BAROTIETEn. 



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A Hdalltf Rvnmrtrr, a modification of the aneruid, WM invented >.y 

 B.. union, in 1850. It consists of an rlastic flatUned tube of meUl, 

 completely exhausted of air, and bent very nearly into the form of a 

 circle. In thi* condition the flattened tube hat the property of ex- 

 panding : the end* separating further from each other when the atmc*- 

 prwcure U diminished, while a contrary effect take* place when 

 A lever U attached to the end of the tube, 



poetic 

 the pr 



and oonnaotod by toitable mechanism to an index hand, which 

 tr.Tcr.rn a divided dial-plate. Thi< plate is gradiuted by placing the 

 instrument, together with a standard barometer, within the receiver of 

 an air-pump, and the poinU of coincidence are determined by varying 

 the pressure. An instrument of thi* kind in well adapted for steam- 

 engine., gasometers, Ac., ainoe it will measure a pressure up to COO Ibs. 

 and upward* on the square inch. 



The tifmpiaomrler (from vvfarttaa, tompnaia*, and plrpor, meattirt), 

 wa invented by Adie, of Edinburgh, in 1819, to serve on a barometer, 

 for the purpose of measuring the pressure of the atmosphere, or of 

 exhibiting the variations of its density near the earth. An account of 

 iu construction is contained in the first volume of the ' Edinburgh 

 Journal of Science.' It consists of a glass tube B about 18 inches 

 long and seven-hundredths of an inch in diameter internally : at the 

 upper extremity is a cylindrical vessel A, half an inch in diameter 

 im.Tinlly and two or three inches long; and the lower extremity, 

 being turned upwards, terminates also in a vessel c. The vessel A and 

 the upper part of the tube B are filled with an elastic fluid, such as 

 hydrogen gas, while the vessel c and the lower part of the tube contain 

 almond-oil, coloured with some tincture such as that of anchusa root. 



In order to introduce the gas and oil, the extremities a and b of 

 the vessels A and c being open, and the former extremity being drawn 

 in the form of a slender tube, the instrument is quite filled with 

 mercury ; when, being held in a horizontal position with the orifice b 

 covered by the finger, that the mercury may not run out, the slender 

 pipe at a is put in communication with the gasometer. The hydrogen 

 gas entering at a, and the instrument being now brought to a vertical 

 position, on removing the finger from b the mercury will flow out till 

 its upper surface is at the same level in the tube B and the vessel c, 

 the gas occupying its place in the former. The extremity a U then 

 hermetically sealed, and, the instrument being inverted, the mercury 

 in C runs out, while that which remains in the 

 tube B prevents the gas from escaping. The 

 instrument being afterwards returned to its former 

 position, the vessel A uppermost, the oil is poured 

 into the vessel c : then, the gaa being heated, by 

 its expansion it drives the mercury in B into the 

 vessel c ; and, the instrument 1 wing now held in 

 an inclined position, the oil is by the pressure of 

 the atmosphere forced into the tube B in propor- 

 tion as the gas by cooling becomes contracted in 

 volume. The remaining mercury U lastly poured 

 out of the vessel c ; and the end b being left open, 

 the pressure of the atmosphere on the oil is, the 

 instrument being again put in a vertical position, 

 a counterbalance to the weight of the column of 

 oil in a and to the expansive force of the com- 

 pressed gas which u in the upper part of the tube 

 nnd in the vessel A. 



In order to form a scale for the sympiesometer, 

 the instrument, together with a good barometer 

 and .-i thermometer, must be placed in a glass 

 n-ceiver in which, by proper syringes, the air may 

 lie rarefied or condensed at pleasure : then, on 

 bringing the air in the receiver successively to 

 such states, with respect to density, that the top 

 of the column of mercury in the barometer may 



3* 



at 27, 28, 29, and 30 inches, and marking the ]>ints on the tube 

 or frame of the sympieaometer at which the top of the column of oi 

 iitood when the air was in those states, the distance between every two 

 of these point* being divided into one hundred equal parts, the scale 

 /> 7 of the instrument is constructed. The particular graduation at 

 which, on the scale, the top of the column of oil utands at any time 

 hould indicate the number of inches and hundrtdtha of an inch at 

 which the top of the column of mercury in a barometer would stem 

 at the same time in the same place. 



But th<- volume of the hydrogen gas changes in consequence o 

 variations in the temperature of the atmosphere; and therefore a 

 correction, depending on the amount of the variation in the volume o 

 gas for given increments of heat, should be applied to the number o 

 the graduation on the scale of the sympiesometer in order to obtain thi 

 true height of tiv mercurial column in a barometer. 



But, to avoid the trouble of computing and applying this correction 

 the scale pg of the sympiesometer is made to slide on one si. 

 soak r j which is graduated like that of a thermometer, by coininnni 

 eating different degrees of heat to the vessel A while the pressure o 

 the atmosphere on the upper surface of the oil in c remains the same 

 (oppose that which corresponds to 294 inches on the scale of a baro 

 meter) and observing the points at which the top of the column it 

 |nds: th point* on the Kale r > are numbered so as to express 

 In degrees, UM temperature of the gas ; and the ncale p i/ being mvw 



till iU index (at 294 inches) coincides with the degree of the soak r 

 corresponding to the temperature of the atmosphere, expressed by the 

 legree of a thermometer I r attached to the instrument, the graduation 

 on p 9 corresponding to the top of the column of oil in B expresses the 

 icight of a column of mercury in a barometer. 



Dr. Robert Gordon, the writer of the article Meteorology in the 

 Edinburgh Encyclopedia,' having made a aeries of observations for 

 he purpose of comparing the indications of the sympieaometer with 

 hose of a barometer, found that the former stood higher th.r 

 alter by quantities varying between 0-03 inch and 0-017 in. 

 emperaturo varying between 42*'3 and 62'6 (Fahr.): he found at 

 the same time that, in general, the difference between the indications 

 of the two instruments increased as the temperature and pressure 

 limiuished. The excess of the indications given by one instrument 

 over those given by the other may be conceived to have arisen fn>m 

 some inaccuracy in the construction of the sympiesometrr : Imt tli. 

 variation of the excess is not so easily accounted for ; it may, however, 

 >e supposed to be caused in part by the expansions of the gas not 

 wing exactly proportional to the increments of heat, a circumstance 

 which is assumed in the subdivisions of the scale r >, between each of 

 he complete inches, being made equal to one another, and in part 

 rom some absorption between the oil and the gas. It is inferred, 

 therefore, that the indications of the sympiesometer cannot always be 

 relied on as accurate measures of the density of the atmo.-i 

 since it has been observed that the instrument is frequently affected 

 jy changes in that density, which are too small to be perceived 

 scale of a barometer ; and since it is well known to be less deranged 

 ,han the latter instrument by the motion of a ship at sea. it I 

 that, in these respects, it possesses great advantages as a marine baro- 

 meter iu indicating the approach of gales of wind. 



M. Pouillet, in the 7th edition of his ' Clements de Physique,' 1856, 

 rives a description of M. Bunten's sympiesometer. It is formed of an 

 Ucohol-thermometer, and an air-tube curved and open at its upper em). 

 The reservoir of the air-tube is in the interior of the cistern 

 thermometer, and completely surrounded by alcohol. The lowt 

 of the reservoir and air-tube contains a column of oil of almonds, acted 

 on by atmospheric pressure, so that the temperature of the air is always 

 indicated approximately by the alcohol-thermometer. This temperature 

 remaining constant, if the pressure is increased, the oil will descend in 

 the open branch, and will rise if it be diminished. Similarly, if the 

 pressure be constant, the oil rises or falls in the open branch, according 

 to the temperature. 



AVith respect to the proper hours of the day for observation. 

 is Professor Doniell remarks, are indicated by the barometer. " Tin 1 

 maximum height of the mercurial column is about 9 A.M., the mean 

 at 12, and the minimum at 3 P.M. If a person have time to i.-i ik. 

 three observations in the day, these are the hours which he -' 

 select ; if circumstances only allow of his observing twice, 9 A.M and 

 9 P.M. are the proper periods ; if only once, noon is the time. These, 

 fortunately happen to be, probably, the most universally conv. 

 hours that could have been selected. In national observatorir- it 

 would not be too much to expect that observations at 3 P.M. K!I .-.!.! 

 be added to the preceding. Even those who merely consult tlir 

 barometer as a weather-glass, would find it an advantage to att 

 these hours ; for I have remarked, that by much the safest progn 

 tions from this instrument may be formed from observing when the 

 mercury is inclined to move contrary to its periodical course. If the 

 column rise between 9 A.M. and 8 P.M., it indicates fine weather ; if it 

 fall from 8 to 9, rain may be expected." This subject is fully f 

 into in books on meteorology. We may especially refer to a p| 

 Meteorology, by Sir John Herochel, in the 'Admiralty Manual of 

 Scientific Enquiry,' and also to a paper in the same work on Atm.>- 

 spheric Waves and Barometric Curves, by W. R. Birt, Esq. 



Some interesting barometrical and meteorological observation 

 recently been mode during four balloon ascents, made by Mr. \\Yl-h. 

 under the direction of the Kew Observatory Committee of the British 

 Association, the result* of which are published in the'Philowi 

 Transactions' for 1853. These ascents were made in the latter | 

 1852, and the objects especially aimed at were the determin.it 

 the temperature and hygrometric condition of the air at dill', i. m 

 heights, analysis of the air, and examination of the light reflected from 

 the upper surface of the clouds. The barometer used was of 

 Lussac'a form (No. 14), Regnault's condensing, and Douiell's dew ]>..int 

 hygrometer. When about 3000 or 4000 feet above the upper surface 

 of the clouds, they were examined with a poloriscope. The light 

 reflected from the clouds next the sun gave no trace of jHilariaation, 

 while those on the side from the sun gave very slight traces, the light 

 of the sky being all the time strongly polariised. The height above 

 the level of the sea attained by the balloon was deduced from the 

 barometric readings by Laplace's formula expressed in English feet 

 .m<l Fahrenheit's degrees : 



i = log |p) x 60159 ( 1 + '-i-j^ )(l + 0-002887 cos 2 L) 



d *2S 



\ 2088 



20886900/ 



I : = height required ; h and A' ; I and t' = height of barometer 1 

 \ and temperature at lower and upper station, L = latitude, 



