RJ Campbell Esqre
Your letter of 17th duly received I am much obliged for the promise of observations. which [sic] will render the record of that district more complete although we have now rain records all round Alectown.
Under separate cover I am sending a supply of the usual forms.
With reference to questions
1 An inch of rain means an inch deep all over the surface. Thus if you had a tin dish or any vessel made with a flat bottom and straight sides 1 inch deep. One inch of rain would just fill it what ever its size whether 1 foot square or one mile.
2 The principle of the raingauge is based upon what has been said in regard to No 1. hence it is that it has an accurately turned brass rim to receive the rain and under this a tunnel to serve two purposes. First it conducts the rain into the bucket and secondly serves as a cover to the bucket to prevent evaporation.
Having thus collected the rain. [sic] you can find the quantity roughly by putting a rule into it and seeing how deep it is. or [sic] you may way [sic] the water. [sic] and find how much there is by proportion as one inch weighs exactly 29 ounces. but the most accurate and convenient way is to have a glass measure carefully made and marked so that the divisions on it each equal to 1/100 of an inch commonly called a point.
The 8 inch gauge is preferred because it has been found that if you put out a number of gauges side by side of all sizes from 1 inch to 24 inches or more in diameter. That the 8 inch gauge catches rain. the [sic] reason is not certainly known: but it is believed to be owing to the wind which is differently affected by blowing over an open mouth like the top of a raingauge.
In some respects it would have been better if the selected diameter had been 7.43 because then 25 ounces of rain would be one inch and ¼ ounce would of course be 1/100 of an inch are [sic] one point.