Page:Handbook of Meteorology.djvu/237

 standard Weather Bureau gauge. In the case of the Friez gauge a readjustment of the stop pins is necessary. With the Short and Mason gauge a pressure brake is provided; the tightening of this will prevent back-tipping.

Sometimes it happens that the stick measurement of the catch is considerably in excess of that registered on the dial or recorded on the paper; indeed, this is pretty apt to be the case in heavy summer showers. Usually this discrepancy is due to the fact that a measurable interval of time elapses while the bucket is discharging its load of water, after it has been filled. The water that flows into the bucket during the interval is therefore slightly in excess of the normal 0.01 inch. During a very heavy shower, aggregating 3 inches, the excess of the measured amount over the recorded amount may be as great as 0.15 inch. In such a case stick measurement rather than bucket measurement should be taken as the total precipitation.

The drip aperture of the Short and Mason registering gauge is very small; although protected against clogging by falling leaves, it may become clogged with dust. Under such circumstances the funnel may fill and overflow with no water running into the buckets. If the gauge receives even ordinary care such a condition is not likely to occur. Cleaning the receiver daily is not absolutely essential, but a conscientious observer will see that the gauge is always in order.

The inside of a gauge is a spot most dear to the heart of the spider, and in many a case the accumulation of spider web has tied up the registering mechanism so completely that registration ceased.

Another source of annoyance in registering and recording tipping-bucket gauges has to do with the condition of the buckets. In regions where the air is very dirty, sediment may cling to the surface of the bucket, and, adding to its weight unequally on opposite sides, prevent a true registration. A still greater source of error may result from handling the inner surface of the buckets with greasy fingers. The water will not cohere to, or “wet” a greasy surface; and this may cause a slight but persistent error in registration.

It is not wise to depend wholly upon a registering or a recording gauge; stick measurement is more certain. Nevertheless, a station of any sort should be provided with two gauges, and a