1' XK I'M A TICS. 



M7 



rales, and also the instruction* drawn op by authority of tho 

 <>f Trade, and utmally engraved on seamen'* barometer*. 



fulW mil-* Hhutild i-rocure a iinall paui|.hl.-t 

 oallutl i in- " Itaromotor Manual," compiled by the late Admiral 

 y tor the Board of Trade : 



The I'. northerly Tho barometer fall* for southerly 



wni'i Inoludlag from north-we*t by wiu<l(iu . iih-wut by 



ih to the eastward), fur dry tho noutli to DID wuntward), (or wot 



or lea* wet, (or less wind, or (or 



m one of these change*; 

 Sxotpt OD a few ooouions when 

 now come* f rum tho north- 

 .>ifj wiud. 



weather, (or stronger wiud, <>r fur 

 more than one of the** change* ; 



Xxcipt on a (ew ooouions when 

 nMxUraU wind with rain or snow 

 comes from the northward. 



i omoter, it ahould be borne in mind, show* what weather 



may be expected, and not what is present. A rising column 



usually indicates fair weather ; a falling column, foul. In both 



oases, the more gradual the rise or fall, tho longer continuance 



weather may be looked for. 



!'!> ([tiuut fluctuations in the height are usually a sign of 

 unsettled and changeable weather. A rapid rise also indicate* 

 unsettled weather, while a rapid fall frequently presage* a storm. 



A high south-west wind nearly always causes the mercury to 

 stand very low, even if no rain ensue ; a north-east wiud pro- 

 diuvH the contrary effect. 



Tiioro are two corrections which have to be introduced into 

 the readings of barometers if we would compare them with 

 those of others at different places. The first is a correction for 

 the height above the level of tho sea at half tide. This is taken 

 as the standard height ; and it is clear that if tho barometer is 

 raised above this level, a portion of tho atmosphere is left below 

 it, and the pressure is therefore less by this amount. 



It is found that an elevation of ninety feet causes a diminu- 

 tion in the height of the column of about 1*5 of an inch, and 

 this amount must therefore be added to the observed reading. 

 This fact in itself shows how worthless tho words on a weather- 

 glass are, for in a valley it may stand at "fair," while on an 

 elevation near by it points to " rain." The fact of its being on 

 the ground floor or at the top storey of a house is quite sufficient 

 to alter the reading if the barometer bo a good one. 



The second correction that has to be made is for temperature. 

 Mercury is very sensitive to heat, and expands considerably by 

 it, thereby becoming lighter ; a longer column will, therefore, be 

 sustained by the pressure of the air. The difference caused by 

 this is found to be about rib of an inch for every 10 P above 

 freezing-point, or 32, and this amount must therefore be de- 

 ducted from the observed reading. 



Apart from the irregular fluctuations which we have been con- 

 sidering, there is a regular daily variation arising from tides in 

 the atmosphere, for the aerial ocean, like the aqueous, has its 

 periods of ebb and flow. The tides, however, are solar tides, 

 being apparently caused rather by the heat of the sun than by 

 its attraction of the air, and hence they occur at a regular time 

 every day, and are always of one uniform height, the phenomena 

 of spring and neap tides not being observed. 



In temperate climates this fluctuation is but little noticed, as 

 it only amounts to yfoj or T&JI of *n inch, and is therefore hidden 

 by the larger fluctuations before mentioned ; if, however, we 

 take the average of a continued series of hourly observations, 

 we shall find that the daily maximum height is attained about 

 nine o'clock in the morning, and the same hour in the evening, 

 while the time of least elevation is about three o'clock. 



In tropical regions the accidental variations of the barometer 

 are very slight, as the wind and the moisture there are not sub- 

 ject to the variations they exhibit in temperate climates. Its 

 fluctuations, therefore, occur with the utmost regularity, and 

 range over about ft of an inch ; and it is of little use there in 

 foretelling the weather, except at rare intervals, when a sudden 

 and great fall of the mercury is the precursor of a terrible 

 storm or hurricane. 



In many places daily registers are kept of the variations in 

 the barometer, the wind, and the temperature, and careful 

 observers are needed in other places, as, by comparison of such 

 tables, many important meteorological questions may not im- 

 probably be solved. The height at noon is usually about the 

 average of that during the day ; if, therefore, only one observa- 

 tion can be taken, it should be at that hour ; but if two observa- 

 tions can be taken, it is better, and then nine and three are 

 about the most favourable times. 



In observatories where it U required to keep a record of thece . 

 variation*, a *elf-regi*tering apparatus U employed. This i* 

 usually adapted to a wheel barometer, the float Mag attaehed ' 

 to the thorter end of a light and well-balanced lerer. The 

 longer end of thu *omtime* oarriee a pointer, which i* caused 

 by clockwork to prick a hole at regular interval* in a ruled card, 

 moved *low!y under the point. The line joining thane mark* 

 how* the change* in the height of the colnmn. 



Most mechanical arrangement* for registering reading* Are, 

 howerer, liable to get oat of order, and have the disadvantage 

 of marking tho height at utatod interval* only, instead of giving 

 a continual line, and thut many minor variation* are loci. The 

 aid of photography ha*, therefore, been called in, and tbi* diffi- 

 culty obviated. The barometer i* placed in a darkened room, 

 and a small mirror i* *o arranged that it U turned by the ruing 

 and falling of tho column. A imall ray of light ic admitted by 

 an opening in a shutter, or, a* i* more commonly the ca*e, the 

 light of a powerful lamp i* concentrated by a lens, and the ray 

 of light from this falls on tho mirror. A *heet of sensitised paper 

 i* BO placed a* to receive the reflected ray, and a* thu paper i* 

 slowly and evenly moved on by mean* of clockwork, a faint 

 trace is left on it, which accurately shown every variation, bow- 

 ever alight, in the height of a column. The trace on the paper 

 can afterwards be developed and fixed, so that it may be kepi 

 as a permanent register of the varying pressure of the air. A* 

 the ray answers tho purpose of a long lever without weight, 

 variation* of ^ of an inch may be distinctly recorded in thi* 

 way. 



Having now seen sot lething of the way in which the baro- 

 meter is used as a weather-glass, we must just glance at the other 

 important use to which it is applied, viz., a* a mean* of ascer- 

 taining the elevations of mountains or other eminence*. This 

 was the first use to which it was put, and it* indication* in 

 this respect are more to be depended on than in foretelling the 

 weather. Pascal's celebrated experiment showed the principle, 

 though he did not understand the mode of calculation. 



If the atmosphere were of equal density throughout, there 

 would be no difficulty whatever in the matter, for then the 

 diminution in the length of the colnmn of mercury supported 

 would be exactly proportional to the elevation attained, and a* 

 we have already seen that an elevation of ninety feet cause* a 

 fall of about ^ of an inch, we should merely have to allow 900 

 feet for every inch, and should thus ascertain the elevation. 



This, however, is not the case, for every portion of the air is 

 compressed by the weight of that above it, and thus the density 

 diminishes rapidly as we ascend. It is usually reckoned that 

 one-half of the entire atmosphere is passed at an elevation of a 

 little less than three miles, though its extreme limit is supposed 

 to be at an elevation of not less than forty-five or fifty miles, 

 and perhaps more. 



The exact calculation of height by means of tho barometer i* 

 a rather complicated process, and involves higher branches of 

 mathematics than we can well introduce here : the following 

 simple rule will, however, give a very close approximation to 

 the true height, especially if it be not greater than 5,000 or 

 6,000 feet : 



Ascertain the height at which the barometer stand* at the 

 level from which the measurement is to be taken, and also note 

 the reading at the highest point attained, the height will then 

 be shown by the following rule of three sum : As the sum of 

 the two readings is to their difference, so 52,000 feet is to the 

 height required. Suppose, for example, that the barometer at 

 the level of the sea stands at 29'76 inches, and at the summit 

 of the mountain at 26-18, wo have the sum of the readings, or 

 29-76 -f- 26-18 = 55'94, and their difference = 3'58, the equation 

 will therefore stand thus : 



As 55-94 : 3-5 : : 52,000 : 3.327. 

 The height of the mountain is therefore 3,327 feet 



As, however, the temperature usually diminishes as we ascend, 

 this will interfere a little with the result. We must therefore 

 notice also the temperature at each place, and allow for the 

 difference as before, by deducting from the reading at the level 

 where the temperature is higher j^ of an inch for every ten de- 

 grees difference in temperature. 



If the weather be at all variable, or the ascent occupy a little 

 time, it is far better to have another observer at the level of the 

 ground, and thus let two simultaneous observations be taken, 



as all risk of variation is thus avoided. 



* 



