B A R O M E T E R. 



291 



meter. 



Passe- 

 mcnt's ma- 

 rine baro- 

 meter. 



Nairnc'* 

 marine ba- 

 rometer. 



Zeiher's 

 marine ba- 

 rometer. 



nometer, (See Manometer), and a thermometer 

 placed together in the same frame. The thermome- 

 ter was affecte i only by the warmth of the air ; the 

 manometer, acting by the expansion and contraction 

 of an included bubble of air, was affected not only 

 by the warmth, but also by the weight of the air. 

 If the two tubes, therefore, are so graduated as to 

 agree wich each other when the air is included, it is 

 it, that when they afterwards agree, the pres- 

 sure of the atmosphere must be the same as when the 

 air was included. And in general, if the thermome- 

 ter be taken as a standard, the difference of ascent or 

 descent in the other will point out the increase or de- 

 of the weight of the air. At the same time, 

 ht to be mentioned, that the condensation and 

 rarefaction of the air, on which this instrument is al- 

 together founded, do not depend solely on the weight 

 of the atmosphere, but are greatly influenced by tem- 

 perature. Hence this instrument cannot, strictly 

 speaking, be called a barometer, but rather a con- 

 trivance for pointing out alterations in the state of 

 the air ; and as such, -according to the observations 

 of Dr Halley, it may be of considerable utility. 

 During his voyage to the South Sea, he had one of 

 these barometers, and " it never failed," says he, 

 " to prognosticate, and give early notice of all the 

 bad weather we had, so that I depended thereon, 

 and made provision accordingly ; and from my own 

 experience, I conclude, that a more useful contrivance 

 hath not for this long time been offered for the be- 

 nefit of navigation." Phil. Trans. 1700-1, N 269, 

 p. 791. 



M. Passement obviated the effects of the motions 

 of a ship at sea on the barometer, by twisting the 

 middle of the tube into a spiral consisting of two con- 

 volutions ; by this contrivance, the impulses which 

 the mercury receives, mutually destroy each other, 

 by acting ir. opposite directions. The effects of the 

 external and momentary impulses may also be dimi- 

 nished by widening the upper part of the tube where 

 the scale is applied. The oscillations, which would 

 be very perceptible in the tube, become scarcely sen- 

 sible when they are thus diffused over a larger ex- 

 tent of surface. 



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 Nairnc 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. Petrop. 1758 and 



rinc baro. 



meter. 



Fig. a 



We shall conclude the account of marine, barome- Baromet 

 ters, with a description of one of the most improved v 



kind as now constructed by MrTroughton. The tube Trough- 

 consists of two parts, joined together about five inches ton s ma " 

 below the top : the bore in the upper part being 

 about -i of an inch, and in the lower part only T |^. 

 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 elfect, before an opposite impulse will be given 

 having a tendency to depress it. 



To counteract more effectually the effects of the p, ATk 

 ship's motions, the instrument is suspended on gim- i,iv. 

 bals, a representation of which is given at Fig. 7. l r ig. 7. 

 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 heavy rolling of the vessel. See 

 Fig. 8. The inner tube carries the gimbals. The 

 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 contri- 

 vance of Mr Troughton 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 y 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 ivoty, 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 tight 

 one, and slides upon the glass tube without touching 

 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- *>g- 9> 10- 

 sent sections of this barometer on a larger scale. 



After it was observed that the different heights of Attempts 

 the mercury served, in some degree, to indicate the t0 er 'l ar g' 

 state of the weather, many attempts were made to en- baro- 

 large the extent of the barometrical scale, in order to 8c; j c- 

 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 



