JOURNAL OF HORTICULTUEE AND COTTAGE GARDENEE. 



[ January 4, 187: 



or unbroken Viy lofty eminences, the chemical nature and colour 

 of the soil, inrtuent-e the result. Subject,"_however, to nu- 

 meroua' fluctuations, we, nevertheless, observejithat there are 

 two stated periods of the day when the temperature reaches a 

 maximum and minimum. The warmest period is generally 

 from two to three hours after the sun 

 has passed the meridian, the difference 

 depending upon season ; the coldest is 

 nearly an hour before sunrise. A mean 

 may be conveniently obtained by re- 

 cording the oscillations of the ther- 

 mometer at suitable intervals, and 

 thus the toils of hourly observation 

 dispensed with. Thus the sum of the 

 indications at 6 .i.m., 2 and 10 p.m., 

 or as suggested by Schouw, at 7 a.m., 

 noon, and 10 p.m., divided by 3, will 

 give a near approximation to the di- 

 urnal temperature. From observations 

 made by Professor Dewey, in North 

 America, it would appear that the 

 result obtained at the homonvmous 

 hours of 10 A.M. and 10 p.m. afforded 

 an approximation of 5-lOOths of a 

 degree to the true mean diurnal tem- 

 jjerature. Though the maximum tem- 

 peratiu'e occurs some time after the 

 sun has culminated, we find that proxi- 

 mity to coasts accelerates the epoch ; 

 and in tropical regions the sea-breeze 

 still farther modifies the result. The 

 same has been found on mountain 

 tops ; nor is it at all remarkable : for, 

 let it be observed, that in the plain, 

 not onl.y is the atmosphere warmed by 

 direct solar rays, but by those radiated 

 from the ground ; upon the mountain 

 top there will be a greater absorption 

 of caloric and less radiation, more 

 heat being abstracted by 

 conduction there than below, 

 where the temperature of 

 the earth is higher and more 

 equaUy diffused. The tem- 

 perature, then, upon those 

 lofty spots will depend 

 chiefly upon the sun's direct 

 calorific action ; and as 

 those rays are most 

 powerful wliich pierce 

 the thinnest atmo 

 sphere — i.e. , when the 

 sun is on the men 

 ,dian, the hottest 



certain height from the gi'ound ; they should all face the north 

 (except those which are required to register the maximum 

 temperature in the sunshine, which should face the south) ; 

 they should not be influenced by radiated heat, by currents, 

 by reflected heat, such as is communicated from an opposite 

 wall, by absorption of heat from the 

 wall or woodwork to which the ther- 

 mometer is attached ; and, in short, 

 they should be situated where there is 

 a free passage of air. 



" With regard to the height that a 

 thermometer is placed above the 

 ground, it is ascertained that an inch 

 occasionally makes a difference of a 

 few degrees. Thermometers placed at 

 various heights do not vary much from 

 each other in cloudy weather ; but 

 mth a cloudless sky the difference is 

 very great ; as an illustration we shall 

 (juote an example — viz., March 13th, 

 1845, 9h. 10m. p.m., a thermometer on 

 the grass had fallen to 4..5°, 2 feet 

 above the grass it stood 15°, and 

 7 feet above the grass at 18.5°. Here 

 is a difference of lOJ', or three times 

 as much between the temperature of 

 the grass and 2 feet above it ; and be- 

 tween the grass and 7 feet, of 14°, or 

 tour times as much. 



" With respect to the aspect of ther- 

 mometers, we shall give an instance 

 which occurred at the same time as 

 the last observation. A thermometer 

 placed 14 feet above the ground, with 

 a south-east aspect was 15°, with a 

 north aspect 17°, with a north-east 

 aspect 18°, and with a south-west 

 aspect 19°. 



" It cannot be expected that ther- 

 mometers, placed as they 

 generally are, may be de- 

 pended upon, some facing 

 the north, others the south ; 

 some the north-east, others 

 the north-west ; some 3 feet 

 to 5 feet above the ground, 

 others 10 feet to 20 feet ; 

 some sheltered by a high 

 wall, others by low 

 palings ; some touch- 

 ing a wall, others dis- 

 tant from it ; and 

 some in the angle of 



period of the day will b; at, or very soon after, culmination. — 

 \Etements nf Meteorology.) 



To obtain accurate comparative observations and records of 

 the maxima and minima temperatures daily, requires not only 

 that the thermometers should be accurately and similarly gi'a- 

 duated, but that they should be similarly exposed and similarly 

 protected in every respect. 



Mr. E. J. Lowe published some years smce, in the " Gar- 

 deners' Magazine of Botany," the foUowiug judicious du-ections 

 upon this subject ; — 



" Thermometers for comparison should fll be placed at a 



a high building (cool as a cellar) , and others exposed to the 

 rays of the sun at one or other hour during the day. 



" ' The Lawson Meteorological Thermometer Stand,' wliieh 

 we shaU descrilie, is so arranged that it may be placed in any 

 ehgible situation ; it commands a true north and south aspect ; 

 the instruments can be read off with the greatest facUity, and 

 they win be at a known distance from the ground ; the instru- 

 ments on the south face will have the meridian sun, and those 

 on the north face will always be in the shade. The instrument 

 is not costly, and were this stand universally adopted, ob- 

 servations, whether recorded here or at the antipodes, could 



