In ascertaining the temperature of the atmosphere, by means of the thermometer, it is of great importance to attend to the situation of the instrument. It ought to be placed so as to be in the shade, not only at the time of observation, but during the whole day, and at such a distance from surrounding objects, as not to be affected by any changes in their temperature. We have repeatedly found that the reflection of the sun's rays from an opposite wall, though at the distance of 50 feet, and where there was a free circulation of air, has raised the thermometer several degrees higher than the actual temperature of the atmosphere. We have also found, that of two thermometers, graduated with the greatest care, and separated only by a thin bar of wood, the one on the upper side of the bar, from be ing more exposed to the sky than the other, has some times, during a clear night, stood several degrees lower. In meteorological observations, therefore, the thermometer should be screened from the sky, as well as from the reflection of the sun's rays. The bulb too should be exposed naked ; and if it is accidentally wetted, it ought to be wiped dry before making an ob servation, otherwise it will indicate a lower temperature than it ought to do, from the influence of evaporation. In consequence of not attending to these sources of fallacy, we suspect that many of the meteorological ta bles that have been published, cannot be relied on as affording an accurate view of the temperature of the atmosphere.
The Barometer, we have already stated, is an instru ment employed for measuring the changes that take place in the weight of the atmosphere. The principle upon which it is constructed may be explained by a very familiar example. If a tube, open at both ends, with a piston exactly fitted to it, have its lower extre mity, to which the piston has been previously pushed down, immersed in water, on drawing up the piston, the water will follow it, and continue to do so till it rises to the height of about 33 feet. The same thing will take place if the tube be immersed in mercury, with this difference, that the mercury will ascend only to about the height of SO inches. The effect in both
cases is produced by the weight of the atmosphere. By drawing up the piston, the pressure of that weight is removed from the portion of the fluid immediately under it, while it continues to be exerted on the sur face of all the rest. The liquid is therefore forced up into the tube, till the column be of such a height as to balance the weight of the atmosphere. If a glass tube of Convenient length, and open at one end, be filled with mercury, and then inverted perpendicularly into a basin of mercury, so that the open end may be under the surface, the mercury will sink down from the upper extremity, and stand at the point to which it would have risen, had a piston been employed as stated above. In this form it constitutes a barometer, and the changes of weight which the atmosphere un dergoes, are indicated by the rising and falling of the mercury in the tube. To measure these variations, a scale may be placed parallel with the tube, and divid ed into inches and decimals, beginning at the surface of the mercury in the basin. It is obvious, however, that if the scale be fixed when the mercury sinks in the tube, the surface ol that in the basin must rise in proportion to the relative width of the basin and the tube, and consequently stand higher than the com mencement of the scale ; and in like manner, when the mercury rises in the tube, the surface ol that in the basin must sink, and stand lower than the beginning ol the scale. In the one case,therefor e, the mercury in the tube will stand at a higher, and in the other at a lower point than it ought to do. This inaccuracy is in part removed, by making the basin very wide re latively to the tube, but still more effectually by con structing it in such a manner that the surlace of the mercury in the basin may be raised or depressed to the commencement of the scale. In general a portion only ol the scale is divided, viz. from the twenty-seventh to the thirty-fast inch, reckoning from the surface of the mercury in the basin, because these are found to be the ordinary limits of the barometrical range. See BARO.