day. Each chart, therefore, carries the record of a full daily period of sunshine.
The Campbell-Stokes recorder is simple in construction and inexpensive. It requires neither clockwork nor electrical mechanism in its operation. The records are unsightly, but they are ineffaceable and permanent. As a piece of mechanism it is practically fool-proof. In the laboratories of Europe its use is general.
Photographic recorders depend on the action of sunlight on sensitive paper. The record sheet is placed within a camera of circular section. A minute aperture permits a spot of light to enter the camera and fall on the record sheet. In one form there are two apertures, one for the period from sunrise till noon, the other from noon to sunset. The changing position of the sun causes the spot of light to traverse the record sheet in an opposite direction. After exposure the sheets are developed and fixed by ordinary photographic processes. Silver paper gives the most legible charts; and when a bit of blue glass is used as a light filter, the line of record is more sharply drawn and clearer. Silver paper is expensive, however, and ordinary blueprint paper is more commonly used.
The time and effort required to prepare the sensitive paper, and to develop and fix the record sheets is the great objection to photographic recorders. In some respects they are more accurate in time measurement than any other form; and in this particular they have possibilities not possessed by any other recorders.
The Marvin thermo-electric recorder is used at United States Weather Bureau stations and in most meteorological laboratories. It consists of a differential thermometer in a vacuum tube and a recording apparatus. The expansion of a volume of air in a blackened tube pushes a column of mercury between two platinum points, the ends of which pierce the tube, thereby making an electric circuit possible in the recording apparatus. The air volume within the blackened tube is exceeding sensitive to heat. Even in the coldest weather, the heat of direct sunshine is sufficient to push the mercury to the circuit-making points; with the absence of sunshine the mercury drops below them. Inclining the tube takes some of the weight off the air chamber and causes the mercury to be more easily lifted. In
