Sidebilder
PDF
ePub

ble of making its way over the fences, in a country where the fences are, in general, less secure than with us.

This breed is distinguished principally by the shortness of the fore-legs, which are always bent like an elbow, and the feet are turned in wards in walking, so that their gait is awkward, and they can neither run nor leap so well as other sheep. Their general organic structure appears, also, to be more infirm, and they do not fatten so well as the common breeds. When put into an inclosure with other sheep, they are observed to separate theniselves into a distinct flock.

The most remarkable circumstance in this account, is, the facility with which this peculiarity of form was propagated, so as to establish a distinct and permanent variety; and the fact is the more important, as it furnishes us with a satisfactory theory of the origin of the varieties which exist among other genera of animals.

XII. Experiments to ascertain the coagulating Power of the

Secretion of the gastric Glands, By Sir Everard Home, Bart. F.R.S. Communicated by the Society for promoting the knowledge of Animal Chemistry.

The property of coagulating animal fluids, which is possessed by the internal membrane of the stomach of animals, is well known; and the object proposed by these experiinents, was, to determine whether this property belonged to the whole of the secretions poured out by the internal membrane, or was peculiar to that formed by the gastric glands, and was communicated by that medium to the other secretions found in the cavity of the stomach. With this view some experiments were first made with the internal membrane of the stomach of several quadrupeds, birds, and fishes. No part of the membrane of the hog's stomach, possessed the coagulating power, except that near the pylorus where the glands are situated. Both the crop and gizzard of birds were found to produce coagulation ; but the gizzard was the most speedy in its effect. The stomachs of the shark, salmon, and thornback, were found to possess the same property, but in different degrees.

The gastric glands were next dissected from the stomach of a turkey recently killed; care being taken to remove them without making any opening through the inner membrane. A comparative experiment was then instituted with equal weights of these glauds, and of the internal membrane of the turkey's stomach, and of that of the calf, in its recent and dried state. They all coagulated milk, but the membrane of the stomach of the turkey, was the most slow and feeble in its action.

From these results Sir Everard thinks himself entitled to conclude, that it is the fluid secreted by the gastric glands that alone possesses this power, which it communicates to all the rest. This inference does not appear quite so well established to us as it does to its Author ; nor can we assent to the truth of the concluding remark, that . Coagulation appears to be the ' first change the food undergoes in the process of digestion ;' because this is a change which seems to belong only to albuminous fluids ; and the first action of the digestive process on alimentary matter, already firm and solid, must undoubtedly be directly solvent. XIV. An Appendix to Mr. Ware's Paper on Vision. By

Sir Charles Blagden, F.R.S. In confirmation of the views of Mr. Ware, that short-sightedness occurs most frequently in the higher classes, particularly among the students at the Universities, Sir Charles Blagden here relates the progress of this affection as it occurred in his own person. At that early period of life when education usually commences, bis vision was extremely perfect; but he became short-sighted as he grew up, though it was in the commencement so trifling as to be corrected by a common watch glass. It afterwards increased so much as to require the use of a concave glass, of low number, which was changed for others, successively, of higher numbers, as the affection became more troublesome. He attributes it entirely to a habit of study, and fondness for reading, acquired in early life, and the influence of which, on his vision, was not corrected by the occasional intervention of any ucoupation or amusement which required the eyes to be directed to distant objects. Sir Charles observes, that

Children born with eyes which are capable of adjusting themselves to the most distant objects, gradually lose that power soon after they begin to read and write ; those who are most addicted to study becomc near-sighted more rapidly; and if no means are used to cousteract the babit, their eyes at length lose, irrecoverably, the faculty of being brought to the adjustment for parallel rays.'

He relates an experiment which be made many years ago, to determine how far the similarity of the images, seen by each eye, contributed to make them impress the eye as one. The objects selected were the alternate cavities and ridges of a fluted marble chimney-piece; and when the optic axes were so adjusted, that the first ridge and concavity of the fluting, as seen by one eye, should fall in with the second ridge and concavity, as soon by the other eye, the fluting appeared perfectly distinct

[ocr errors]

A

1

and single, but it appeared to be about double the distance that
it really was from the eye, and, consequently, to be magnified
in proportion.
XV. A Method of drawing extremely fine Wirer. By Wi-

liam Hyde Wollaston, M. D. Sec. R. S.
The contrivance recommended by Dr. Wollaston for this
purpose, is extremely simple, and of very easy application in
practice. A wire of gold or platina, is to be introduced into
the centre of a rod of tine silver, which is then drawn into five
wire by the usual means. As silyer wire used for lace and em-
broidery, is frequently as fine as the sto of an inch in diameter,
if the gold wire introduced into the centre of the rod has to
the diameter of the silver, then, when it is drawn into wire of
sts of an inch, the diameter of the gold will be yoóo of an
inch, and of such wire 550 feet will weigh only one grain. By
these means, however, Dr. W. reduced platina to the extreme
tenuity of yoooo of an inch in diameter, but the tenacity seemed
to be impaired when the fineness exceeded meso of an inch,
and wire of this diameter supported of a grain before it broke.
The silver coating is easily removed from these wires by nitric
acid; but, as when they exceed in fineness the poor, or moóc of
an inch, they are managed with difficulty, from being easily dis-
turbed by slight currents of air, and from being nearly invi-
sible, and not at all perceptible to the touch ; Dr. W.
mends that the silver

coating should not be removed from the
oxtremities, and by this means they are kept stretched, and are
easily applied to the purposes for which they are wanted.
XVI. Description of a single lens Micrometer. By Wil-

liam Hyde Wollaston, M. D. Sec. R. S.
This instrument is admirably adapted for the purpose of mea-
suring the diameter of the extremely fine wires, which are
occasionally employed in the construction of philosophical in-.
struments. Its external form is that of a common telescope,
onsisting of three tubes. The scale by which the object is
measured, occupies the place of the object glass, and consists
of a series of small wires about to of an inch in diameter, equi-
distant from each other, and formed into divisions by a regular
variation in the length of the wires with a view to facilitate
the computations of the observer. This then forms a scale
o equal parts. The lens is placed at the smaller end of
the instrument, and having a focal length of only It of an inch,
it admits a small perforation to be made in the brass mounting
at the distance of about 5 of an inch from its centre, through

recom

which the divisions of the scale can be seen distinctly by the naked eye, on account of the smallness of the aperture through which it is viewed. The object to be measured is placed between a pair of plain glasses which slide before the leps, and which admit of adjustment by means of a screw, and the lens also has a small motion by means of the cap, for the purposes of adjustment. As the indications of the scale must be different according to the distance to which the tube is drawn out, it is necessary to determine these with precision, before the instrument is completed. In Dr. W's, instruments each division of the scale corresponds to rorooo of an inch when it is at the distance of 16-6 inches from the lens, and since the apparent magnitude in small angles, varies in the simple inverse ratio of the distance, cach division of the scale will correspond to setor of an inch at the distance of 8-10 inches, and the intermediate fractions too, noor, will be found at intervals of 1-66 inch. These intervals should be marked on the outside of the tube.

In order to determine the value of each division of the scales with accuracy in the first instance, on which the excellence of the instrument must depend, it is necessary to employ a wire, of which the diameter has been determined with great care, for any error in this process, will, of course, pervade all the future admeasurements for which the instrument may be employed. Dr. W. recommends, for this purpose, that the diameter of this wire should be deduced from the specific gravity of the metal. The specific gravity of gold, for example, being 19-36, a cylindrical inch will weigh 3,837 grains, and consequently a wire of pure gold, drawn out fifty-two inches in length, shall weigh five grains, and will be of the diameter of

of an inch. The accuracy of the instrument will be still greater, if this method be pursued with several wires of different diameters, but weighed with equal care, and the subdivisions of the exterior scale made to correspond to the average of their indications.

XVIII. On the Tusks of the Narwhale. By Sir Everard

Home, Bart. F.R. S. Much uncertainty has prevailed on this subject, and the general report of those persons who are employed in the Greenland fishery has been, that the female Narwhale is destitute of tusks, and that the male has one only. From one of these persons (Mr. Scoresby, jun.) Sir Everard Home received the skull of a female, in which the sutures were firmly united, and yet there was no appearance of tusks, though a male skull. which appeared to be about the same age, had a tusk four feet long.

These facts then afforded some evidence of the truth of the general opinion of those employed in the Whale-fishery; but on reference to Anderson's description of Iceland, Greenland, and Davis's Straits, published in 1684, Sir E. found an account of a female skull which had been brought to Hamburgh, and which had two tusks, the left being seven feet five inches long, and the right, seven feet. And in another work, published in 1700, by Tycho L. Tychoricus, he found an account of a skull, having the left tusk seven feet long, and the right, imbedded and completely concealed in the skull, nine Danish inches in length. In consequence of these contradictory statements, the skulls of the Nar-whale, in the Hunterian Museum, were examined by means of the saw, when the rudiments of the tusks, not yet protruled from the bony substance, were discovered. In two male skulls, in which the left tusk was seven feet nine inches, and four feet respectively, the right tusk (about ten inches long) was completely imbedded in the bone, and was still more than seven inches distant from the front of the skull. In one of the specimens there was an external opening, leading down to the point of the tusk. Sir E. considers these as milk tusks; they are perfectly solid throughout, while the full grown ones are hollow nearly through their whole length. The left task, therefore, appears much earlier than the right; and so rare is it to meet with an instance in which they are both visible, that a captain of a Greenland ship, who had been thirtyfive voyages, informed the Author, that he had once only, and that from the mast head, seen a male Nar-whale with two tusks. The female skull, sent to Sir E. by Mr. Scoresby, when cut into, was found to contain two milk tusks, similar to those in the male : they were about eight inches long, and had advanced to within two inches and a quarter of the front of the skull, and there was a canal leading from the point of each to the external surface; the tusks, therefore, appear much later in the female than in the male. These facts prove that the name Monodon Monoceros, given to this species by Linnæus, is an improper one.

[merged small][ocr errors]
« ForrigeFortsett »