, invented by M. Secchi, of Rome, which appears to be a good invention, and worthy of more general attention than it has yet received. Suppose the mercury bowl of a barometer to be placed on a table and the glass tube so arranged as to admit of its being lifted by hand, the force that will be required to lift the tube will be equal to the weight of mercury in the tube, or, in other words, to the amount of atmospheric pressure exercised on the mercury of the instrument. We shall therefore be able to weigh the pressure of the atmosphere by attaching the tube of the barometer to the one end of a balance, and a weight to the other, for it is evident that at every change in atmospheric pressure, a corresponding increase or decrease in weight will have to be made at the other end of the balance to maintain equilibrium. To ascertain the value of absolute pressure on a unity of surface, it will be necessary to take into consideration the weight of the tube, and also the weight of that portion thereof which is immersed in the mercury of the barometer bowl, and especially the internal sectional area; the force which actuates the instrument ma also Ije increased, and permit of more minute and exact readings. If the sectional area be ten square centimeters (a centimeter is '39370 of an English inch), and as the pressure varies by centimeters in hight, the weight to be placed at the other end of the balance, will be that of nineteen cubical centimeters of mercury, or one hundred and thirty five grammes (a gramme is 154440 English grains), while, if the sectional area had been equal to one square centimetor only, the weight would have been but 13*5 grammes. Starting from these observations, M. Secchi constructed his balance barometer, which has been successfully used for some time in the Roman Observatory. The tube of the barometer is attached to one end of a steel yard or balanced lever, which carries at the other end a counterbalance weight and a small pointer, which is reflected in a mirror. There is also a graduated scale reflected in this mir- \ ror, so that a very minute variation of the | pointer is indicated by a movement on the reflected image. As the atmospheric pressure is thus weighed, as it were, and not indicated by the hight of the column of mercury, the tube may be made of cast iron instead of glass, as the iron is not liable to become amalgamated with the mercury, if the tube is of equal bore throughout. By increasing the sectional area of the tube, sufficient motive power will be given to the lever, to operate a pencil attached to one end, to record the variations of atmospheric pressure on a piece of paper. Other fluids beside mercury may also be used in iron tubes to obtain the same results. It has been suggested that this barometer may be employed in ships and in mines for signalizing dangers of atmospheric pressure, such as approaching wind storms on the ocean and the Sowings of fire-damp in the mines. For this purpose this barometer should have a pointer of iron or some metal, insulated from the rest of the instrument, but in communication with one of the poles of a battery. The dial, over which the pointer has to travel, should be of glass or ivory, having metallic points inserted at those gradations which indicate dangerous variations of atmospheric pressure. These points being placed in communication with the other pole of a battery will, when the pointer comes into contact with one of them, close to the electric circuit, and operate a signal by an electro-magnet, and thus give sensible warning of approaching danger. It is not a little remarkable that a similar instrument to the above was designed by an American in the early part of last summer. His invention was subsequent to that of Secchi, but was made without the least know- ledge of the Roman professor's efforts. It will j be observed by our readers that the foregoing I barometer is similar in the principle of its op- eration to the one illustrated on page 52, this volume, Scientific American, but it is very different in its details. From the description given of it, some of our readers will no doubt be enabled to construct such barometers, and test their qualities.
