45, COBNH1LL, E.G., AND 122, REGENT STREET, W., LONDON. 89 



temperature of from about 40 to 46 Fahrenheit, by the thermometer enclosed 

 in the tube (and exposed to view by means of a piece of glass inlet), ascends 

 through the funnel, and gradually melts the snow, etc., as it falls in. Too 

 high a temperature should not be employed, as being conducive to evaporation. 



An additional protection may also be afforded by the employment of a self- 

 acting valve midway in the heating tube, to expand with any access of heat, in 

 so doing partly to close the way, and at the same time to push open a small 

 trap-door, letting out some of the hot air and admitting some cooler air from 

 the outside. As soon as the proper temperature has been by this means 

 restored, the valve would contract into its normal position. This arrangement, 

 however, owing to the delicacy of the adjustment, is very apt to get out 

 of order. 



Messrs. Negretti and Zambra construct the above apparatus to order ; but 

 no exact prices can be quoted, as the cost would vary greatly according to the 

 extent and nature of the work and the position in which it is to be placed. 



A small piece of very open wire- work might be placed across the receiver, 

 halfway down, in case of leaves, &c., falling in. In the construction of the 

 Gauge, Copper should be employed. 



124. Marine Bain Gauges. Negretti and Zambra have constructed 

 several different forms of Rain Grange for use on board of ship. One arrange- 

 ment having gimbal mountings similiar to a steering compass has been found the 

 best, but the records obtained by their use at sea have been unsatisfactory and 

 not considered of any scientific value. 



EVAPORATION. 



125. Mr. R. H. Scott, of the Meteorological Office, writing on this subject, 

 remarks that it "is one of very great importance, especially as regards its 

 connection with Rainfall and Water supply, and well deserves especial atten- 

 tion ; but it cannot as yet be said that the results hitherto obtained merit much 

 confidence as regards their applicability to the evaporation occurring in nature, 

 owing to the exceptional manner in which the observations have been made." 



Atmometers of many forms have been invented and constructed, both in 

 this country and on the Continent, but, at present, nothing satisfactory has 

 been devised ; hence the difficulty of making any very accurate observations 

 in connection with evaporation from the surface of water. 



126. Evaporation Gauge, (fig. 103), (Evaporometer), for showing the 

 amount of evaporation from the earth's surface. This gauge consists of a brass 

 vessel, of eight inches diameter, corresponding with Glaisher's Gauge, the area 

 or evaporating surface of which is accurately determined ; and also a glass 

 cylindrical measure, graduated into inches, tenths, and hundredths of inches. 

 In use, the Evaporating Gauge is nearly filled with water, the quantity having 

 been previously measured by means of the glass cylinder ; it is then placed out 



