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[Vol. IV., No. 99. 



busy providing for their issue, or preparing for 

 winter quarters. 



October. — This is the month when spiders of all 

 kinds are most noticeable, their gossamer threads 

 glistening high up in the air, or their webs disfigur- 

 ing shrubs and buildings. Immigrant plant-lice 

 come on the wing to store away the winter egg on 

 congenial trees ; and the other insects most notice- 

 able are those which hibernate, and are getting 

 ready to do so. The buck moth (Hemileuca maia) 

 flies quietly, with its delicate crape-like wings, 

 among the dropping leaves of the forest, and is 

 the species most peculiar to the month. 



November. — In this month most insects are 

 hushed in death or torpor ; but the fall canker-worm 

 moths will rise from the ground after a severe frost, 

 and many hibernating Hymenoptera and Coleoptera 

 will take an airing when the weather is mild. The 

 cluster-fly (Pollenia rudis) holds out against the 

 cold much longer than the house-fly, which it so 

 much resembles. 



December. — Nothing peculiar marks this month ; 

 but most of the species mentioned for both No- 

 vember and January may be seen in December, 

 when the temperature and circumstances favor. 



WEATHER FORECASTS. 



" Afiother storm brewing ; I hear it 

 Sing i' the wind" 



Shakspeare. 



The methods by which weather forecasts are 

 made are based almost wholly upon facts of ob- 

 servation rather than upon established deductions 

 of science. This is unavoidable, because atmos- 

 pheric movements are very complicated, and 

 because the science of meteorology is not yet 

 sufficiently advanced to satisfactorily explain them 

 in the detail necessary for successful forecasting. 



The leading fact upon which predictions depend 

 is that atmospheric conditions advance in a direc- 

 tion generally easterly. The motion may vary in 

 velocity, but in direction is usually between north- 

 east and south-east, rarely towards any other 

 point of the compass. During this advance, 

 changes in condition may occur ; and it is neces- 

 sary to foresee the character of these changes, as 

 well as the direction, and rate of motion. The in- 

 dications of the barometer are the chief aid in 

 understanding the weather conditions themselves, 

 and the changes which may be expected. At any 

 given moment there exist, in the territory occupied 



by the United States, differences in the atmos- 

 pheric pressure which may amount to two inches 

 in the height of the barometer. Usually there are 

 one or more areas of pressure above the average, 

 and one or more below the average, the pressures 

 at intermediate points lying between the highest 

 and lowest values. Each of these areas of hig-h 

 and of low pressure is accompanied by its peculiar 

 conditions, and is moving towards the Atlantic 

 coast with varying velocity. Thus the low area, if 

 its centre is more than two or three tenths of an 

 inch below the average pressure, is accompanied 

 by clouds, and rain or snow, and forms a storm. 

 The area of high pressure is usually attended by 

 clear skies ; and the radiation of solar heat to the 

 earth during the day, or from the earth at night, is 

 unchecked by clouds : consequently in summer, 

 when the days are long, the temperatures which 

 accompany an area of high pressure are above the 

 average; while in winter, when the nights are long, 

 low temperatures are found with high pressures. 

 Many similar facts have been learned from the 

 study of meteorological observations, upon which 

 dependence is placed in weather-predicting. 



Under the auspices of the U.S. signal-service, 

 observations are made three times each day at a 

 hundred and twenty-nine stations suitably located. 

 Each of these observations is made at the same 

 moment (seven a.m., three p.m., and eleven p.m., 

 Washington time), and includes determinations of 

 the atmospheric pressure, the temperature and 

 humidity of the air, the direction and velocity of 

 the wind, the kinds and motion of clouds, and 

 other meteorological data. The results are at once 

 telegraphed to the central office, and maps formed 

 which show graphically the conditions at the mo- 

 ment of observation, and the changes which have 

 occurred in the past few hours. From these maps 

 a detailed prediction is made for the twenty-four 

 hours following, based upon the conditions which 

 exist at the time, the changes which have occurred, 

 and the changes which, former experience shows, 

 usually follow similar conditions. 



The weather prediction thus assumes that com- 

 ing changes will agree with the changes noted in 

 former times under like circumstances. This is 

 true on the average ; but, whenever exceptions 

 occur, the prediction fails. Increased skill in pre- 

 dicting depends upon increased skill in anticipat- 

 ing these exceptional cases. At the present time 

 the government predictions are verified in eight 

 cases out of ten. Reliable forecasts cannot be 

 made for a period longer than twenty-four hours, 



