METEOROLOGY. 



BO much jrrruUT than tho regular ones, that thU cannot be done 

 by a ingle observer. By means, however, of a staff of observers 

 scattered over Western Europe, and able to communicate by 

 telegraph, most storms can be predicted, and notice be sent 

 us ports, so as to warn sailors of their approach. 



A system of this kind was commenced and carried out by the 

 late Admiral Fitzroy, and though many errors crept in at first, 

 tin- majority of the warnings were correct, and the per-centage 

 -10 steadily increased. 



" places on or near the west coast were, of course, un- 

 favourably situated for receiving the warnings, since the storms 

 usually commenced in that quarter ; and, in the cose of a few 

 storms not foretold, the reason usually was that a constant 

 watch was not kept at Valentia, in the west of Ireland. The 

 great difficulty in this matter is not the foretelling of tho storm 

 itself, but the ascertaining in what direction it is travelling, and 

 thus only sending the warnings to those places where it is likely 

 to be felt. In more recent years the study of meteorology in 

 Great Britain, the United States, and foreign countries, has 

 wonderfully advanced, and storm warnings are now published 

 iu all the leading newspapers. 



A very important point in connection with the climate of any 

 place is the mean temperature of the air. This is ascertained 

 by reading the temperature indicated by a thermometer every 

 hour, and taking the mean of the twenty-four observations thus 

 made in the day. If we take the mean temperature at any two 

 hours of the same name, we arrive at a result differing only by 

 a fraction of a degree. This is most accurate if we take the 

 mean of 9 a.m. and 9 p.m., or of 10 a.m. and 10 p.m. The two 

 former are usually chosen as being most convenient ; when 

 practicable, four equidistant observations should be made. 

 Several years' observations must be compared to obtain the 

 true mean. 



Another important thing to notice 

 is the extreme temperatures registered 

 during tho day. These are very easily 

 noted, as thermometers are now made 

 which register tho maximum and mini- 

 mum temperature. The maximum ther- 

 mometer usually employed is a mercurial 

 one, with a small steel index moving in 

 the tube above the column. As the 



mercury expands, it drives this before it, and leaves it at the 

 highest point attained, the mercury not attracting the steel 

 enough to draw it bock again. 



In the minimum thermometer (Fig. 1), the tube is usually 

 filled with spirit, and a similar index is placed in it. As the 

 spirit contracts, it drags this index with it ; but when the tem- 

 perature rises again, the spirit passes it, and it is thus left 

 lying at the point indicating the lowest temperature reached. 

 Both these thermometers are set by bringing the index to the 

 top of the column, either by inclining the instrument, or by 

 means of a small magnet. They are read off and set at a regular 

 time each day, the highest and lowest temperature being entered 

 in a book provided for the purpose. If we take the mean of these 

 two readings, we shall find that it differs very slightly from the 

 mean daily temperature obtained as already explained. 



When daily records of temperature are kept, great care is 

 required to ensure that the thermometers employed are cor- 

 rectly graduated and properly placed. Many common thermo- 

 meters, when compared with a standard one, are found to be 

 very inaccurate. The instrument employed should therefore bo 

 carefully verified ; and most good instruments are now sent to a 



upon the mean temperature, for the range of temperature that 

 is the difference between the maximum and minimum readings 

 exert* a very important influence on it. Tbui, for instance, 

 Madrid, and Mentone on the Gulf of Genoa, were observed to 

 have the same mean temperature, viz. 72-8 daring September, 

 1865. The climates were, however, widely different, for, in the 

 case of the former, the means of the hottest and the coldest 

 periods of each day were 86'2, and &9'5 respectively. Those 

 at the latter place were 77-6 and 68*0 only. The importance 

 of observing this point is farther shown by the fact that th 

 rate of mortality is found to vary with the range. 



An insular climate usually has this range very limited in 

 extent. The specific heat of water being very high, it varies 

 but little in temperature through the year, and the changes that 

 do occur in it are very gradual. The result is that in localities 

 close to the sea, the hot winds are cooled and the cold ones 

 warmed by passing over the water, and thus the temperature is 

 much more uniform than in situationsfurther inland. 



The following list shows the difference between the extreme 

 temperatures recorded at a few places : 



Borne . 

 Copenhagen 



78 

 90 s 



Greenwich 



I iri.s . 



99 

 110 



Petersburg 



Moscow 



117 

 126 



J5) 



I I I I I I I rtct^t^ 

 10 20 80 40 60 60 70 80 90 WO 



In some localities the range is even greater than any of these. 



By taking a long series of observations at different places, the 

 mean temperature of each is ascertained, and lines can then bo 

 drawn on a map, passing through those places which have the 

 same mean annual temperature. These lines are called isothermal 

 lines. They were first noticed by Humboldt, and serve to show 

 the general distribution of temperature. On the sea they are 

 almost parallel, but on the land they are somewhat irregular, as 

 elevation above the sea-level greatly influences the temperature. 

 The line showing the highest mean 

 temperature, or, as it is usually called r 

 the warmth equator, is almost entirely 

 in the northern hemisphere, attaining 

 on the eastern side of the African 

 desert the latitude of 17 N. The 

 mean temperature of the place is 87 

 or 88. The fact of this isothermal 



Pig. l. being thus situated is owing to the 



great preponderance of the land in the 



northern hemisphere over that in the southern, the surface oi 

 the land absorbing the heat more readily than that of the sea. 



Besides the general causes which account for temperature, 

 there are local ones which exert a great influence on that of any 

 particular place. The most important of these are the alti- 

 tude above the sea-level, the direction of the prevailing winds, 

 and the proximity of the sea. 



In many places, the climate of which is remarkably salu- 

 brious, the mean temperature is often found to be scarcely 

 at all higher than that of the surrounding district, but very 

 often some local cause, such as a range of hills, affords a 

 shelter from cold or injurious winds. Ventnor, in the Under- 

 cliff of the Isle of Wight, is a good illustration of this fact, 

 and hence it is a favourite winter residence for invalids. Not 

 only is it sheltered from the cold east winds, but its proximity 

 to the sea raises its winter temperature several degrees. 



Forests likewise exert a considerable influence on climate and 

 mean temperature. Trees, like all other bodies, become greatly 

 heated by the sun's rays. They do not, however, acquire 

 their maximum temperature till a little after sunset, while the 

 maximum temperature of the air is attained about two or three 



Government Observatory for that purpose before they are sold, i o'clock. They also change their temperature much more slowly 



To ensure accurate readings, the instrument should be shielded 

 from the direct or reflected rays of the sun, but at the same 

 time be exposed to a free current of air. The minimum thermo- 

 meter should also be placed at a distance of four feet above the 

 surface of the ground, as the readings are otherwise considerably 

 affected by radiation from the earth. In a spirit thermometer 

 there is frequently a tendency for some of the spirit to condense 

 in the upper part of the tube, and thus to render all the readings 

 too low by this amount. Many of the very low readings which 

 are sometimes noticed by newspaper correspondents may be 

 accounted for in this way. The thermometer should therefore 

 be occasionally examined and compared with another to guard 

 against errors of this kind. 



The climate of any place, however, does not depend alone 



than the air around them. Hence they make the days cooler 

 and the nights warmer, and thus render the climate more mild, 

 imparting to it somewhat of the insular character. They also 

 exert a considerable influence on the evaporation from the earth, 

 and increase the humidity of the air. In this way they serre 

 to increase the rainfall of any district. 



In some places, where a large expanse of country has been 

 cleared of most of the forests, the difference in the rainfall has 

 been shown by the partial drying up of rivers and hikes. 



Not only is the influence of the mean temperature upon man 

 distinctly seen and felt, but it also exerts a great influence 

 on the distribution of plants, the limits to the cultivation of 

 various trees and varieties of grain being almost coincident 

 with some of the isothermal lines already referred to. 



