UM 



METEOROLOGY. 



BAROXETERS. 



the month, at 8 A.M. , make a clot, corresponding to the 

 barometric height, on the dark vertical line, na on the 

 previous day, and then draw a line to it from the dot of 

 the previous evening or morning observation, according 

 as to whether a single or doable one lias been made. 

 Proceeding in this manner, a lino will daily appear to 

 progress from the left to the right of the scale, indicating 

 the changes which have occurred throughout the month. 

 The barometric lino should be made both dark and th ;!:, 

 to distinguish it from the thernwmetric line, which should 

 be.riru, for the same reason. 



The indications of the thermometer are to be similarly 

 marked. Supposing, for instance, that on the 1st of the 

 month the thermometer stood at 62 look on the right 

 lian 1 of the diagram for GO , and, at two lines above it, 

 vill be found the division corresponding to 02. By the 

 eye, trace this lino until it comes under figure 1 of the 

 vertical scale, and there make a dot Continue in the 

 same manner as directed for the barometer, only making 

 a /iu< line between the morning and evening, or morning 

 aii'l morning, divisions from day to day. Thus, the indica- 

 tions are to bo traced by linos throughout the month, as 

 is done with the barometer, and the two scales will be 

 simultaneously marked for each day, or each twelve 

 hours, as tho observations are made and registered. 



The first division and a-h:ilf, on tho left hand of the 

 diagram, are intended to indicate the respective heights 

 of both instruments for the preceding 30 hours ; that is, 

 an entire division is given for the last day of tho pre- 

 ceding month, and half a one for the day previous. The 

 lines are to be connected witli those of the current month, 

 in the manner already described, and thus tho observa- 

 tions of the two months will run on consecutively ; and, 

 from an inspection of the diagram, the probable weather 

 fur the enminencemeut of the month may be predicted, 

 without further reference to the plan of the past one. 

 The place, month, and year of observation should bo 

 filled up at the top of the diagram ; and space may be 

 left at the bottom, to receive notes of any special 

 meteorological occurrences. 



To still further illustrate this method of registering, 

 we may trace a portion of the folio plate. The indica- 

 tions have been Drought from the previous mouth, and 

 on the 1st of the current one, we notice that the height 

 of the barometer (dark line) stands at 30-30; it then 

 passes to 30*17, on the 2nd ; the line then progresses 

 to 30-05, and so on. Similarly, the thermonietric fine 

 line indicates 55 on the 1st of the month ; on the 

 2nd, 59, <tc. ; tho heights of each instrument being 

 simultaneously marked, and their variations clearly 

 seen by the relative position of tho dots and connecting 

 lines on the diagram. 



This plan is remarkable for its simplicity, and the ease 

 with which any one may map out tho observations 

 made daily. For forecasting it has great advantages ; 

 for instead of appealing solely to the mind for the 

 calculation of numerical differences, tho eye is at once 

 instructed, and, at a glance, the observer can see whether 

 the barometer and thermometer are rising and falling 

 together, or oppositely ; whilst tho rules for predicting 

 weather, given at page 1170, may bo immediately 

 applied in connection with the observations on tho 

 diiejtion of tho wind blowing at tho moment, or 

 previously. 



Although a full description has been given, in our 

 previous pages, of the qualities which should characterise 

 a good barometer, it may be useful here to mention some 

 particulars in reference to the Standard Barometer, as 

 supplied to the Navy by the Board of Trade ; and the 

 care which is shown in their construction, and subsequent 

 verification, will bo best understood by the foil. 

 account of tho mode of testing them, as described in tho 

 Barometer Manual. Our readers may thus learn some 

 raliiable lesson* in respect to the purchase of instruments 

 for their own use. 



The barometer* to be tested, are placed, together with 



On* that bu betn alnadj full 7 UtUd, la ertry ropect, at the Kew 

 OWnratory. 

 t TO* egolmUott at UM marine bvooeter-tabe prarenu what U 



I a Standard,* in an air-tight chamber, to which an air- 

 pump is applied, so that by partially exhausting tho 

 air, tho Standard can be made to read much lower 

 the lowest pressure to which marine barometers are likely 

 to be exposed ; and by compressing the air, the mer- 

 cury may also be read higher than it ever stands at 

 the level of the sea. The tube of the Standard is 

 contracted similarly to that of the marine barometer.t 

 but a provision is made for adjusting the mercury in the 

 cistern to the zero point. Glass windows are inserted in 

 the uppej part of the iron air-chamber, through which tho 

 scales of the barometers may be seen ; but as the verniers 

 cannot be moved, in the usual way, from the outside of 

 the chamber, another method is adopted for reading the, 

 exact height of the rnorcu/y, independently of tho 

 verniers attached to the scales of the respective baro- 

 meters. At the distance of some five or six feet from tho 

 air-tight chamber, a vertical scale is fixed ; and the 

 divisions on this scale correspond exactly with those on 

 the tube of the Standard barometer. A vernier and 

 telescope are made to slide on tho scale, by means of a 

 rack and pinion. The telescope has two horizontal wires 

 one fixed, and the other movable by a micrometer screw ; 

 so that tho difference between the height of tho column 

 of mercury, and the nearest division on the scale of the 

 Standard, and also of all the other barometers placed by 

 the side of it for comparison, can be measured either 

 with the vertical scale and vernier, or the micrometer wire. 

 The means are thus possessed of testing barometers for 

 index-error in any part of tho scale, through the whole 

 range of atmospheric pressure to which they are likely to 

 be exposed ; and the usual practice is to test them at 

 every half-inch from 27*50 to 31 inches, which is about 

 tho extreme average range of the barometer in almost 

 any latitude. 



In this way various other descriptions of barometers 

 have been tested ; and some errors were found to be so 

 large, that a few instruments read half an inch and 

 upwards too high, while others read as much too low.* 

 In some cases, those which were correct in one part of 

 the scale, were found to be from half au inch to an inch 

 wrong in other parts. In some tho mercury would not 

 descend lower than about 29 inches, owing to a fault 

 very general in the construction of many common 

 barometers till lately, in frequent use namely, the 

 cistern was not large eninnjh to hold the mercury 

 which descended from the tube during a low atmospheric 

 pressure. 



In the Government Marine Barometers, tho diameter of 

 the cittern is about an inch and a quarter, and that of 

 the tube about a quarter of an inch. The scale, instead 

 of being divided into inches, in the usual way, is 

 shortened, hi the proportion of about 0-04 of an inch for 

 every inch. The object of this is to avoid tho necessity 

 of applying a correction for the difference of capacity 

 between the cistern and tho tube. Tho perfection with 

 which this is done may be judged of from the fact, that 

 of tho first twelve barometers tested at the Liverpool 

 Observatory, with an apparatus exactly similar to that 

 used at Kew (whence those instruments were sent by 

 railway, after being tested and certified), the index cor- 

 rections, in the pressures from 28 to 31 inches, of three 

 of them, wore tho same ; two differed O'OOl of an inch ; 

 and, for tho remainder, the differences ranged from 0*002 

 to O'OOO of an inch. The corrections for capacity wero 

 therefore considered perfect, and, with one unimportant 

 exception, agreed with those given at Kew. 



From the above account, it will bo seen how frequently 

 common barometers may lead the observer astray an 

 occurrence which, whilst of comparatively slight impor- 

 tance on land, might cause the destruction of a vessel, 

 and serious loss of life, if such indications of an imperfect 

 instrument wero relied on at sea. Tho peculiar nature 

 and sudden advent of cyclonic storms, which are always 

 accompanied by a barometric fall, render it of the utmost 

 importance that the barometer on board of a vessel should 



called " pumping ;" that U, the extreme oecillation caused by the motion 

 of the ibip in a rmigh tea. 

 } See anlt, p. Hit). I Sec ante, p. 1 175. 



