THE value of a series of observations on the weather depends, among other things, on the length of time over which it extends. The phenomena observed during a single day are of but little value when compared with those of a whole month. So the observations for one year will give but a slight idea of the climate of a place, compared with what might be obtained by combining the results obtained for a series of years. The greater the length of time during which the observations are continued, the more valuable will the results become, and the nearer will they approach to the constants, or fixed quantities, representing the absolute climate of the place at which they are taken. It is only by a long-continued series of observations that the mean or average temperature of any place can be obtained; and it is by a comparison of the corresponding temperature of any particular time with the mean temperature that we may perceive the amount of deviation above or below the usual degree. So with the pressure of the atmosphere, the quantity of rain, the moisture in the air, and all the other elements which unite to make up what is generally known as "the weather." To illustrate this subject, the two tables which follow are given. The first contains an abstract of the observations made at Philadelphia for one year, 1861; and the other, the results of the observations made at the same place for eleven years and a quarter. The latter may be said to be an approach to the constants referred to above; the averages altering but little as new years are added. By reference to these tables, it will be seen that the temperature of the spring of 1861 was almost identical with that of the same season for more than eleven years; while the summer was a little colder, and the winter and autumn a little warmer, than the average for those seasons for the whole period. The observations were taken at the hours of 7 A.M., 2 P.M., and 9 P.M., because it has been found that the arithmetical mean of the results so found is very near what it would be if the observations were taken continuously through the whole twenty-four hours.

By comparing the hourly averages of temperature of 1861 with those for the whole period, it will be seen that at 9 P.M. they are identical, but in 1861 the temperature at 7 A.M. was five-tenths of a degree higher, and at 2 P.M. seven-tenths of a degree lower, than usual. The average tem- | perature for the year 1861 was less than half a degree higher than for the whole period.

The column under Temperature, headed "Range," shows the difference between the highest and lowest temperature attained in each month. The column headed "Average Oscillation" shows the average difference between the highest and low

est degree for each day, and that headed "Mean daily range," the average difference of temperature between two successive days. There is no doubt but that the healthiness or unhealthiness of a climate depends, in some measure, upon the suddenness and extent of the daily changes of temperature indicated in the two last-mentioned columns. It will be seen that there is a more equable temperature throughout the whole day in winter than in either of the other seasons, while the daily range, or change of temperature from day to day, is less in the summer than in any other season.

If the barometric observations are compared, it will be interesting to notice that the hourly results for 1861 are almost identical with those for the whole period, while those of the months differ considerably. It will also be observed that the barometer is lowest in the month of June; that it then rises, at first slowly, and afterwards quickly, until September, when it descends until November; it then rises until January, when it reaches its maximum or greatest height, and finally falls, at first quickly and afterwards slowly, until June; thus showing two well-defined maxima, in January and September, and two minima, in June and November. Other peculiarities of the pressure of the atmosphere might be noticed, did time and space permit: let it suffice to call attention to the remarkable closeness of the averages of the 9 P.M. observations to the general average for the months, seasons, and years. This is evident not only for the whole period, but appears also in the observations for one year.

The quantity of sky covered with clouds is estimated by the eye at the hours of observation. By comparing the two tables, it will be seen that the quantity of rain which fell in 1861 was about 11⁄2 inches more than the average amount; and by examining the amount indicated for the several seasons, it will be seen that the excess occurred in the spring and autumn, while the quantity registered during the summer months was less than usual.

The Force of Vapor and the Relative Humidity are calculated from observations of the different temperatures indicated by two thermometers, one of which is kept dry, while the other is constantly wet. It will be seen that the former increases very regularly with the temperature, while the latter, indicating the quantity of moisture in the air as compared with entire saturation, appears, so far as the months are concerned, to follow no fixed law.

The regularity of the winds, and the correspondence of the columns in the two tables indicating the general direction from which they blow, are very remarkable.