I 



BAROMETER. 



BAKCCMETKR. 



id OB hore to be always little behind an 









barouoUr, varying in amount according to the rate at which the 

 mercury is rising or falling, and ranging from 0-00 to 0-02 of an inch. 

 It is supposed that as the motion of the ship at Ma causes the mercury 

 to pan* more rapidly thruugh the con- 

 tattled tube, the reading* are about the 

 aame an they would be U the tube were 

 not contracted. 



: 



by Maura. Nogretti and Zambra on 

 Kortin'H prinoi]>le (No. 15), in which the 

 reading U made from on ivory point in 

 the cUtern, U represented in tlie adjoining 

 figure. The tube, which U filled by l.il- 

 ing, U enclosed within a bran tube, B, 

 fnrnuhed at the upper part with two 

 longitudinal opening* opposite each other; 

 ,, H idi> of tin- front o|-ning in tin- 

 Male of English inches divided to show, 

 l.y means of a vernier, jjujth of an inch ; 

 on the opposite side is sometime* divided 

 a scale of French millimetre*, reading 

 also by a vernier to ,',th of a millimetre ; 

 the reservoir, or cistern of the barometer 

 in of glass, closed at bottom by means of 

 a leather bag, acted upon by a thumb- 

 screw, r, passing through the bottom of 

 on arrangement of brass-work, by which 

 it is protected. This instrument is sent 

 out in three parts, packed separately for 

 safety in carriage. They consist first, of 

 the barometer-tube and cistern, filled with 

 mercury ; secondly, the brass tube, with 

 its divided scale and thermometer, o; 

 and thirdly, a mahogany board with a 

 bracket at the top, and a brass ring with 

 three adjusting screws at the bottom. 

 The board is to be fixed as nearly vertical 

 as possible ; the barometer tube and its 

 cistern are screwed into the brass tube ; 

 and an experiment is made to see whether 

 the tube is free from air. For this pur- 

 pose the screw at the bottom of the 

 cistern is lowered three or four turns, so 

 that the mercury in the tube when held 

 upright may fall 2 or 8 inches from the 

 top. The mercury U then made to strike 

 the top of the tube as already noticed ; 

 if a sharp tap be heard, the instrument 

 is perfect ; but if the tap be dull, or not 

 audible, there is air aliove the mercury, 

 which must be driven into the cistern by 

 inverting the instrument and gently 

 tapping it by the hand. The Inrometer 

 U next suspended on the brass bracket, 

 and the cistern passed through the ring 

 at the bottom, ami when it has settled 

 in a vertical position it is clamped by 

 means of the three thumb-screws, n. 

 Before making an observation, the mer- 

 cury in the cistern must be raised or 

 lowered, by means of the thumb-screw, x, 

 until the ivory point, E, and its re8ectwl 

 image, are just in contact; the vernier 

 is then moved by means of the milled 

 head, until its lower termination just 

 excludes the light from the top of the 

 mercurial column, when the rending ix 

 taken by means of the scale on the limns 

 and vernier. 



Hrlf-nyiitrri'j Bvomtttn. Various at- 

 tempts have been made so to construct 

 barometers that they shall register tli. 

 own proceedings In 1830, Dr. Trail 1 

 constructed an apparatus consisting of 

 two diagonal barometers, one inverted 

 and resembling] in principle tho rect- 

 angular barometer; both were attached 

 to the same frame, but before 1>. n.lin^ 



sad (111 inn ** "prifrhl one, a piece of smooth steel win- w:i intriKlueiil 

 into esrh, which served as indices by being driven forwards by the 

 mercury so as to indicate the m im " m or minimum for a given time, 

 tod ~ii"i.g stationary in the tube when the mercury retreated 



In Krith* sipbnn barometer. a* improved by Bryan, the register 

 iiinsMslii of aa ivury AW. and a kneed wire ending in a small knife-edge. 

 By the aid* of this in a vwtical cylinder of tinned iron, japanned, made 

 by means of clock-work to move round on iU axis once in 24 hours ; 



face of this cylinder being thinly coated with a mixture of chalk 

 and water, tho knife-edge is made to touch the cylinder every hour. 

 and by cutting through the chalk shows a black line on a white 

 ground. At the end of 24 hours the first cylinder is removed, and a 

 second one substituted, while the marks on the first can be read off 

 and recorded by means of a scale of equal parts- 

 Messrs. Negretti and Zambra register the highest and lowest readings 

 in the following manner : To the longer leg of a siphon barometer, 

 8 inches from the top, is joined a bent jrU tube, carried up for 

 inches parallel to the principal tube, and joining it at 2 inches from 

 the top. The mercury flows freely in and out of this tube, and 

 maintains the same level as that in the larger tube. This bent tube 

 contains a small piece of steel, which is kept in position by means of 

 fine glass springs : as the mercury rises the steel is pushed up, and 

 remains stationary when the mercury recedes. The lowest readings 

 are indicated by a similar contrivance in the shorter leg. 



Mr. Yeates' registering apparatus consists of a revolving cylinder 

 4 inches in length, around which is paper ruled into thirty-one vertical 

 lirtins, and horizontally into lOtha of inches, and numbered from 

 27 to 31 inches. To the receiver is attached a pencil for marking the 

 paper. The ivory point is adjusted to the surface of the mercury by 

 means of a plunger. 



Of late yean, the self-registration of the barometer has been effected 

 in meteorological observatories by means of photographic apparatus. 

 The principle of the contrivance is to direct the light of a lamp through 

 the Torricellian vacuum upon a surface of sensitive paper, as it is 

 gradually unwound from a cylinder. In this way a broad trace is left 

 on the paper, the breadth of which varies with the height of the 

 mercury in the tube. 



Attempts at self-registration have also been made by furnishing the 

 upi*T part of the tube with wires, connected with certain voltaic 

 arrangements, in which the currents are closed by the ascent of the 

 mercury in the tube. M. Hardy, an instrument maker of Paris, has 

 fitted a siphon barometer with an apparatus consisting of a Boat in the 

 open limb, which Boat acts upon an aluminium rule ; this is furnished 

 with a pencil, and is made to move up and down before a \ 

 cylinder, but the pencil is at the distance of one millimetre from the 

 cylinder. The cylinder is moved by clockwork, which every five 

 minutes sends an electric current through an apparatus which 

 sets a hammer in motion, and this striking on the barometer 

 tiilw performs the office of the finger of the observer in tapping the 

 instrument so as to produce a more exact level of the mercury. 

 Half a minute after the hammer has ceased, the clock sends a current 

 through an electro-magnet, the armature of which strikes on the 

 aluminium rule, and forces the pencil up to the register paper on the 

 cylinder. 



Mi*cc/laeoxa. Various forms of barometer were brought under the 

 notice of the Jury (Class X) of the Great Exhibition of 1851. Mr. 

 John Griffith exhibited a barometer so constructed as to give the power 

 'at all times of securing a vacuum above the mercury. The top of the 

 tube is furnished with a crook for the purpose of trapping all the air 

 which may be above the column ; the tube is bent at the lower part, 

 while near the middle of tho tube is a joint with a trap; 

 there is also a stop-cock, and a stretcher for closing the 

 open part of the glass tube when necessary. The reading is 

 by means of a brass bar, carrying two cylinders at the dis- 

 tance of 29-722 inches, and moved up and down by means 

 of a steel screw, with twenty-five threads to the inch, 

 working in a matrix by means of a milled head. The 

 bar has a pointer for reading to lOOths of an inch, ami 

 there is also suitable apparatus for reading to 1000th of an 

 inch. 



Messrs. Negretti and Zambra had a barometer with an 

 air-trap glass cistern, to be read off by means of a sliding 

 scale adjustable to the surface of the mercury by means of 

 a fine ivory jioint. The tube and cistern were blown 

 together, and at intermediate junctures were three points 

 and three small tubes or traps, communicating with each 

 other to prevent the admission of air. Tho construction 

 of these air-traps will be understood by referring to ih.- 

 accompanying figure, in which it will be seen that air, 

 creeping up the tube, would evidently pass up into the 

 hemispherical part or trap, where it would be retained 

 instead of pawing up that part of the tube which is drawn 

 to a point, and leads directly into the Torricellian 

 vacuum. 



The same excellent makers also exhibited a folding I 



consisting of a tube with a steel stop-cock in the centre, 

 which when MoM up, carries with it the two valves of the tube 

 with which it is connected. 



In Mr. Abraham's- barometer the scale w suspended over a pulley 

 by means of a counterpoise, the lower end of the scale being con- 

 to a float, in the shorter leg of the siphon. Mr. Ross had a 

 self-eompCTlKiting barometer on the name principle. 



Among the numerous forms of barometer which arc ln'ing constantly 



i. we m:iy mention tli;it l.y M. Trouesmrt. in whirl, Hi. 

 portion of a siphon "barometer is made of caoutchouc, which allows the 

 observer to augment the capacity of the barometer chamber at will, and 



