The foregoing facts will doubtless apply to about one half of Illinois. This climate also is subject to sudden changes from heat to cold; from wet to dry, especially from November to May. The heat of the summer below the 40° of latitude is more enervating, and the system becomes more easily debilitated than in the bracing atmosphere of a more northerly region.
At Marietta, Ohio, in lat. 39° 25' N. and at the junction of the Muskingum river with the Ohio, the mean temperature for 1834, was 52 degrees, four-tenths; highest in August, 95 degrees,—lowest, January, at zero. Fair days 225,—cloudy days 110.
At Nashville, Tenn. 1834, the mean temperature was 59 degrees and seventy-six-hundredths; maximum 97, minimum 4 above zero. The summer temperature of this place never reaches 100°. On January 26th, 1832, 18 degrees below zero. February 8th, 1835, 10° below zero.
The putting forth of vegetation in the spring furnishes some evidence of the character of the climate of any country, though by no means entirely accurate. Other causes combine to advance or retard vegetation. A wet or dry season, or a few days of heat or cold at a particular crisis, will produce material changes.
The following table is constructed from memoranda made at the various dates given, near the latitude of St. Louis, which is computed at 38° 30'. The observations of 1819 were made at St. Charles and vicinity, in the state of Missouri. Those of 1820, in St. Louis county, 17 miles N. W. from the city of St. Louis. The remainder at Rock Spring, Illinois, 18 miles east from St. Louis. It will be perceived, the years are not consecutive. In 1826, the writer was absent to the eastern states, and for 1828, his notes were too imperfect to answer the purpose.
In the columns showing the times of the first snows, and the first and last frosts in the season, a little explanation may be necessary. A "light" snow means merely enough to whiten the earth, and which usually disappears in a few hours.
Many of the frosts recorded "light" were not severe enough to kill ordinary vegetation.
table continued
| Oaks and other | First snow on | ||||
| Grass green in | forest trees put | approach of | Last frost in | First frost in | |
| Year. | prairies. | forth leaves. | winter. | Spring. | Autumn. |
| 1819 | April 18. | Half size | Oct. 8, | May 18, | Sept. 23. |
| May 19. | few flakes. | very light. | |||
| 1820 | April 10. | April 22. | Oct. 24. few | June 1, | Sept. 20. |
| full size May 7. | flakes. Nov. 11 | very light | Oct. 8, ice. | ||
| 3 inches. | |||||
| 1821 | April 26. | Ap. 26 to May 3. | Nov. 8. | April 18, severe. | Oct. 8. |
| f. grown 22. | 2½ in. | May 9, light. | |||
| 1822 | April 10. | April 29. | Nov. 16, light. | April 16, | Oct. 13. |
| full size May 14. | severe, ice. | ||||
| 1823 | April 10. | April 23. | Nov. 1, light. | April 24. | Sp. 21-2. |
| Ice 23. | |||||
| 1824 | April 14. | April 30. | Nov. 7. | May 5. | Oct. 21. |
| hard freeze. | |||||
| 1825 | Mar 16. | April 3. | Dec. 11, | Feb. 22. Next. | Oct. 2-3. |
| 3 inches. | Ap. 20, ice. | 27th, ice. | |||
| 1827 | Mar. 25. | April 10. full | Nov. 25, light. | May 7, light. | Sept. 23, light. |
| size April 30. | |||||
| 1829 | April 24. | April 27. | Nov. 12, | Not noted. | Sept. 17. |
| 4 inches. sleet. | |||||
| 1830 | April 1. | begin Ap. 5. | |||
| f. size May 1. |