85. Juncus effusus, (L.,) Common or Soft Rush, (p.) There are numerous species of this worthless native weed, but this is the best known, and perhaps the most objectionable, as it has a constant tendency to form unsightly bunches. or tussocks, in moist low grounds. Mr. Elliott, an eminent botanist, says that in South Carolina this Rush occupies and almost covers rice fields as soon as they are thrown out of cultivation.

86. Cyperus phymatodes, (Muhl., Nut.,) Grass of Florida, (p.) This species is fortunately somewhat rare, as yet, in the northern and middle States, but it is a great pest to the agriculture of the South.

87. Cyperus hydra, (Mx.,) Coco grass, nut grass of South Carolina, (p) This is regarded by the southern planters as the most intolerable pest of their agriculture. Mr. Elliot says: "It shoots from the base of its stem a threadlike fibre, which descends perpendicularly eight to eighteen inches, and then produces a small tuber. From this horizontal fibres extend in every direction, producing new tubers at intervals of six or eight inches; and these immediately shoot up stems to the surface of the earth, and throw out lateral fibres to form a new progeny. This process is interminable, and it is curious to see what a chain or net-work of plants and tubers can, with some care, be dug up in a loose soil. The only process yet discovered by which this grass can be extirpated is to plough or hoe the spots in which it grows every day through the whole season. In their perpetual efforts to throw their leaves to the light the roots become exhausted and perish; or, if a few appear the next spring, they can easily be dug up."

88. Carex tentaculata, (Muhl.,) Many-beaked Sedge, (p.) A very common species, in swampy low ground, of the large and unprofitable genus of sedges. 89. Carex stricta, (Lam.,) Tussock Sedge, (p.) This is one of the most common, and most difficult to manage, of all our sedges. Its roots are apt to form large dense tufts or "tussocks" in swamps. The careful farmers sometimes get rid of those tussocks by digging them out, and, when dry, collecting them in large heaps, burning them, and using the ashes as a manure. Of this remarkable and very numerous genus, (Carex,) Dr. F. Boott, an accomplished botanist of London, has now in hand one of the noblest and most elaborately illustrated monographs ever issued from the press.

90. Panicum sanguinale, (L.,) Crab grass, finger grass, (a.) In the middle States this troublesome grass abounds in gardens in the latter part of summer, and is frequent also in Indian corn fields, but may be kept in tolerable subjection by the early and faithful use of the instrument known as the "cultivator." The crab-grass is regarded as a serious pest in the plantations along the lower Mississippi.

91. Panicum capillare, (L.,) Hair-like Panicum, "Old Witch" grass, (a.) This worthless species flourishes best in a light sandy soil, but is usually more or less abundant in corn-fields. In autumn the dry culms break off and the light-spreading panicles are rolled over the field by the winds, until they accumulate in great quantities along fence and hedge rows.

92. Panicum crus-galli, (L.,) Cock-foot Panicum, barn-yard grass, (a.) This coarse homely grass is said to be an inhabitant of all quarters of the globe. It is usually found in the latter part of summer, rather abundant along drains of barn-yards and other waste places.

93. Sitaria glauca, (Beauv.,) Bristly Fox-tail grass, (a.) All our weed-like species of this genus are believed to be naturalized strangers here. This one usually makes its appearance in abundance among the stubble, after a wheat crop, and is frequent in pastures, orchards, &c., when not kept down by a mere valuable growth. The S. viridis, (Beauv.,) called green fox-tail or bottle grass, is about equally worthless, but not quite so prevalent.

94. Sitaria virticellata, (Beauv,) (a.) The adhesive bristles of this species,

frequenting gardens and neglected lots, are calculated to make it something of a nuisance if permitted to become abundant.

95. Cenchrus tribuloides, (L.,) Bur grass, hedge-hog grass, (a.) This pestilent nuisance is quite abundant in the sandy districts of New Jersey and along the great northern lakes.

96. Cynodon dactylon, (Pers.,) Dog's-tooth grass, Bermuda grass, (p.) Of this grass, which has found its way from Europe into Virginia and other southern States, Mr. Elliot remarks: "The cultivation of it on the poor and extensive sand-hills of our middle country," (viz., in South Carolina,) "would probably convert them into sheep-walks of great value; but it grows in every soil, and no grass, in close rich land, is more formidable to the cultivator. It must, therefore, be introduced with caution."

97. Bromus secalinus, (L.,) Cheat, chess, broom grass, (a.) This is a wellknown intruder among our crops of wheat and rye, and often appears in the same fields for a year or two after those crops, but it is soon choked out by the perennial grasses.

This plant is an annual, and easy to overcome by care in sowing clean wheat, by keeping fence corners and field borders clear, and in establishing a proper rotation in cropping. The vulgar error, that this grass is merely transmuted wheat, came to us with the earliest immigrants, and, notwithstanding the boasted "march of mind," it yet prevails among a certain class of farmers to a considerable extent.

98. Triticum repens, (L.,) Couch grass, Quitch grass,' (p.) This species of triticum, which is quite distinct in habit from the cultivated wheat, has found its way into some districts of our country, and is a pernicious intruder, when fully introduced, by reason of the exceeding tenacity of life in its rhizomas, or creeping subterranean stems.

99. Andropogon nutans, (L.,) Wood grass, Indian grass, (p.) This and two or three other species of native Indian grasses are common in our sterile grounds, and are no better than mere weeds.

SERIES II.-CRYPTOGAMUS, or Flowerless Plants

100. Pteris aquilina, (L.,) Brake, bracken of the Scotch, (p.) This large fern is often abundant in moist woodlands and borders of thickets, and in our wild forests it affords a favorite shelter, or hiding-place, for deer and other game, but it is little better than a weed on the farm.

Having thus disposed of the most prominent weeds in our agriculture, it remains merely to mention, very briefly, three or four of the injurious cryptogams, among the lower order of the fungi, viz:

Merulus lachrymans, (Schum.,) Dry-rot. This fungus, with some others which infest timber in places where a damp air is confined, as in houses and ships, is very injurious. It is said to be remedied by a wash of diluted sulphuric acid.

Ascophora mucedo, (Link.,) Mould, bread-mould. This minute fungus usually abounds on moist decaying substances, and is well known to housewives as growing plentifully on bread and pastry which have begun to "spoil;" yet it is probable that many of them have never suspected it of being as genuine a plant as any weed that grows on the farm.

Uredo segetum, (Pers.,) Blight, smut, brand. This is usually found within the glumes and fruit of wheat, barley, and other grasses, speedily filling the whole with a profuse black dust.

Puccinia graminis, (Pers.,) Mildew, rust. This often operates injuriously on wheat crops in warm, close, foggy weather, near harvest time; especially where the crop is a little backward and mingled with grass or herbage.

OBSERVATIONS ON ATMOSPHERIC HUMIDITY.

BY J. S. LIPPINCOTT, HADDONFIELD, N. J.

To a large and intelligent class of readers of the Agricultural Department reports, grape-growing has become an object of absorbing interest. Those of this class who may have read a paper on "The Climatology of American Grape Vines," in the report for 1862, and its continuation under the title of "Geography of Plants," in that for 1863, will have observed that success was promised in certain zones of summer temperature, provided the atmospheric humidity were not there deficient, either permanently or for the season. This element, so variable, seems scarcely less important than that of the mean temperature of the growing season. Experience derived from the failure of the grape crop of 1864 and 1865, over wide regions deficient in humidity, and its success in others where this element must have been abundant, have set its value in a clearer light than heretofore.

In seasons not marked by extreme fluctuations of atmospheric humidity, and accompanying reductions of temperature in midsummer, the isotherms which bound the grape-growing belts, as heretofore described, limiting the regions adapted to certain varieties of grapes, may still be esteemed as normally correct and reliable; but in seasons of exceptional character, when extremes of humidity occur, and, with them, extreme high temperatures followed by great reduction of atmospheric moisture, (and oftentimes accompanying sudden decline during the night to near the freezing point,) such isotherms cease to be the guiding clews to the regions adapted to any special variety of grape, or, indeed, to indicate that any grape can be therein successfully cultivated. There are few physical laws which can be realized with mathematical exactness, but they are generally approximations, more or less false, in each particular case. "These laws are ideal truths towards which nature tends, but which are never fully reached. Even as respects the law of gravitation, there always have been residual phenomena unexplained by the law; and so, probably, there always will be as our generalizations widen towards the great Presence of which all natural phenomena are the direct manifestation."

We have hitherto regarded the conditions of temperature as of primary importance. Though the amount of moisture in the atmosphere of each locality may be of nearly equal value, we have not the data for determining the proportions of this ingredient demanded for the successful culture of many of our garden and field products.

With regard to the grape, we are better prepared to discuss the question of the climatic value of excessive or diminished relative humidity. The very favorable reception awarded our former efforts, encourages the hope that the present will prove suggestive, if not instructive.

As a necessary consequence of the evaporation continually going on over the entire surface of the earth, the atmosphere at all times contains a proportion of vapor of water, the amount of which is perpetually varying. This amount is almost always below the proportion which experiment has shown to be the greatest degree possible at the observed temperature. It is owing to this circum

stance that the air is rarely fully charged with vapor-that wet bodies become dry, and that the surface of the soil, although saturated with moisture, yet in a few hours becomes parched and dusty. By the process of evaporation from the surface of the land, as well as of the ocean, a natural distillation is thus continually carried on, and a perpetual circulation of waters maintained-those conveyed by the rivers into the sea being returned by invisible channels through the atmosphere to form clouds, which shall restore to the streams, by means of rain, their perpetual tribute to the ocean.

Upon variations in the quantity of moisture present in the atmosphere, many of the great peculiarities of our climate mainly depend. The frequency of rain, and many other meteorological phenomena of the highest interest and importance, are greatly influenced by the proportions of humidity present in the atmosphere of any locality. To attain an accurate knowledge of the quantity of aqueous vapor which exists at any given time in a certain bulk of air, becomes, therefore, a problem which is constantly requiring solution. Instruments employed for this purpose are termed hygrometers. Various methods have been devised for ascertaining the proportion of moisture in the air; and the simplest and the most accurate of these consists in the determination of the dew-point, or temperature to which the air must be reduced so that its moisture shall begin to separate and condense upon cold surfaces. This difference, alone, is sometimes used to express the dryness of the air, or the reduction of its moisture below the point of saturation. The determination of the dew-point may be readily made, on a summer's day, by noting with a delicate thermometer the exact temperature of water in a glass, at the moment deposition of vapor ceases to be made. From this temperature, and that of the air at the same time, the tension (pressure, or force) of the aqueous vapor present in the air, as exerted on the column of mercury in a barometer, may, by means of tables constructed for this purpose, be readily ascertained; and the corresponding proportion of moisture (or the relative humidity or percentage of saturation) be easily learned.

The above method, apparently so simple, is not readily employed in general practice, and has given place to the wet-bulb thermometer, or August's Psychrometer, which for simplicity and ease of manipulation leaves nothing to be desired. This consists merely of two similar delicate accurate thermometers, placed side by side on the same stand, the bulb of one covered with thin muslin, which is supplied with moisture and kept continually wet by capillary conduction from a vessel beneath. The action of this instrument may be readily understood by the uninitiated observer, who, with one hand wet and the other dry, will expose them equally to a gentle current of air, on a drying day. He will not need a thermometer to indicate which hand is most rapidly cooled, and that the drier the day, the more his wet hand will become chilled below the other. Thus it is with the "wet and dry bulb" thermometers. The wet bulb thermometer will exhibit decline in temperature if the air be not already saturated with moisture, and evaporation thereby prevented. The rate of evaporation, and consequently the depression of temperature in the wet-bulb instrument, will be greater in proportion as the air is further from the point of saturation. To determine the exact amount of vapor present, and the proportional degree or percentage of saturation, tables have been prepared which greatly facilitate the study of hygrometry; the best of which are those published by the Smithsonian Institution, at Washington. Without such tables, the indications of the Psychrometer, except when very near saturation, can be but vaguely defined, since the amount of vapor contained in the air, at any time, is reduced by a fall in temperature, more rapidly than in direct proportion to the fall; for while the temperature changes in arithmetical, the humidity varies in geometrical progression. It should be understood that the amount of vapor held in the air over any district is very variable—perhaps con

stantly changing in amount. The colder the air, the less the vapor it can hold; and the warmer the air, the more it may contain. But it does not follow that there must necessarily always be more vapor in the air at a high temperature than at a lower one. Air at a given temperature will hold a certain quantity of vapor, and no more; but it may hold any quantity less. If heated, it may absorb more, (if not already full or saturated,) if it can gain access to it, or to water. If already full, it will lose a part of it on being chilled. The definite quantities of vapor which air will hold at certain temperatures, by Fahrenheit's thermometer, are as follows: At zero the weight of vapor in a cubic foot of saturated air has been estimated at about three-quarters of a grain; at 32°, 23 grains; at 40°. 3 grains; at 50°, 4 grains; at 60°, 57 grains; at 70°, 8 grains; at 809, nearly 11 grains; at 90°, 14 grains; at 95°, 17 grains, and at 160°, nearly 20 grains of vapor in each cubic foot.

When an atmosphere of very high temperature is loaded with all the vapor it can hold, as at 95°, saturated with 17 grains for every cubic foot, it becomes very oppressive to the people of the district sustaining it, and sometimes destruetive to life. A consideration of the above numbers will explain the general extreme humidity of the climates of warm countries. The amount of vapor in the air is not generally expressed in grains in each cubic foot, but in inches of pressure on the barometer, and in degrees of relative humidity, 100 being taken to represent saturation. This ever fluctuating element varies from hour to hour through each day, according to the changing temperature of the air, the action of the sun's rays, the presence or absence of clouds, and the force of the wind. It may be reduced almost, if not quite, to a nullity, or may rise at high temperatures until it presses upon the barometer with a force measured by two inches of its column. Throughout the year it is generally least or lowest in the morning about sunrise, when a portion has been deposited as dew or frost; and greatest or highest at 21 p. m., or about the period of greatest heat; and declines again in the evening, but not to the low measure of the morning. These are the mean average conditions, but it may be, and it often is, greatest in the morning, lowest at noon, and lower in the evening than at the morning observation. It is at its lowest point, generally, in January, when we have observed about one hundredth of an inch; increases in February, and advances in quantity as the season progresses, until it reaches its greatest amount in August, during the periods of greatest heats; then declines with the decline of heat, the humidity of autumn being in advance of that of spring. The highest we have observed the pressure of vapor was on the 7th of July, 1864, when it affected the barometer to the extent of 1.235 inch, the thermometer being at 90° at 2 p. m., and on the 26th of June, 1864, 1.053 inch at 2 p. m., thermometer at 960, both of which were followed by rain in one to three hours-the last with lightning and tornado blasts of wind