MANOMETER, or MANOSCOPE, an instrument to show or measure the alter ations in the rarity or density of the air. The manometer differs from the barometer in this, that the latter only serves to measure the weight of the at mosphere, or of the doltimn of air over it : but the former, the density of the air on which it is found ; which density depends not only on the weight of the atmosphere, but also on the action of heat and cold, &c. Authors, however, generally confound the two together; and Mr. Boyle himself gives us a very good manometer of his contrivance, under the name of a statical barometer, consisting of a bubble of thin glass, about the size of an orange, which, being counterpoised when the air was in a mean state of density, by means of a nice pair of scales, sunk when the atmosphere became lighter, and rose as it grew heavier. Other kinds of manometers were made use of by Colo nel Roy, in his attempts to correct the er rors of the barometer. " They were," says he, " of various lengths, from four to upwards of eight feet : they consist ed of straight tubes, whose bores were commonly from one-fifteenth to one-twen ty-fifth of an inch in diameter. The ca pacity of the tube was carefully mea sured, by making a column of quicksil ver, about three or four inches in length, move along it from one end to the other. These spaces were severally marked, with a fine-edged file, on the tubes ; and transferred from them to long slips of pasteboard, fbr the subsequent con struction of the scales respectively be longing to each. The bulb, attached to one end of the manometer at the glass house, was of the form of a pear, whose point being occasionally opened, dry or moist air could be readily admitted, and the bulb sealed again, without any sen sible alteration in its capacity. The air was confined by means of a columm of quicksilver, long or short, and with the bulb downwards or upwards, according to the nature of the proposed experi ment. Here it must be observed, that, from the adhesion of the quicksilver to the tube, the instrument will not act truly, except it be in a vertical position; and even then it is necessary to give it a small degree of motion, to bring the quicksilver into its true place, where it will remain in equilibria between the exterior pressure of the atmosphere on one side, and the interior elastic force of the confined air on the other. Pounded ice and water were used to fix a freezing point on the tube ; and by means of salt and ice, the air was further condensed, generally fbur, and sometimes five or six degrees below zero. The thermometer and manometer were then placed in a tin vessel among water, which was brought into violent ebullition ; where, having remained a sufficient time, and motion being given to the manometer, a boiling point was marked thereon. After this the fire was
removed, and the gradual descents of the piece of quicksilver, corresponding to every twenty degrees of temperature in the thermometer, were successively marked on a deal rod applied to the manometer. It is to be observed, that both instruments, while in the water, were in circumstances perfectly similar; that is to say, the ball and bulb were at the bottom of the vessel. In order to be certain that no air had escaped by the side of the quicksilver during the ope ration, the manometer was frequently placed a second time in melting ice. If thu barometer had not altered between the beginning and end of the experiment, the quicksilver always became stationary at or near the first mark. If any sudden change had taken place in the weight of the atmosphere during that interval, the same was noted, and allowance made for it in afterwards proportioning the spaces. Long tubes, with bores truly cylindrical, or of any uniform figure, are scarcely ever met with. Such, however,ts were used in these experiments, generally tapered in a pretty regular manner from one end to the other. When the bulb was downwards, and the tube narrowed that way, the column of quicksilver con fining the air, lengthened in the lower half of the scale, and augmented the pressure above the mean, In the upper half, the column being shortened, the pressure was diminished below the mean. In this case, the observed spaces both ways from the centre were diminished in the inverse ratio of the heights of the ba rometer at each space, compared with its mean height. If the bore widened to wards the bulb when downwards, the observed spaces, each way from the cen tre, were augmented in the same in verse ratio ; but in the experiments on air less dense than the atmosphere, the bulb being upwards, the same equation was ap plied with contrary signs; and if any ex traordinary irregularity took place in the tube, the corresponding spaces were pro portioned both ways from that point, whe ther high or low, that answered to the mean. The observed and equated ma. nomorical spaces being thus laid down on the pasteboard containing the mea sures of the tube ; the 212° of the ther mometer, in exact proportion to the sec tions of the bore, were constructed along side of them : hence the coincidences with each other were easily seen; and the mn ber of thermometrical degrees answering to each manometrical space, readily transferred into a table prepared for the purpose."