BARO-METF.R. 



llARrCMETKR. 



Mi 



Boueteic pinyessinn whose ratio u r. This, we may add, is easily 

 provtd u. be the case ; see Goodwin'* ' Course of Mathematic*,' p. 333, 



m 

 from which we deduce the expression r = 1 j, when g = force of 



gravity. Let the height* of the barometer at the two station* be A A', 

 which are proportional to the atmospheric pressures, so that 



(*-*) 



!* 



The rule* for finding the height* by the barometer we summed up in 

 the following word* : 



1. Correct the length of the mercurial column at the upper station, 

 adding to it the product of its multiplication into twice the difference 

 between the degrees on the attached thermometer*, the decimal point 

 being shifted four place* to the left. 



2. Subtract the logarithm of UiU corrected length from that of the 

 lower column, multiply by 6, and more the decimal point four places 

 to the right; the result i* the approximate elevation expressed in 

 i . 



1. Correct the approximate elevation by shifting the decimal point 

 three place* back to the right, and multiplying by twice the sum of 

 the degree* on the detached thermometer* ; thia product being now 

 aildni will give the true elevation. 



The annexed figures represent a convenient form of portable 

 barometer manufactured by Messrs. Negretti and Zambra. The first 



figure represent* the instru- 

 ment packed in its case with 

 a strap c, and the other 

 figure mounted for an ob- 

 servation ; D D is the tripod 

 stand, and o the barometer 

 tube. 



Standard Barometer. In 

 preparing a standard baro- 

 meter for the Royal Society, 

 the late Professor Daniell, of 

 King's College, endeavoured 

 to get rid of the objections 

 to the boiling of the mercury 

 in the tube by first abstract- 

 ing the air by means of an 

 air-pump, the presence of 

 moisture being avoided by 

 the use of sulphuric acid. 

 Accordingly a barometer 

 tube was fitted with a stop- 

 cock, which was screwed 

 into the under surface of 

 the pump-plate ; while, on 

 the upper surface, was a 

 glass dish perforated in the 

 centre, and containing the 

 acid. In this dish was placed 

 a stand with glass legs for 

 receiving a funnel, the stem 

 of which, being drawn out 

 into a capillary tube, passed 

 down into the mouth of a 

 small paper cone resting 

 upon the tube. The aper- 

 ture at the upper part of 

 the stem was closed by an 

 iron plug, between which 

 and the capillary opening 

 cotton was placed. The 

 glass funnel was filled with 

 carefully prepared mercury, 

 and the whole was covered 

 with a gla** receiver, furnished at the top with a sliding rod, the lower end 

 <>f which could be screwed into the plug so as to draw it up and replace 

 it. The air was now exhausted to '5 inch, and the plug being with- 

 drawn, the mercury was allowed to trickle into the tube. In its fall it 

 wan, however, broken into globules, which formed minute cavities as 

 the tube was filled. As these cavities could not entirely be got rid of, 

 the apparatus wa* varied by passing a small tube to the bottom of the 

 barometer tube, so that the mercury could trickle down this and deliver 

 iteelf slowly into the barometer tube; 'by this means the mercury rose 

 in the latter in perfect and uninterrupted contact with the gluts. The 

 suoces* of this experiment wan such, that umlcr "ulinary circumstances 

 it would not have been thought necessary to boil the mercury in the 

 tube, but the prejudice* of the day required that this troublesome and 



luardou* process should be gone through. Many tube* were destroyed 

 in the boiling, after all the trouble of measuring and filling had been 

 taken. The tube finally selected was 33} inches long, and -.ISO inch 

 bore. The boiling wa* not performed, however, in the ordinary manner, 

 over a charcoal fire, but during the process of filling under diminished 

 pressure, as already described. When 17 inches of mercury had been 

 introduced, the tube was gradually heated before a fire, and a large 

 spirit-lamp name was applied In the upper part of the mercury ; when 

 this had reached the point of ebullition tin- Uiiliiu; was slowly continued 

 downward*, and when it had reached the bottom it was again as 

 gradually conducted to the top. " The bubbles of vapour freely passed, 

 with the assistance of a slight degree of agitation, from one end of the 

 column to the other ; and very bright flashes of green light accompanied 

 their extrication. One minute glolnilc of air alone was detected during 

 the heating, notwithstanding the diminished pressure, and this was 

 readily extricated ; and there was not the slightest condensation of 

 moisture visible in the cold portion of the tube." The cooling was 

 conducted very slowly ; when the second portion of mercury was intro- 

 duced, making the column equal to 294 inches, the air was abstracted, 

 and the boiling begun as before from the top and carried downwards to 

 about two inches below the union of the two portions of mercury. 

 The last 34 inches of the tube were filled up with hot mercury, and 

 the whole was left for 48 hours. When the tube was inverted in the 

 cistern, this great body of mercury did not descend until it had 

 received two or three smart concussions ; a good proof of the displace- 

 ment of air. By vibrating the mercury in the tube, beautiful green 

 flashes of barometric light were seen in the vacuum, and the crackling 

 sound of electrical excitation was heard on approaching the finger. 



This fine instrument was not destined to remain long a faithful 

 representative of the atmospheric pressure. The mercurial column, 

 which was originally perfectly bright and compact, became, in the 

 course of some months, dull and speckled, arising from the pressure of 

 minute air-bubbles ; the air, in fact, had insinuated itself between the 

 metal and the tube where the latter dipped into the cistern, and creeping 

 up into the Torricellian vacuum, opposed a resisting medium to the 

 oscillations of the column ; which, consequently, no longer represented 

 the atmospheric pressure, but the difference between that pressure and 

 that of the air contained in the space at the top of the tube. I 

 it was evident that all barometers, constructed with a straight tube 

 dipping into an open cistern of mercury, must be liable to a constant 

 process of deterioration ; and consequently all the registers which had 

 been so industriously kept by scientific men in various parts of the 

 world, were no longer trustworthy. The effect of this deterioration 

 would be to produce a gradual lowering of the mean annual height of 

 the barometer ; but in order to trace it, it was necessary to compare 

 periods of five or six years or more with each other. The ' Ephemerides ' 

 of the Meteorological Society of the Palatinate furnished the materials for 

 this comparison. Taking the series of observations made at Mannheim 

 for the twelve years 1781 to 1792 inclusive, and dividing them into 

 two periods of six years each, the mean of the last period was found to 

 be '62 inch lower than that of the first period. A similar comparison 

 for the observations made at Padua gave '044 inch lower for the second 

 period than for the first. For Rome, the difference was "114 inch ; for 

 Buda, -035 inch ; for Brussels, -044 inch ; for Munich, -026 inch ; for 

 Peisenberg, in Bavaria, -026 inch ; and for Mount St. Gothard, '020 inch. 

 These examples establish the fact of the gradual depression of tin- 

 mercurial column by the infiltration of air; and it may further be 

 seen, that the amount of the effect depends in some degree upon the 

 elasticity of the atmosphere in which it takes place. The five series of 

 observations, the mean pressure of which is 29'235 inches, show an 

 average depression of OSS inch in 12 years ; while the three series, the 

 mean pressure of which is 25'977 inches, show a depression of only 

 026 in the same period. 



The remedy proposed by Professor Daniell against this serious source 

 of deterioration, which would render barometrical observations com- 

 paratively worthless, was to weld to the barometer tube near it* open 

 end a ring of platinum, so that when the tube was dipped into the 

 cistern, the mercury, by wetting the platinum, would produce such 

 perfect adhesion as effectually to exclude the infiltration of atmospheric 

 air. Accordingly this plan was adopted, and for many years all the 

 best barometers were furnished with a platinum guard. It was thus 

 supposed that the evil complained of, was most effectually cured ; I nit 

 time, which is as delicate a tost aa any that a chemist can offer, showed 

 that the platinum guard was not free from defects. The mercury 

 formed an amnlgnTn with the platinum, which, during the oscillations 

 of the barometer, got sucked up into the vacuum, and, clinging about 

 the tube, impeded observation. The next remedy was the simple and 

 apparently obvious one of turning up the end of the tube, as represented 

 in the annexed figure ; but a* this involved a new method of filling, 

 we must, in order to describe it, again recur to the difficulties of 

 boiling, especially when the tube is of large dimensions. 



In the years 1853 and 1854 several attempt* were made, under the 

 8U|-rintendence of the Kew Observatory Committee of the British 

 Association, to prepare a barometer tube of large size in the usual 

 manner. Mr. Negretti succeeded in filling tubes fully one inch in 

 internal diameter ; but many of these, lfore they could be mounted, 

 broke spontaneously, some within a few hours, and others after an 

 interval of several days after the boiling. Two or three tube* were 



