11 -v, 



METEOROLOGY. 



[EFFECTS op HF.AT. 



surfaces of mercury are there appai 

 OIM at A, another at 15. Now it is evident that the 

 columnar interference at A will be exactly equal to 

 Uie columnar interference at B, j-r.'M i. <1 the m'mlar 

 diameter be equal thruughout. II. i,..-. \\ii.-ilii-r the 

 degree of elevation be counted from the summit of 

 the lower to the summit of the upper meniscus, or 

 from the base line joining the two corners of the lower 

 meniscus to the base line joining the two corners of tho 

 up|<vr one, the two resulting columnar estimations 

 will be strictly equal and comparable. But it is 

 different when the observer has to do with barom- 

 eters constructed on the original, or Toriccllian, plan 

 of a separate reservoir ; in this case the meniscoid eleva- 

 tion of the mercury in the reservoir is practically 

 ignored, being so inconsiderable that it amounts practi- 

 cally to nothing, as will Fig. 19. 

 be seen by reference to 

 the diagram (Fig. 15). 



The following is a small 

 table of depressions due 

 to capillarity. Larger 

 and more elaborate 

 tables have been calcu- 

 lated ; but the one given 

 will suffice for, perhaps, 

 every occasion. The 

 meteorological student 

 cannot have the fact too 

 strongly impressed upon 

 him, however, that the 

 barometer is confessedly 



a very imperfect instru- ~^B^^^^H 



meut ; therefore correct 

 results from barometric 

 observation!* are to be looked for only as the mean 

 resultant of a number of accumulated observations. 



DEPRESSION DUE TO CAPILLARY ACTION. 



THE ANEROID BAROMETKU Of comparatively recent 

 date is the invention of the Aneroid barometer. It 

 consists of two discs of metal, accurately joined to- 

 gether, so as to enclose a space, from which the air is 

 removed, and a vacuum produced. An increase or 

 decrease of atmospheric pressure will cause the contrac- 

 tion or expansion of the sides of this vessel ; and by 

 means of tooth-wheels and rack-work, an index is thus 

 moved on the face of the instrument, so as to indicate 

 the pressure on a circular plate, divided into inches, <tc., 

 corresponding with the gauge of the mercurial barometer. 

 The "aneroid" has been so greatly improved, that, 

 according to Mr. Olaisher, a good one may be consi'i 

 correct at a pressure so low as that equal to five inches 

 of mercury. (See Chapter V ) 



THERMAL EXPANSION. We will now examine the 

 function of thermal expansion, which not only intimately 

 concerns the barometer, but is the fundamental basis of 

 the thermometer ; and besides, it enters as an element 

 into no many meteorologic calculations, that a thorough 

 investigation of it* laws cannot be omitted. We shall, 



therefore, embody the laws of tli-rnvil expansion in a 

 few propositions for successive demonstration. 



Heat may be regarded in tliu two senses : one signify- 

 ing temperature, or that sort of heat which is recog- 

 nisable to the sense of touch, and which utlucts the 

 mometer; and heat, which is devoid of these manifesta- 

 tions, which neither creates the sensation of warmth, nr 

 is amenable to thennometric demonstration. Tim former 

 we may express by the term lentible heat ; and the second 

 by the tern. r insensible heat. 



On the supposition that all the functions of heat, sen- 

 sible as well as insensible, are referable to a real \>}\\ 

 agent, the term caloric has commonly been applied as the 

 representative of such agent ; but though the term be iu 

 general use, it is perhaps objectionable modern science 

 leading us to infer that the functions of heat are due to 

 a condition of matter, rather than to a separate agency. 

 It is to evident heat, recognisable to the touch, th:it we 

 shall now direct the reader's attention. 



I. Heat affects the Volume of all Bodies. The general 

 effect of heat on bodies is to cause their expansion. 

 So general is this rule, that we shall do well to consider 

 it as universal ; treating all deviations from it hereafter 

 as so many exceptions. 



II. The Volume of all Solids is Increased by Increase of 

 Heat. This proposition is demonstrated by so many in- 

 stances commonly occurring, that specific experiments 

 are hardly required. The wheelwright takes advantage 

 of this property to bind tightly together the wood-work 

 of his carriage-wheels. He heats the annular tire, by 

 which he expands it ; he then slips the tire over and 

 around the wood-work, and, allowing the tire to cool, the 

 wood-work is tightly braced together by an immense force. 



Some years since, the walls of the Conservatoire des 

 Arts et Metiers, at Paris, were found to be diverging 

 from the perpendicular, and were restored to their ori- 

 ginal lines by the following beautiful expedient : They 

 were perforated transversely ; copper bars were thrust 

 through the perforations, each bar at either extremity 

 being supplied with a nut and screw. Every alternate 

 bar was heated by means of a spirit-lamp flame ; being 

 heated, the bars expanded, and the screw-nuts being 

 now turned close up to the wall on either side, the bars 

 were allowed to cool. By cooling they contracted pulled 

 the walls to some extent together, leaving the ends of 

 the unheated bars protruding ; their screw-nuts were 

 now turned close up to fie wall on cither side, and the 

 heating process repeated. Thus, little by little, the walls 

 were restored to their original position. 



An exemplification of the expansion of iron by heat 

 sometimes occurs to the laundress. Occasionally she is 

 surprised to find that the heater of her Italian iron will 

 not enter its corresponding sheath when red-hot, though 

 it enters readily enough when cold. This is attributable, 



Fig. 16. 



as the student will perceive, to the effect of thermal ex- 

 pansion on the iron (Fig. 16). 



Thousands of familiar instances might be cited, aU 

 illustrative of the same property. We shall leave their 

 consideration to the reader, concluding the remarks on 

 this part of the subject by bringing before his 1 notice a 

 common lecture experiment, illustrative of solid thermal 

 expansion. Let G (Fig. 17) be a gauge, into which tho 

 metallic bar, A, accurately 

 fits, whilst cold ; it will be 

 found that when tho bar A 

 in In -a tod moderate heating 

 will suffice, such as may be 

 accomplished by means of 



Fig. 17. 

 A 



