thought to deferve attention, it is because the standard deducible from it, is not fubject to the three laft mentioned, defects.

THEOREM.

If there be a cubic veffel with an aperture in the bottom,. which aperture is in a given ratio to the base of the veffel; and if the ratio between the weight of the water which this veel contains when full, and the weight of the water discharged from it, through this aperture, in a given time be given, the cube itself is given.

DEMONSTRATION.

No other fimilar veffel will difcharge a proportionate quantity through a proportionate aperture in the fame time; for if fo, the weights of water discharged by two fuch cubes in the fame time, would be in the fame ratio to their difcharging cubes refpectively, and confequently the weights. of the difcharged waters must be to each other, as the weights of the cubes, that is in a triplicate ratio of their altitudes; but by the laws of Hydraulicks the weights of the water discharged in the fame time from different altitudes and through different apertures will be to each other in a ratio. compounded of the fimple ratio of the apertures, (which is equal to the ratio of the basis, which is equal to the duplicate ratio of the altitudes) and the fubduplicate ratio of the altitudes; that is, the weights of the water discharged muft be to each other in the triplicate ratio of the altitudes, which is abfurd.

In making an experiment according to this theorem, it is to be obferved that the edges of the orifice should be as then as poffible, and that the vessel should not be suffered

to

to discharge so much of its contents that the surface would approach the bottom.

It is evident, that by this method, a ftandard may be formed for any length, fuperficies, or capacity, and alfo for any weight, if the specific gravity of the water made use of be given.

No. XL.

Defcription of a SPRING-BLOCK, defigned to affift a vesset in Sailing. By FRANCIS HOPKINSON, ESQ. of Philadelphia.

Honoured with the Magellanic Gold Medal, by an award of the Society, in December, 1790.

T is an acknowledged fact, that when the fhrouds of a veffel are braced very tight, fo as to prevent the mafts from having any play or fpring, fhe will not fail as fast as when her mafts are permitted to bend a little to the impulfes of the wind. The reafon is, that the wind is feldom uniform in its force for any length of time; and it is impoffible that a fudden encrease of impulse should inftantaneously communicate a proportionable velocity to fo heavy a body, placed in a refifting medium of fo great density.

In fuch cafe the veffel is forcibly preffed into or against the water, and is obliged to heel from the blaft, until a progreffive motion, adequate to the force impressed can be communicated to the whole mafs. But thefe fudden preffures against the water and this heeling of the veffel, are great obftacles to faft failing: in as much as they oc

cafion an unprofitable expenditure of the force which fhould push her forward.

When a veffel first hoifts her fails, although she may be. in a fituation to receive the full impulfe of the wind, yet fhe will not immediately proceed with the velocity which. she will afterwards acquire from the fame force: having not yet got under way, as the feamen exprefs it. Upon the fame principle, when a veffel is failing at the rate of five knots, if a fudden blaft of wind fhould come, which would enable her to make feven, knots, fhe may be confidered as being perfectly stationary with respect to the twoadditional knots, and will require fome time to get under way as to them. Now the effect produced by relaxing the fhrouds is, that the maft, receiving the first impreffion of the wind upon the fails, acts as a fpring, and yielding to the impulfe, gradually communicates motion to the whole veffel, giving her time to get under way, and occafioning an eafy transition from one degree of velocity to another fo that her way is not checked by her being fuddenly urged against the refifting medium on the accef fion of every new force impreffed.

But the misfortune is, that this advantageous ufe of the maft can be exercised only to a small extent; for, if it is allowed too much play, it will be in danger of breaking. The object of the prefent propofal is to enjoy the fame benefit to a greater extent and with more fecurity.

The masts, yards and rigging of a ship receive the first impulfes of the wind. All these are in fome degree elastic Every twisted rope is a spiral spring, and every fpar capable of being a little bent. But let us fuppofe that the ropes were all compofed of ftraight ftrands, that the mafts and yards were inflexible, and the fails made of thin light wood; in fuch cafe, I fuppofe, that the veffel in failing by the wind would make but little head-way, whatever

the