receiving surface, hence no attention need be paid to orientation. - Th Eppley pyranometer has two concentric silver rings for receiving surface. One of which is coated black and the other white. The temperature difference between the two rings is measured with thermojunctions. - The Dirmhirm-Sauberer pyranometer uses black and white segments alternately mounted in the form of a star. Again the temperature difference is measured with thermojunctions which are in good thermal contact with the segments. The above mentioned thermopile instruments are covered by hemispherical glass domes of mm thickness. They are usually well sealed and provision is made that the air inside be kept dry with silica gel. The time required for about 90$ response to a sudden change varies between 20 to 30 seconds. The temperature coefficient does not exceed -0.2$ per degree
