356 



SCIENCE OF AGRICULTURE. 



Part II. 



absence of light is necessary to tKe formation of sugar in the germination of seeds ; and 

 its presence to the production of sugar in fruits. The following is the late Dr. Murray's 

 ingenious explanation of these remarkable facts. The seed consists chiefly of farinaceous 

 matter, which requires oxygen to convert it into sugar. Now living vegetables appear to 

 absorb oxygen in the dark : unripe fruits usually contain an acid, that is, have an excess 

 of oxygen ; and light is favourable to the evolution of oxygen from living plants. (J*.) 



2345. Those changes in the atmosphere which constitute the most importa^it meteorological 

 phenomena may be classed under five distinct heads ; the alterations that occur in the 

 weight of the atmosphere ; those that take place in its temperature ; the changes produced 

 in its quantity by evaporation and rain ; the excessive agitation to which it is frequently 

 subject ; and the phenomena arising from electric and other causes, which at particular 

 times occasion or attend the precipitations and agitations alluded to. All the above 

 phenomena prove to demonstration that constant changes take place, the consequences of 

 new combinations and decompositions rapidly following each other. 



2346. With respect to the changes in the weight of the atmosphere, it is generally known 

 that the instrument called the barometer shows the weight of a body of air immediately 

 above it, extending to the extreme boundary of the atmosphere, and the base of which is 

 equal to that of the mercury contained within it. As the level of the sea is the lowest 

 point of observation, the column of air over a barometer placed at that level is the longest 

 that can be obtained. 



2347. The variations of the barometer between the tropics are very trifling ; they increase gradually as 

 the latitude advances towards the poles, till in the end it amounts to two or three inches. The following 

 Table will explain this gradual increase: 



2348. The range of the barometer is considerably 

 less in North America than in the corresponding 

 latitudes of Europe, particularly in Virginia, where 

 it never exceeds Tl. The range is more considerable 

 at the level of the sea than on mountains; and in the 

 same degree of latitude it is in the inverse ratio of 

 the height of the place above the level of the sea 

 Cotte composed a table, which has been published in 

 the Journal de Physique, from which it appears 

 extremely probable, that the barometer has an in- 

 variable tendency to rise between the morning and 

 the evening, and that this impulse is most con- 

 siderable from two in the afternoon till nine at night, 

 when the greatest elevation is accomplished ; but the 

 elevation at nine differs from that at two by four twelfths, while that of two varies from the elevation of 

 the morning only by one twelfth, and that in particular climates the greatest elevation is at two o'clock. 

 The observations of Cotte confirm those of Luke Howard; and from them it is concluded, that the 

 barometer is influenced by some depressing cause at new and full moon, and that some other makes it 

 rise at the quarters. This coincidence is most considerable in fair and calm weather ; the depression in 

 the interval between the quarters and conjunctions amounts to one tenth of an inch, and the rise from 

 the conjunctions to the quarters is to the same amount. The range of this instrument is found to be 

 greater in winter than in summer ; for instance, the mean at York, during the months from October to 

 March inclusive, in the year 1774, was 1-42, and in the six summer months r016. 



2349. The more serene and settled the weather, the higher the barometer ranges : calm weather, with a 

 tendency to rain, depresses it ; high winds have a similar eflfect on it ; and the greatest elevation occurs 

 with easterly and northerly winds ; but the south produces a directly contrary effect. 



2350. The variations in the temperature of the air in any particular place, exclusive of 

 the differences of seasons and climates, are very considerable. These changes cannot be 

 produced by heat derived from the sun, as its rays concentrated have no kind of effect on 

 air ; these, however, heat the surface of our globe, from which heat is communicated to 

 the immediate atmosphere ; it is through this fact that the temperature is highest where 

 the place is so situated as to receive with most effect the rays of the sun, and that it 

 varies in each region with the season ; it is also the cause why it decreases in proportion 

 to the height of the air above the surface of the earth. The most perpendicular rays 

 falling on the globe at the equator, there its heat is the greatest, and that heat decreases 

 gradually to the poles, of course the temperature of the air is in exact unison ; from this 

 it appears that the air acquires the greatest degree of warmth at the equator, whence it 

 becomes insensibly cooler till we arrive at the poles; in the same manner the air 

 immediately above the equator cools gradually. Though the temperature sinks as it 

 approaches the pole, and is highest at the equator, yet as it varies continually vdth the 

 seasons, it is impossible to form an accurate idea of the progression without forming a 

 mean temperature for a year, from that of the temperature of every degree of latitude for 

 every day of the year, which may be accomplished by adding together the whole of the 

 observations and dividing by their number, when the quotient will be the mean tem- 

 perature for the year. The " diminution," says Dr. Thomson, " from the pole to the 

 equator takes place in arithmetical progression ; or to speak more properly, the annual 

 temperature of all the latitudes are arithmetical means between the mean annual tem- 

 perature of the equator and the pole ; and, as far as heat depends on the action of solar 

 rays, that of each month is as the mean altitude of the sun, or rather as the sine of the 

 sun's altitude. Later observations, however, have shown that all the formulae for cal- 

 culating the mean temperatures of different latitudes, which are founded on Mayer's 



