BAROMETER. 



113 



conveniently taken as .22 inch. Strictly, this 

 difference is composed of two parts 1, the 

 true index error for the aneroid ; 2, the special 

 difference of the two instruments in different 

 latitudes, clue to the facts that the mercurial 

 barometer is independent of changes in the 

 force of gravity, while the aneroid is sensible 

 to the increase of gravity in going northward. 

 Within the limits of 66 and 75.3, K, this 

 variation in the force of the earth's attraction 

 amounts to 0.014 of an inch. 



The diurnal variation of the barometer is 

 discussed at. length. This is slight, and is to be 

 traced only by combination of a great number 

 of observations ; besides, it is often masked by 

 irregular fluctuations of the atmospheric press- 

 ure. To determine the diurnal variation, the 

 observations of two years are considered ; for 

 Baffin Bay, mean latitude of 72. 5, those of 

 September, 1857 e to August, 1858, inclusive; 

 and for Port Kennedy, latitude of 72, those of 

 September, 1858, to August, 1859, inclusive; 

 while the observations made at Van Rensselaer 

 Harbor, latitude of 78. 6, in the years 1853- 

 '54-'55, were introduced for the sake of com- 

 parison. The curves of diurnal variation for 

 the two former have in common a maximum at 

 about 71 P. M., and a minimum at about 4 A. M. 

 The mean range of the diurnal fluctuations is 



In Baffin Bay, 028 Inches. 



At Port Kennedy, 048 " 



At Van Bensselaer Harbor, 010 " 



Hence, between latitudes 72.2 and 78.6, K, 

 the diurnal range diminishes by .028; and, at 

 this rate, it would become insensible (less than 

 001) in about 81 N. 



The computed annual range of variation, i. e., 

 the difference between the highest and the low- 

 est monthly mean, is 



In Baffin Bay, 0.94 inches. 



At Port Kennedy, 0.41 " 



At Van Kensselaer Harbor, 0.21 " 



The maximum effect of any one wind or calm 

 does not exceed 0.04 inch. In Baffin Bay, the 

 barometer is higher with the wind from N"., 

 N. E., and E., and lower with the wind from 

 S. W., W., and N. W. ; whereas, at Port Ken- 

 nedy, where the wind -is much subject to local 

 influences [and the barometer also, the great 

 amount of its daily range appears to show], 

 nearly the opposite law holds true. 



The Earth's Rotation, and the Surfs Dis- 

 tance. Of a series of papers by Mr. Pliny 

 Earle Chase, relative to aerobaric (air-pressure) 

 phenomena, which, originally appearing within 

 the past three years in the publications of the 

 American Philosophical Society, have been re- 

 published in the American Journal of Science, 

 and to some extent in other journals, and which, 

 though in certain parts of a highly abstruse 

 character, are nevertheless the vehicles of some 

 important physical hypotheses, the first [of those, 

 at least, with which the writer of this has met] 

 bears the title, " On the Barometer, as an Indi- 

 cator of the Earth's Rotation, and the Sun's Dis- 

 tance." In making extracts from these articles, 

 as elsewhere, the language of the original will 

 VOL. v. 8 A 



be adopted so far as it appears most concisely, 

 and for the general reader satisfactorily alsp, to 

 convey the author's meaning. 



That daily barometric tides exist, has been 

 known for more than 150 years; their cause 

 has been thus far in dispute. They are not to 

 be explained by variations of temperature 

 [alone] ; since, 1, they are detected after elimi- 

 nating all the known effects of temperature ; 2, 

 they occur in all climates and seasons ; 3, they 

 often show opposite effects, under average tem- 

 peratures which are the same. 



Now, the combined rotation (daily) and revo- 

 lution (yearly) of the earth imparts to each 

 particle of the air a velocity in the direction 

 of the earth's orbit, which must vary with 

 changes of latitude and hour of the day, and, 

 at the equator, from about 65,000 miles per 

 hour at noon to 67,000 at midnight. The ratio 

 of the force of rotation at the equator to that 

 of terrestrial gravity, as known by the amount 

 of effect each is capable of producing in 24 

 hours, is ^VfiVorr .00109 ; and this ratio, 

 representing the proportionate elevation or de- 

 pression of the barometer above or below its 

 mean height that should (by theory) be caused by 

 the earth's rotation, corresponds very nearly 

 with the actual disturbance at stations near the 

 equator. From O h (noon) to 6 h , the air has a 

 forward motion greater than that of the earth, 

 so that it tends to fly away; its pressure is 

 therefore diminished, and the mercury falls. 

 From 6 h to 12 h , the earth's motion is greatest ; 

 it therefore presses against the lagging air, and 

 the barometer rises. From 12 h to 18 h , the earth 

 moves away from the air, and the barometer 

 falls. From 18 h to 24 h , the increasing velocity 

 of the air urges it against the earth, and the 

 barometer rises. 



The theory of the forces in operation would 

 give a maximum height of the barometer at 9 h 

 and 21 h , and a minimum and at 3 h and 15 h . 

 Three years' hourly observations (1844-'45-'46) 

 at St. Helena, however, show the actual maxima 

 at 10 h and 22 h , and the minima at 4 h and 16 h ; 

 and this retardation, of one hour throughout, 

 agrees with that due to the inertia of the mer- 

 cury, as shown by comparison with indications 

 of the water-barometer. 



The author argues that the varying centrif- 

 ugal force to which the earth is subjected by 

 the ellipticity of its orbit, must likewise pro- 

 duce annual tides ; while from a comparison of 

 certain elements of the earth's daily and yearly 

 revolutions he deduces a formula for the sun's 

 distance. The following are his results, in part : 



The effect of latitude on the aerobaric tidea 

 is indicated in the following table : 



