# Explanation of Rain Gauge, Summer 1809

#### From Daily Record, 1 November 1802-25 May 1826 (Massachusetts Historical Society).

#### In Jefferson’s hand in available spaces in margin alongside entries for June-August 1809.

I shall begin with the next month to use my rain gauge. The diam. of it's tube is <*.9875 I. that of the glass funnel 3.255 consequently as 3.2552 : .9752 or as*> .9875 I. that of the glass funnel is 3.2187 consequently as 3.21872 : .98752 or as 10.36 : .975 :: <*so is the depth of water in the graduated tube : to the depth fallen*> so is the area of the funnel to the area of the tube & so is the depth of water in the tube to the depth fallen on the earth. <*therefore the depth in the tube divided & .975 : 10.36 : : 10.87 therefore the depth in the tube divided by 10.87 = the depth fallen. The tube being but 7. I. deep will be filled by a fall of .644 I. of rain. The area of the mouth of the tube is to the area of the tube as 2.066 : .975*>

the diam. of the tube of the rain gage being .9875 & that of it’s mouth 1.4375 I. the square of 1.4375 will be to that of .9875 or 2.066 : .975 as the height of rain in the tube to that fallen on the ground. then in the tube

.1 | =.0471 |

.2 | =.0943 |

.3 | =.1415 |

.4 | =.1887 |

.5 | =.2358 |

.6 | =.2830 |

.7 | =.3302 |

.8 | =.3774 |

.9 | =.4245 |

1. | =.4717 |

the diam. of the tube of the rain-gage being .9875 & that of the small glass funnel <*exactly double to wit [...]*> 2. I the height of water in the tube will be <*exactly 4 times*> to that fallen on the earth, to wit

.1 | =.02437 |

.2 | =.0487 |

.3 | =.0731 |

.4 | =.0975 |

.5 | =.1218 |

.6 | =.1462 |

.7 | =.1706 |

.8 | =.195 |

.9 | =.2193 |

1. | =.2437 |

1. | =.2437 |

2. | =.4875 |

3. | =.7312 |

4. | =.975 |

5. | =1.2187 |

6. | =1.4625 |

7. | =1.7062 |

8. | =1.95 |

9. | =2.1937 |