METEOROLOGY. 



149 



(Vnt 'gride 



qnisite that the number in each be known, though it is cer- 

 tainly desirable that a uniform scale should be adopted. 

 Accnrdirig to the division generally used in this 

 country, the space between the free/ing and fouling 

 points consists of 180 degrees. Fahrenheit, who first 

 employed this scale, and whose name it still bears, 

 imagined that the greatest cold that could be produced, 

 is that which results from a mixture of snow and sea- 

 salt, and he adopted tliat tem|erature as the com- 

 mencement of the scale, which he accordingly marked 

 zero, or o. He then exposed the instrument to the 

 beat of boiling mercurv, and having marked tin- point 

 to which it rose, he divided the .veen that 



point and the lower extremity into ()OO equal parts. 

 He afterwards found that the melting point of snow 

 corresponded with the 3-2<\ divi-ion, and the I 

 point of water with the il^th. Hence, in what are 

 called Fahrenheit's thermometer*, the freezing |Miint is 

 Si, and tli ditl'.-rence ! 



them being I .->>. I \,<- mode of graduating the scale 

 originally adopted by Fahrenheit, h.i.s fot-n long ago 

 abandoned for th.it stated above ; but the . 



it-eil. tilou;.-il il il>i,- to TM ii , ' i ; !-, i- -llil rrt.lillcd. 



and it i* to this scale tliat all our references in this ar- 

 ticle are made. This graduation is represented on the 



\> of the scale, 



Among the various thennometrical scales that have 

 at ditierent times been proposed, that wli 

 generally used in France, and known by the name of 

 the centigrade, is perhaps upon the whole the most eli- 

 gible . -tance between the freezing and boiling 

 .point* is divided into 100 equal parts, the freezing 

 point being marked 0, and the folding point 1 00. \\ c 

 must refer to our article TIIEHMO r a particu- 

 lar account of the methods In indications of 

 different thermometers may be co 

 here remark, that when any giv 

 pressed in degrees of the centi) 

 verted into those of Fahrenheit, 

 pie rule. Multiply the degrees c 

 divide the product by 5, and ad 

 Thus when the centigrade ctandi 

 the same circumstances, will indi 



by 9, and divided by 5, give- Ii added to 



32 i- . divisio v thermometer are 



numbered from zero downwards, in the same way as 

 from aero upward* ; but to distinguish them, the far- 

 mer are written with the negative sign or i 

 fixed. '1 hiii 1 ~> means a temperature 15 degrees be- 

 low 0. The degrees, according to this graduation, are 



d of al> 



air. r<*- 



spsjnti ; but we may 

 n temperature is ex- 

 rade, it may be con- 



- 



Fahrenheit, in 

 nte 77, for .'.". imilti- 



f**r*f*. P ec * < to mercury, is on certain 

 with'sJeo** *^ indeed necessary, as for exa 

 bol imtrarf ** nl P er *ui'e below the point 

 if BIIUUI i. ' n *" alcohol thermometer, the f 

 ascertained by plunging it into m 

 above, but the other divisions m 

 comparing it with a mercurial on 



m swruiiimg 

 mercury freezes, 

 ng point may be 

 | snow, as stated 



.1 by 



. . nriously graduat- 

 ed, as alcohol passes into a state of vapour long before 

 it arrives at the heat of boiling 

 rial thermometers are frequently graduated in this way, 

 the tube in many of them fo-ing too short to a<! 

 the instrument being exposed to so elevated a tempera- 

 ture. 



rrgt*. In meteorological observations, it has been found 

 ih*r- very desirable to have some method of ascertaining the 

 "" greatest degree of heat and cold, during any given in- 



terval, in the absence of the observer. Various con- Mcteorolo- 

 trivances have accordingly been employed for accom- *? 

 pli-hing Uiis, by means of what are called self-regis- p~ A T ~ 

 tering thermometers. Those commonly in use are of a CCCL x X1T . 

 MTV simple construction. A mercurial thermometer, Fig. 8. 

 A 15, (Fig. 2.) of a somewhat wider bore than ordinary, 

 with a small bit of steel wire a over the mercury, so as 

 to slide easily up and down in the tube, is placed in a 

 horizontal position. As the temperature increases, the 

 mercury pushes forward the bit of steel or index ; but 

 when the mercury again retires, in consequence of a 

 diminution of temperature, the index remains behind 

 at the highest point to which the mercury has risen. 

 A thermometer AlJ (Fig. 3.) of a similar bore, but fill- Fig- * 

 ed with spirit of wine, and having a small thread of 

 glass a, about half an inch in length, immersed in the 

 fluid, is also placed in a hori/.ontal position. As the 

 temperature diminishes, and the spirit sinks in the 

 tube, it is found that the surface of the liquid does not 

 pass the glass index, but carries it along with it, 

 though when an increase of temperature again takes 

 e, nnd the spirit rises in the tube, the index is 

 behind at the lowest point to which the liquid had 

 sunk. To prepare the instruments for new obser- 

 vation, they are both inclined so as to bring the index 

 in each to the surface of the liquid, when they are 

 again placed in a horizontal position. In referring to 

 these instruments in the course of this article, we shall 

 denominate the first a maximum, and the last a minimum 

 thermometer. For a more particular account of the 

 mi-thud of constructing these, and others of the same 

 kind, see the article THERMOMETER. 



In ascertaining the temperature of the atmosphere, Sourrei of 

 by meant of the thermometer, it is of great importance fallacy to 

 to attend to the situation of the instrument. It ought '* guarded 

 to be placed so as to be in the shade, not only at the """' '" 



time of observation, but during the whole day, and at .".P.J" 1 ' 1 



. . *' theinermo- 



.1 distance trom surrounding objects, as not to be m etcr to 



affected by any changes in their temperature. \Ve mctcorolo- 

 have repeatedly found that the reflection of the sun's glcl pur- 

 rays from an opposite wall, though at the distance of P K> - 

 50 feet, ami 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 ol the bar, from be- 

 ing more exposed to the iky than the other, has Home- 

 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 ii accidentally 

 wetted, it ought to be wiped dry before making an ob- 

 servation, otherwise it will indicate a lower tempera- 

 ture than it ought to !. from the influence of evapo- 

 ration. In consequence of not attending to i 

 sources of fallacy, we stuped that many of the meteo- 

 rological tables that have been published, cannot be re- 

 lied on as affording an accurate view of the tempera- 

 ture of the atmosphere. 



The narometrr, we have already stated, is sn instru- Of the bs- 

 ment employed for measuring the changes that take rorccter. 

 place in the weight of the atmo-phcre. The princ pie 

 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, 



