| Inches of Rain | Tons per Acre | Gallons per Acre |
| 0·1 | 10 | 2262 |
| 0·2 | 20 | 4525 |
| 0·3 | 30 | 6787 |
| 0·4 | 40 | 9049 |
| 0·5 | 50 | 11312 |
| 0·6 | 61 | 13574 |
| 0·7 | 71 | 15836 |
| 0·8 | 81 | 18098 |
| 0·9 | 91 | 20361 |
| 1·in. | 101 | 22623 |
The instruments called Rain Gauges or Pluviometers are, as their name implies, constructed to measure the amount of rain falling in any given locality, and those in most general use have this principle in common: that the graduated glass always bears a definite relation to the area of the receiving surface. A very extraordinary and hitherto unexplained fact in connection with the fall of rain, and which justifies the opinion that its formation is not limited to the region of visible cloud, is that a series of rain gauges placed at different elevations above the soil are found to collect very different quantities of rain, the amount being greater at the lower level. Thus, twelve months’ observations by Dr. Heberden determined that the amount of rain on the top of Westminster Abbey was only twelve inches, that on a house close by but much lower eighteen inches, and on the ground during the same interval of time twenty-two inches. Accordingly, ten inches is the height at which meteorologists have agreed the edge of the rain gauge should be placed from the ground. The spot chosen should be perfectly level, and at least as far distant from any building or tree as the building or tree is high, and, if the gauge cannot be equally exposed to all points, a south-west aspect is preferable. It is also important that the rain gauge should be well supported, in order to avoid its being blown over by the wind; and, should frost follow a fall of rain, the instrument should be conveyed to a warm room to thaw before measuring the collected contents. The graduated glass furnished with each instrument should stand quite level when measuring the rain, and the reading be taken midway between the two apparent surfaces of the water.
The best form of rain gauge is that in use in the Meteorological Office.
48.
Howard’s Rain Gauge.
Scale about 1/5.
Howard’s Rain Gauge consists of a vertical glass receiver, or bottle, through the neck of which the long terminal tube of a circular funnel, five inches in diameter, is inserted. A metal collar or tube fits over the outside of the neck of the receiver, and aids in keeping the funnel level, while the tube extends to within half an inch of the bottom, thus ensuring the retention of every drop of rain which falls within the area of the funnel. The glass vessel furnished with the instrument is graduated to 100ths of an inch. A modification of this instrument is made with a glass tube at the side graduated to inches, 10ths, and 100ths, showing the amount of rainfall by direct observation, thus dispensing with the use of a supplementary graduated measure.
In Glashier’s Rain Gauge special provision is made, in two ways, to prevent possible loss by evaporation, even in the warmest months of the year. 1. The receiving vessel is partly sunk beneath the soil, thus keeping the contents cool. 2. The receiving surface of the funnel, accurately turned to a diameter of eight inches, terminates at its lower extremity in a curved tube, which, by always retaining the last few drops of rain, prevents evaporation. The graduated vessel, in this instance also, is divided to 100ths of an inch, having due regard to the larger area, 8 in. of the funnel. For use in tropical climates, where, as has been shown, the rainfall is excessive, a modification of this instrument is supplied by the instrument makers, having an extra large receiver and tap for drawing off the collected rain.
Luke Howard, in his “Climate of London,” says: “It must be a subject of great satisfaction and confidence to the husbandman to know at the beginning of a summer, by the certain evidence of meteorological results on record, that the season, in the ordinary course of things, may be expected to be a dry and warm one, or to find, in a certain period of it, that the average quantity of rain to be expected for the month has fallen. On the other hand, when there is reason, from the same source of information, to expect much rain, the man who has courage to begin his operations under an unfavourable sky, but with good ground to conclude, from the state of his instruments and his collateral knowledge, that a fair interval is approaching, may often be profiting by his observations, while his cautious neighbour, who ‘waited for the weather to settle,’ may find that he has let the opportunity go by.” This superiority, however, is attainable by a very moderate share of application to the subject, and by the keeping of a plain diary of the barometer and rain gauge, with the hygrometer and vane under his daily notice.
49.
Symons’s Rain Gauge.
Scale about 1/7.