140 Instructions Jar Making and Registering 



the correspondence of its movements and affections over great regions of the 

 earth's surface, or even over the whole globe, the Committee have resolved 

 to recommend, that four days in each year should henceforward be especially 

 set apart by Meteorologists in every part of the world, and devoted to a most 

 scrupulous and accurate registry of the state of the Barometer and Thermo- 

 meter; the direction and force of the Wind; the quantity, character, and 

 distribution of Clouds; and every other particular of weather, throughout 

 the whole twenty-four hours of those days, and the adjoining six hours of the 

 days preceding and following.* The days they have been induced to fix on 

 and recommend for these observations are, the 21stof March, the 21st June, 

 the 21st September, and the 21st December, being those or immediately ad- 

 joining to those of the Equinoxes and Solstices, in which the Solar influence 

 is either stationary, or in a state of most rapid variation. But should any one 

 of those 2\st days fall on Sunday^ then it will be understood that the observations are 

 to be deferred till the next day^ the 22d. The observation at each station should 

 commence at 6 o'clock a. m. of the appointed da^'s, and terminate at G o'clock 

 P.M. of the days following, according to the usual reckoning of time at the 

 place. During this interval, the Barometer and Thermometer should be 

 read off, and registered hourly, and the precise hour and minute of each read- 

 ing should be especially noted. 



For obvious reasons, however, the commencement of every hour should, if 

 practicable, be chosen, and every such series of observations should be accom- 

 panied by a notice of the means used to obtain the time, and, when practicable, 

 by some observation of an astronomical nature, by which the time can be in- 

 dependently ascertained within a minute or two.f As there is scarcely any 

 class of observations by which meteorology can be more extensively and es- 

 sentially promoted, it is hoped that not only at every station of importance 

 in this colony but over the whole world, and on board ships in every part of 

 the ocean, individuals will be found to co-operate in this inquiry. Every 

 communication of such observations addressed by channels as secure and as 

 little expensive as possible to the Secretary of this Institution, will be consi- 

 dered as highly valuable- 

 Ill. Of Meteorological Instruments^ and first of the Barometer and its 

 attached Thermometer. 



The Barometer is the most important of all Meteorological instruments. Its 

 office is to measure the actual pressure of the atmosphere on a given hori- 

 zontal surface at the time and place of observation. Its fluctuations are ob- 

 served to have considerable relation to changes in the weather, and especially 

 of the wind. Hence its use as a weather-glass. 



* This is necessary by reason of the want of coincidence of the day in different 

 parts of the globe, arising from difference of longitude. la order to obtain a complete 

 correspondence of observation for twenty-four successive hours over the whole globe, 

 it must be taken into account that opposite longitudes differ twelve hours in their 

 leckoninw of time. By the arrangement in the text the whole of the astronomical day 

 (from noon to noon) is embiaced in each series, and no observer is required to watch 

 two nights in succession. 



•^ For example, the first appearances and last disappearances of the Sun's upper and 

 lower border, above and below the sea-horizon, if at sea or on the coast, — or on^land 

 the exact length of the shadow of a vertical object of determinate lengthen an horizon- 

 tal level, at a precise moment of time, (not too near noon}, &c. 



