OBSERVATIONS ON THE PLATES BELONGING TO LECTURE XI. PLATE I. Represents the feveral forts of SPRINGS, as they are ufually divided into, (1) thofe which run continually, i. e. PERENNIAL; or (2) thofe which run only at times, i. e. TEMPORARY. (3) INTERMITTING SPRINGS, which flow and stop alternately; sometimes very frequently, and at small intervals, by regular intermiffions. (4) RECIPROCATING SPRINGS, whofe waters ebb and flow, by rifing and falling alternately. These feveral kinds of fprings are fuppofed to be fituated in the fame mountain or hill, and a fection of the fame exhibited to infpection. No. 1. The rains which have fallen on the hill's top, are fuppofed to have trickled through crevices, or channels, wherever they could find a way, till at length, fettling in a bafon, or collection of water, they are disposed to iffue in a ftream, and form a spring; it is evident, that fo long as the water in this bafon rises so high as B, it will furnish the outlet A with a regular fupply of water, and thus a spring may arife from the upper parts of any hill whatever: but if the refervoir be fmall, or the duct B A be large, this fpring may fail, or at least be much diminished, without perpetual fupplies from rains. There There is an odd phenomenon of such springs fometimes obferved, i. e. they run in dry weather, and are dry in wet weather; which is eafily folved, by obferving, that during rain they are gathering waters, and that it requires fome time for the water to rife fo high as B, fo that, probably, when the rain ceafes, the refervoir being full, may iffue by its channel B. A. and laft during the dry weather; but be exhausted again by the time of following rains. The fame idea applies to Temporary Springs. No. 2. Shews the principles of Intermitting Springs; which fprings iffue from a cavity where the water is collected, and which is continually receiving fresh fupplies from fome other fources, by the channel C. At H, on the lower part of the bafon, is an outlet, H K E, which is of a curved form, rising at K: here it is evident, that only when the water arrives at the level D K, will it fife in the channel high enough to run beyond K, and when it rifes to that level, it will continue running through H to K, and fo on to E, till it has drained the cavity dry: it will then ceafe of course, till the water brought in by C, has again collected to the level D K. No. 3. Reciprocating Springs, are formed by a combination of the foregoing; for if we fuppofe that befide its own inlet of water L, the stream of the bafon, No. 2, instead of issuing at E, was added, as at M, it is clear that the fpring would run conftantly by the fupply of water from L, but 0 would would run with much greater force and plenty whenever it received the waters of the intermitting fpring, by means of the conduit HK M; and its times of abundance would anfwer to the periods of the intermitting spring. No. 4. Shews the principle on which water rifes in wells, &c. which is always to that height, S, which is equal to the level of the water in the cavity :—the stream running from R to Q, and rifing from Q to S, which is the utmost it will rise to, be the height of the well P what it may. This property of water to rife to its level, and alfo, in fome circumstances to empty itself, is explained by No. 5, where abcdis fuppofed a veffel containing water to the level ef, from the bottom of this veffel iffues a pipe of any form, as g bi; the water rifing in this pipe to g and h, will there run over, and rife again to its original level ef, as at i; where it will again run over, and if the pipe i k be somewhat longer than the depth c d e f, the water will run out at k, and will not ceafe, till it has drawn the veffel a b c d dry to the very bottom. This principle affords an inftrument very useful in drawing off many kinds of fluid commodities, called a Syphon; if the fyphon be inferted into the veffel above the bottom, it is evident it will draw off the upper parts of the fluid, and leave its fediment, feculencies, &c. undisturbed. XI, 3 C PLATE PLATE II. T I DE S. THIS fcheme illuftrates the lecture, p. 256. We obferve, first, that were the globe of the earth to be covered with water of any fuppofed depth, that water of itself would always maintain the fame level, and form a ring round the globe, as a b c d. This would be little, if at all, fenfibly changed, by any effect of the diurnal rotation of the earth; but fome fmall effect would arife from the attraction of the fun, S, which would conftantly draw together those parts of the water adjoining where it was vertical, as at A: and this rifing of the water would follow its verticality in its daily course, but the immense distance of the object S, being fo many millions of miles, would render this tide very trifling. But if a celeftial body, as M, be supposed pretty near to the earth, the influence of fuch body muft needs be very confiderable. M1 is the moon in conjunction with the fun, confequently, the combined forces of these bodies, have now the greatest effect on the fluid element. Immediately beneath M 1, as at A, the waters appear affembled to the greatest depth, and gradually become fhallower as we advance to a, which is the point of genuine depth of water; it being here nei |