470 



NEW ENGLAND FARMER. 



Oct. 



THE BAROMETER AND ITS USE. 



The attention of the agricultural community 

 having been called to this instrument the past 

 season, more directly than ever before, mainly by 

 the advertisements in our columns and in other 

 Agricultural Journals, of the instruments manu- 

 factured by Messrs. John M. Merrick & Co., of 

 Worcester, we propose in this article to explain 

 briefly the character of the instrument, and its 

 utility as a weather-guage. 



The word Barometer is derived from two Greek 

 words, signifying weight and to measure. The in- 

 strument, therefore, is to measure weight, as ap- 

 plied to the atmosphere, or is, in other words, to 

 determine the pressure of the atmosphere. It 

 is well known that the weight of a column of air 

 one inch square, and of the height of the atmo- 

 sphere above the earth, is equal to about 15 lbs. 

 This column will then sustain a column of water, 

 mercury or any other fluid of similar size, and 

 high enough to make it equal in weight. The col- 

 umn of water which can thus be sustained is 32 

 feet in height, the column of mercury about 29 

 inches. Galileo commenced, near the close of his 

 life, experiments to ascertain why a column of 

 water could be raised in a vacuum only to the 

 height of 32 feet. Without completing these ex- 

 periments he died, and left his pupil, Torricelli, 

 to pursue the investigations. He used mercury ; 

 filling a tube, closed at one end, with the fluid, he 

 placed his finger over the other end, which he 

 then immersed in a basin of mercury, holding the 

 tube upright. Upon removing his finger, the mer- 

 cury in the tube sunk and finally rested at a 

 height of 28 inches above the level of that in the 

 basin. Repeated experiments resulted in the 

 same way, and thus was discovered a principle in 

 natural law which is as unchangeable as the na- 

 ture of the Creator. 



The Barometer thus being invented, it was not 

 long before the changes of the atmosphere were 

 discovered to afi'ect it, Pascal, a French philos- 

 opher, also inferred, and established the truth of 

 his inference by experiment, that the Barometer 

 would indicate a lessened pressure of the atmo- 

 sphere by ascending with it above the ordinary lev- 

 el of the earth's surface. These experiments were 

 conducted in the years 1642 to 1648. Numerous 

 attempts have been made to modify the form of 

 the barometer, but those now most approved are 

 essentially the same as Torricelli's, a straight in- 

 verted tube, about thirty-two inches in height, 

 and mercury as the measuring liquid. Some have 

 been made where no liquid was used, but the 

 weight of the atmosphere was determined by its 

 pressure upon the sides of a metal box from which 

 the air had been exhausted. The mercurial ba- 

 rometers are, however, the standard, and much 

 preferred by scientific men. 



It is obvious that the weight of the atmosphere 

 is not always the same. The presence of mois- 

 ture, the variations of heat and cold, and the force 

 of the winds, all operate to make the weight of 

 the atmosphere at any point, an almost constantly 

 changing quantity. Observations for now more 

 than two hundred years have enabled us to judge 

 with much accuracj', by these changes in the 

 weight of the atmosphere as indicated by the ba- 

 rometer, of approaching changes in the weather. 

 It has been found that almost invariably certain 

 atmospheric changes precede a change of the 

 weather from wet to dry, or from dry to wet, and 

 that the barometer never indicates so high an al- 

 titude during a storm as during dry weather. 

 These observations have established a set of rules 

 for judging of approaching changes of weather, 

 which we shall give at the close of this article. 



The index affixed to some barometers, marking 

 upon a circular disc the height of the mercury, 

 with the alleged corresponding condition of the 

 atmosphere, is of no value, and is calculated to 

 mislead. In fact the altitude of the barometer at 

 any given time is not alone a guide to the weath- 

 er which may follow, but the true guide is — 

 whether the height indicated is more or less than 

 that indicated a short time previously ; whether 

 the mercury is declining, or rising, at the time of 

 the observation. 



There are also changes of the barometer in our 

 latitude, which indicate no corresponding change 

 of weather. For instance, the mercury during fair 

 weather may stand at a high altitude in the morn- 

 ing and decline a little towards the middle of the 

 afternoon, with no variation of the weather follow- 

 ing. But should this change continue, and the mer- 

 cury still decline, and not return towards midnight 

 to its altitude in the morning, then a change is sure 

 to follow. We notice this change in a Timby's 

 Barometer which hangs by our side as we write this 

 — Tuesday P. M., (Sept. 4th,) — which has declined 

 one-tenth of an inch since 9 o'clock this morning. 

 As it fell in a similar manner yesterday, and rose 

 again at nightfall, we apprehend no change unless 

 this decline continues. It was a knowledge of 

 these changes which led us last week in noticing 

 this instrument, to caution people against imput- 

 ing to that, faults which exist only their in own 

 ignorance of its principles, and to advise them 

 to obtain an understanding of the laws which 

 govern it, before judging falsely of its indications. 

 Mr. Timby's Barometer is a simple instrument, its 

 indications plain to be read and easy to under- 

 stand. We believe it to be thoroughly made, as 

 the mercury has that brilliant lustre which be- 

 speaks its purity, and in inclining the tube it 

 strikes against the top with a sharp click which 

 indicates a perfect vacuum. These points show 

 a correct construction ; and as the principle of 



