the weather is not ferene, it is impoffible to affign any rule for their variation, as no regular correfpondence can then be perceived with the different hours of the day, nor with the various modifications of the air. The reafon is evident; when contrary and variable winds reign at different heights, when clouds are rolling over clouds, thefe winds and clouds, which we cannot perceive by any exterior fign, influence, however, the strata of air in which we make our experiments, produce those changes of which we only fee the refult, without being able to affign either the caufe, or it's relation. Thus, in ftormy weather, we fee the electricity ftrong, then null, and in a moment after arife to it's former force; one inflant vitreous, the next refinous; without being able to affign any reason for thefe changes. Mr. de Sauffure fays, that he has feen thefe changes fucceed with fuch rapidity, that he had not time to note them down.

When rain falls without a ftorm, thefe changes are not fo fudden; they are, however, very irregular, particularly with refpect to the intensity of force; the quality thereof is more conftant. Rain or fnow almoft uniformly give vitreous electricity.

The ftate of the air, in which the electricity is ftrongeft, is foggy weather; this is always accompanied with electricity, except when the fog is going to refolve into rain.

and thofe

The most interefting obfervation which throw the greatest light upo modifications of electricity in our att those that are made in ferene weather. (during which most of Mr. de Sauffure, tions were made) and ferene weather, t

tricity was generall

the dew had

rifing; it's i

Liter

various

gre, are

nan evening,

inter,

tent of the
ited by

grees, fometimes fooner, and fometimes later; but generally before noon it attained a certain maximum, from whence it again declined, till the fall of the dew, when it would be fometimes ftronger than it had been during the whole day; after which, it would again gradually diminish during the whole night; but is never quite deftroyed, if the weather is perfectly ferene.

Atmospherical ele&ricity feems, therefore, like the fea, to be fubject to a flux and reflux, which caufes it to increafe and diminish twice in 24 hours. The moments of it's greatest force are some hours after the rifing and fetting of the fun; thofe when it is weakest, precede the rifing and fetting thereof.

The electricity of ferene weather is much weaker in fummer than in winter, which renders it more difficult to obferve thefe gradations in fummer than in winter; befides a variety of accidental caufes, which at the fame time render them more uncertain. In general, in fummer, if the ground has been dry for fome days, and the air is dry also, the electricity generally increases, from the rifing of the fun till 3 or 4 in the afternoon, when it is ftrongeft; it then diminishes till the dew begins to fall, which again reanimates it; though after this it declines, and is almoft extinguished during the night.

But the ferene days that fucceed rainy weather in fummer, generally exhibit the fame diurnal periods or ftates of electricity, as are to be obferved in winter.

ON PROGNOSTIC SIGNS OF THE WEATHER.

There is no part of meteorology which interests mankind fo much, as the predictions it furnifies of the change of weather. The theory of it only

engages

engages the attention, by animating us with the hopes of thereby bringing the knowledge of thefe predictions to perfection.

And the far greater part of those who purchafe meteorological inftruments, buy them, not fo much to know the actual ftate of the elements, as to foresee the changes thereof. This fcience is, however, very imperfect; for it is but of late years that we began to make observations on the changes of the weather; and that it's progrefs has been rapid and fuccessful, may be seen in the works of De Luc, De Sauffure, Jones, Marshall, and Kirwan. But thefe obfervations will be ftill more valuable to pofterity; for we can scarce expect them in fufficient number in our own age, to deduce from them a general and perfect theory.

To attain this end, it will be neceffary to multiply obfervations on as great a number of figns as poffible; for it is only by their combination and concurrence that we can expect to remove the uncertainty infeparable from each in itfelf. Thus the barometer is not always a certain fign; the fame may be faid of the thermometer, the hygrometer, and the action of winds. But if they all concur together, there is but little chance of being deceived; and there would be ftill lefs, if to these were joined other figns, which are easy to observe, and which by their combination would render our prediction certain.

No fign, nor any inftrument of obfervation, fhould therefore be neglected, either from a love of ideal perfection, or fears of inaccuracy. Thus, though the hygrometer be at prefent a very imperfect inftrument, yet one certain fign has already been obtained from it's indications, and more may be reasonably expected. Even the words very dry, very moift, moderately dry, moderately moift,

though

though of vague determination, may throw much light on the state of the atmosphere.

It is neceffary that the obferver should enter into a precife detail of the various states of the sky, and the clouds. What can we learn from the words covered, and cloudy, or half covered fky, &c.? Nothing, fince it is well known, that a covered fky, in one cafe, is almoft as certain an indication of fine weather, as in another, it is an indubitable prefage of rain. The accurate obferver piques himfelf on a thermometer, with which he can obferve within a degree, and a barometer that he can depend upon to lefs than the roodth of an inch; but is filent on the tranfparency of the air, on dews, on the elevation, the form, the fign, the difpofition, the colour, and the denfity of the clouds; things that may be observed with cafe, and described without trouble; being attended with no other inconvenience than that of extending the fize of our meteorological tables.

There is a phenomenon, which has not been fufficiently attended to, namely, the undulating motion of the firmament, or that diurnal tumult in the air, which is kept up by the heat of the fun. What the fun raifes from the earth by the heat of the day, is fuftained in the atmosphere by it's heat, and the agitation, or expanfive undulation of the air. This motion is often vifible to the naked eye, but in the field of a powerful telefcope it is very confpicuous; all objects appear in violent agitation, and the line of the fenfible horizon, which ought to be clear and well defined, is waved like a field of corn in the wind, or the furface of the fea in a ftorm. long as this agitation continues, the vapours ftay in the air; but when it fubfides, and the fun departs, they are condenfed, and fall down to the earth in the night as dew.

So

In the prefent ftate of this part of science, when we are unacquainted with fo many phenomena, and ftill more ignorant of their caufes, general rules will often be found to fail, and particular ones will, without much circumfpection, prove to be a fource of error. Amongst the variety of means for predicting the changes of the weather, the barometer is undoubtedly one of the beft; and is in this, as well as many other refpects, one of the greatest acquifitions to natural philofophy.

The ufual ranges of the mercurial column, in this latitude, are comprized between 28 and 31 inches; of which the middle, or 29, is confidered as the variable: I think it fhould be placed fomewhat higher. Near the pole, the variations of the barometer are much greater.

OF PROGNOSTICS BY THE BAROMETER.

Ever fince the barometer has been invented, philofophers have endeavoured to account for the variations in the height of a local barometer, but hitherto in vain. Mr. de Luc, in the first volume of his "Recherches fur les Modifications de l'Atmofphere," has given a critical and very interefting account of the various phyfical opinions that have been invented for this purpofe by Pafcal, Beale, Wallis, Garden, Halley, &c. &c. and fhewn that they are all imperfect, and inadequate to the folution of the phenomena. He then propofes one of his own; which, with that candour that ever diftinguishes a lover of truth, he has fince abandoned. To give a particular account of the various hypothefes, would occupy a volume, and that to little purpofe. As I know of none that can be depended on, I fhall content myfelf with only relating the bare phenomena. The two great fources of error on this fubject have been, ift, the difficulty