254 ANNUAL OF SCIENTIFIC DISCOVERY. 



the sectional area had been equal to one square centimetre only, the weight 

 would have been but 13'5 grammes. 



Starting from these observations, M. Secchi adopted the following construc- 

 tion for his barometer ; which has been made and used in the observatory 

 at Home. The barometrical tube is attached to one end of a steelyard or 

 balanced lever, which carries at the other end a counterbalance weight and a 

 pointer, fifteen millimetres, or five-eighth inch diameter, which is reflected in 

 a mirror. This mirror also reflects a graduated scale, so that the variation 

 of one-tenth of a line of the pointer is indicated by a movement of six lines 

 on the reflected image. The writer enumerates the following advantages 

 peculiar to his invention : 1. As the atmospheric pressure is weighed, and 

 not indicated by the height of the column of mercury, the tube may be con- 

 structed of any non-fragile material, such as iron, which docs not amalga- 

 mate with the mercury, provided the bore be of equal diameter throughout. 

 2. By increasing the sectional area of the tube, the additional weight will 

 give sufficient motive power to a pencil attached to the other end of the lever 

 to mark the variations of atmospheric pressure. 3. By the intervention of 

 suitable gearing, the scale of observations maybe augmented without incon- 

 venience or danger to the exactness of the instrument. 4. The new construc- 

 tion is independent of the form of the miniscus, of the purity of mercury, of 

 its specific gravity, and of the temperature and difference of gravity peculiar 

 to different latitudes, for all these qualities exercise influence upon the volume 

 of mercury, and on the height of the column of mercury in the tube, which 

 has to be measured to obtain the weight of atmospheric pressure, whereas, 

 with the new instrument the weight is given at once. Another advantage 

 of employing iron for barometrical tubes is, that there is no danger to fear 

 from the adhesion of air, or moisture, and the mercury may be boiled with- 

 out fear of bursting. Iron barometrical tubes will likewise permit of other 

 fluids being employed, and probably advantageously, instead of mercury. 

 M. Secchi states that he invariably found his new barometer indicate varia- 

 tions of atmospheric pressure before ordinary barometers did so, and that by 

 avoiding loss by friction, most exact instruments may be produced. 



It has been suggested that this invention may be applied to the construc- 

 tion of audible or danger-signalizing barometers, which, if placed on board a 

 ship, would tell the captain and whole crew of the approach of storms ; or 

 which, if placed in mines, would warn miners and inspectors of the presence 

 of fire-damp, as well as indicate its precise locality. For this purpose it 

 would be only necessary to employ a weighing barometer, the pointing of 

 which was composed of some suitable conducting material, but insulated 

 from the rest of the instrument. The pointer should be in communication 

 with one of the poles of an electro-magnetic battery. The dial, over the face 

 of which the pointer would have to travel, should be composed of glass, or 

 other suitable non-conducting material, with metallic points inserted at those 

 gradations, which indicate dangerous variations of atmospheric pressure. 

 These points would have to be placed in communication with the other pole 

 of the battery. The reader will readily see by this arrangement, that when 

 the pressure of the atmosphere shows the presence of danger, either at sea 



