ombroscope

from The Century Dictionary.

• noun A rain-gage arranged to make the rate of rainfall immediately observable; specifically
• noun Symon's ombroscope, in which the rain over a large area is led into a narrow vertical tube which fills up rapidly so that the rate per minute is easily perceived;
• noun Fassig's attachment to tho ordinary self-registering rain-gage, by which the first few drops of a shower make a blotch or other mark, so that the exact time of beginning and the occurrence of very slight showers may be recorded.