190 EIGHTH ANNUAL REPORT OF 



The instruments for indicating the variation of the hygrometrical con- 

 dition of the atmosphere, consist of two thermometers, of the same 

 dimensions, accurately graduated. The bulb of one of these is envel- 

 oped in a covering of muslin moistened with water, and that of the 

 other is naked. •# 



The rain and snow gauges, and also the wind vanes, are made under 

 the direction of the Institution, by Messrs. Pike & Son, 166 Broadway, 

 New York. The rain gauge is an inverted cone of sheet zinc, of which 

 the area of the base is exactly one hundred square inches. This cone 

 ox funnel terminates in a tube which carries the water into a receiving 

 vessel. The water which has fallen is measured by pouring it from 

 the gauge into a cylinder, so graduated as to indicate hundredths of 

 inches. A smaller cjdinder is also provided, which gives thousandths 

 of inches, and may serve, in case of accident, as a substitute for the 

 large cylinder. The rain gauge is placed in a cask sunk in the earth, 

 with its mouth near the level of the ground. 



The snow gauge is a cylinder of zinc of the same diameter as the 

 mouth of the rain gauge. The measurement is made by pressing its 

 mouth downward to the bottom of the snow, where it has fallen on a 

 level surface, then carefully inverting it, retaining the snow, by passing 

 under it a thin plate of metal. The snow is afterwards melted, and 

 the water produced is measured in one of the graduated glass cylinders 

 of the rain gauge. 



The wind vane is a thin sheet of metal, (it might be of wood,) about 

 three feet long, carefully balanced by a ball of lead, and attached to 

 the top of a long wooden rod, which descends along the wall of the 

 building; to the sill of the observer's window, tt terminates in the 

 centre of a fixed dial-plate, and its movements indicate the direction 

 of the wind by a pointer attached to the rod. 



The observer is by this arrangement enabled to determine the course 

 of the wind, by looking down on the dial-plate, through the glass of 

 the window, without exposing himself to the storm. 



Besides the full sets of instruments furnished by the State of New 

 York, from the appropriation of the regents of the University, the 

 Smithsonian Institution has furnished a number of sets to important 

 stations ; and, in order that they might be more widely disseminated, 

 we have directed Mr. Green to dispose of sets to individuals, at a re- 

 duced price, on condition that they will give us copies of the results of 

 their observations ; the remainder of the cost being paid by this Insti- 

 tution. A number of persons have availed themselves of this privi- 

 lege. 



To accompany the instruments, and for the use of those who take 

 part in the Smithsonian system of meteorological observations, a series 

 of minute directions, prepared by Professor Guyot, has been printed by 

 the Institution. It occupies forty octavo pages, with wood-cut repre- 

 sentations of the instruments, and is accompanied by two lithographic 

 engravings, to illustrate the different forms of clouds, and to facilitate 

 their notation in the journals, in accordance with the nomenclature 

 adopted by meteorologists. A set of tables has also been furnished for 

 correcting the barometrical observations, on account of variation of 

 temperature. A set of hygrometrical tables, to be used with the wet 



