. Transactions - American Philosophical Society. interval between the0th, and 280th vibration, between the 10th, and 190th, etc., and between the40tli and 320th; and the mean of these is taken as the result. At Dorchester, Princeton, and Philadelphia, the times were noted by a chro-nometer. At the other stations, a lever watch was used. At Hudson, the watchwas compared with the Observatory clock, immediately before and after theobservations. At the remaining stations there is a little uncertainty withreo-ard to the time, yet, it is thought, its influence upon the results will not be great. No
. Transactions - American Philosophical Society. interval between the0th, and 280th vibration, between the 10th, and 190th, etc., and between the40tli and 320th; and the mean of these is taken as the result. At Dorchester, Princeton, and Philadelphia, the times were noted by a chro-nometer. At the other stations, a lever watch was used. At Hudson, the watchwas compared with the Observatory clock, immediately before and after theobservations. At the remaining stations there is a little uncertainty withreo-ard to the time, yet, it is thought, its influence upon the results will not be great. No correction has been applied for the arc of vibration. In order to deter-mine the correction for temperature, the apparatus was placed upon a largeearthen plate, covered by a bell glass, alternately heated from below by a lamp,and surrounded by a freezing mixture. The usual mode of observation wasemployed, and the results are shown in the following table, the first column ofwhich indicates the time of commencement of each series of AT SEVERAL PLACES IN THE UNITED STATES. 63 The mean of the preceding observations furnish, Time ofVibration. Hansteen, 869^64 Sabine, 747 .03 Henry, Then, by the usual formula a = — Temp. 88^885 .687 .6 T—T T{t-r) Time ofVibration. .38 we obtain:— °.923 .526 .0 For Hansteens needle, a=--^^^ ^^^ X = .000191. For Sabines needle, a= — = .000328. For Henrys needle, a = = .000116. The standard temperature, to which the following results are reduced, is 60°Fahrenheit. No correction is applied for the diurnal variation of intensity, butthe hours of observation are always stated. To test the permanency of themagnetism of the needles, I have been furnished, by Prof Renwick, with twoseries of observations, made at New York, besides those made in September,1839. The results are as follows: Needle. Date. Time of 280Vibrcitions. Temp. CorrectedTime. Hansteen 1
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