M. Passement accordingly constructed marine ba rometers upon this principle ; and Mr Nairnc, an in genious artist in London, made one of a similar kind, for Captain Phipps, in his voyage to the north pole. Mr Nairne also suspended his instrument on gimbals, by means of which the effects of the ship's motions were almost entirely counteracted. M. Zeiher has suggested another marine barometer, depending in principle on the variable elasticity of the air. He has proposed a hollow cylinder completely freed from air, with two moveable ends ; in the inside of the cylinder, and between the ends, is placed a spring which keeps them separate, and resists the pressure of the air, so that the degree of its compression af fords a measure of that pressure. When the pres sure of the air is increased, the two ends of the cy linder approach each other; and when it is diminish ed, they recede. Consequently the distance between them will indicate the atmospherical pressure in some inverse ratio. See Mem. Acad. Petrol). 1758 and 1739.
We shall conclude the account of marine barome tors, with a description of one of the most improved kind as now constructed by Mr Troughton; The tube 1 consists of two parts, joined together about five inches 0 below the top : the bore in the upper part being about o of an inch, and in the lower part only T.L-' By this construction, partly from the difference of the bores, and partly from the greater friction in the lower end, the motion of the mercury is so much retarded, that any impulse given by the ship having a tendency to raise it, will scarcely have produced a sensible effect, before an opposite impulse will be given having a tendency to depress it.
To counteract more effectually the effects of the / ship's motions, the instrument is suspended on gim bals, a representation of which is given at Fig. 7. 1 The whole is attached to the side of the cabin by two tubes of brass, which slide one within the other, and render the instrument capable of being suspend ed at different distances from the place of support, that the bottom of it may not strike the side of the cabin, during any heart' rolling of the vessel. See Fig. S. The inner tube carries the gimbals. The i external frame of the barometer is a cylindrical tube of wood, on which slides a brass socket ; and in this is inserted the innermost pair of pivots of the gimbals or universal joint, which furnishes the instrument with a moveable point of suspension. The top is termi nated with a knob of brass, of a weight nearly equal to that of the mercury, &c. at the lower end. With respect to the position of the point of suspension, no general rule can be given, applicable to every case ; though it is a circumstance on which the oscillations of the mercury greatly depend. It is obvious, how
ever, that though this point were accurately determined for one particular height of the mercury, it would not correspond to every other. By the ingenious contd.. vane of Mr Trdughton of placing a knob at the top, as a counterpoise to the weight of the mercury, the centre of gravity of the whole will be about the middle ; and if the instrument were of the same spe cific gravity throughout, the point of suspension that would produce the smallest oscillations in the mer cury would be about ÷ of the length of the instru ment from the top, considering the lower part as a fixed point. But as this is not strictly the case, the point of suspension is best ascertained by experiment. The graduation is on two scales of ivory, about four inches long, for the reception of which, two opposite quarters of the cylindrical frame are crossed Out through that length, their planes pointing towards the centre of the tube. The index is a very light one, and slides upon the glass tube without touckng any other part. At the bottom is the usual screw, which pressing up the leather bag, prevents the mer cury from moving when the instrument is carried from one place to another. Fig. 9, and 10. repre sent sections of this barometer on a larger scale.
After it was observed that the different heights of .i the mercury served, in some degree, to indicate the t state of the weather, many attempts were made to en- large the extent of the barometrical scale, in order to measure the smallest variations in the weight of the atmosphere. These attempts soon gave rise to a con siderable variety of barometers, differing in form from the common barometer, and whose scales though less accurate, were so much increased in extent, as to point out the most minute changes in the pressure of the air.
Descartes was the first who thought of enlarging the barometrical scale, and for this purpose he invent ed a barometer which still retains his name. It con sisted of a tube of about four feet and a half long, swel led out towards the middle, as represented by CD, ; AC was filled with water, the point C being about 31 inches above the surface of the mercury in the cistern B. The diameter of the tube AC may be to that of the part CD in any ratio. By this con struction, the range of the scale would be nearly as great as if the whole fluid were water, or about 40 inches, if we neglect the weight of the water, which is inconsiderable compared with that of the mercury. Huygens constructed a barometer of this kind ; but, owing to the escape of the air from the water, or to the vapour produced in the vacuum at the top of the tube, the variations were not nearly so sensible as he expected.