same alluvial deposits, as that in which the remains of the rein-deer were found. In the year 1848, four canoes were found at Springfield, and all of them within a few yards of each other. These canoes were covered by a deposit of 17 feet of finely laminated sand. In other cases canoes have been found in the same alluvium, but much farther inland or remote from the modern channel of the river, as at London Street, and under the site of St Enoch's Church, and in the two last quoted instances the canoes are probably of greater antiquity than the remains we have described.

On certain New Applications of Prisms. By the Rev. W. HODGSON, Old Brathay, Ambleside. With a Plate. Communicated by the Author.

It has long been well known that when objects are viewed by means of pencils of light, which are refracted through an isosceles prism, and suffer an internal reflexion at the base, the objects are seen quite free from chromatic dispersion. Little attention however seems to have been paid to the fact that there is a position in which the pencils of light, after being thus refracted and reflected, may be made to enter the eye along with other pencils proceeding directly from the object. The position referred to is that represented in figure 1. If P Q and P E be contiguous rays of a parallel pencil proceeding from a distant object, one ray will follow the course P Q R S E, while the other will pass directly from P to E. In this position the image and the object coincide exactly; but if there is the slightest deviation from this position on either side, then the object and the image may be seen separately.

The application of this principle to the measurement of angles is easy and obvious. For this purpose the prism merely requires mounting, with its axis perpendicular to a movable graduated circle, which is capable of being adjusted at any required inclination to the horizon.

The form of prism best adapted for an instrument of this description seems to be one of which the principal section is an equilateral triangle. For as an observation can be

taken in the direction A C as well as in the direction C A (fig. 1), and as each face of the prism may be used in succession, there are altogether six observations so arranged that their errors will correct each other. Figs. 2 and 3 are rough sketches copied from an instrument fitted for observing any horizontal angles and some altitudes. The table for supporting the prism and its frame consists of a stout square brass plate, adjusted for parallelism to the horizon by the two spirit levels A A, and by the three screws B B, of which two only are shewn in the figures. The upper surface of the table of the instrument is graduated; and the frame which contains the prism is clamped by a screw to the movable circle which carries the verniers. With this arrangement the horizontal angle between two objects is readily found. It is not material that the objects should be nearly at the same height above the horizon; for the only effect of viewing an object obliquely, instead of directly through the prism, is a very slight displacement in a vertical plane, which does not interfere with the horizontal angle.

In fig. 3 the back of the same instrument is shewn in a state adapted for finding the time, &c., by the method of equal altitudes. On the face of the movable circle already referred to, there are two small holes to receive the points G G, by which the axis of the prism is kept parallel to the plane of the table. The screw CC which passes through the nut D, gives the power of raising or depressing that face of the prism through which the observation is being taken. The nut D is movable upon its axis, and the point F of the screw CC, may be inserted successively into the several holes which are prepared for it. By this means any required number of altitudes may be taken, and as these can be all repeated in the reverse order, these will be the pairs of observations which are necessary.

If the actual altitude in any case is desired instead of the mere fact of there being pairs of equal altitudes, this may be obtained by a slight modification of the instrument. Let the nut D be rigidly fixed so that the axis of the screw CC shall be perpendicular to the face of the prism used in taking the observation. Then the tangent of the altitude to the constant

radius G F is given by the variable portion DF of the screw below the nut D.

If the table of the instrument (fig. 2) be permanently fixed in a position parallel to the plane of the equator, and if the degree and minute on the graduated circle, correspondent with each face of the prism coinciding with the meridian be observed and registered, the true time may be found within a second or two at any period during the day when the sun is visible, or during the night from observations of known. stars of the first, second, or even third magnitude.

If the instrument be placed with its table horizontal, the observations would give azimuths instead of hour angles. Or, suppressing the graduated circle, and fixing the prism with one of its faces parallel to the meridian, and its axis perpendicular either to the horizon or to the equator, there will be six known directions in which the transits of the heavenly bodies may be accurately determined.

The rectangular isosceles prism is capable of being employed in this instrument; and in the course of investigating its advantages, some uses of a prism of that form were suggested which appear hitherto to have passed unnoticed.

In Coddington's Reflexion and Refraction, and possibly in some earlier optical works, it is stated that a feeble but very distinct second internal reflexion may take place in a triangular prism, when one angle is a right angle, and the other two each half a right angle. It is not, however, stated that the feebleness of this image may be remedied by silvering the base of the prism. If this plan be adopted, the rectangular prism becomes at once a camera lucida. Fig. 4 shews the course of the ray in this case, and it is obvious from fig. 5 that there is another, though for some purposes less convenient direction in which the prism will serve the purpose of the camera lucida. There is also this advantage in the rectangular prism, that if it is desired to take a reversed copy of the object, this is given by the single internal reflexion, and either may be taken at pleasure.

In this, and in Dr Wollaston's camera, the incident and the emergent rays are accurately perpendicular to each other, and it is somewhat strange that a little instrument termed

an optical square should have been introduced, when the purpose for which it was constructed is answered either by the camera lucida or by the rectangular prism.

There remains still one other application of the rectangular prism, which is too important to be overlooked. If a prism of this form be placed over the eye-piece of a compound microscope, or of an astronomical telescope,* so that its axis is inclined to that of the microscope or telescope at an angle of 45°, the rays, after suffering two internal reflexions, will emerge without chromatic dispersion, and in such a manner as to do away entirely with the inversion produced by those instruments. There is, however, a great loss of light, in consequence of the obliquity at which the rays enter the prism. With a view to remedy this defect, various plans were tried. Two equilateral prisms cemented together, with their axes perpendicular to each other, were found to erect the image without loss of light, and to allow of its being seen as if projected on a screen placed to receive it: but there was a deviation to the right or to the left, which was unsatisfactory. The regular tetrahedron also was tried, and after trial rejected for a similar reason. At length the solid represented in fig. 6 was adopted, and found to answer completely. The rays are incident upon the first surface perpendicularly, the internal reflexions are both total, the solid is of a form which admits of easy adaptation to the eyepieces, the movements of the image correspond exactly with those of the object, and the image may be traced upon a screen placed behind it, as readily as with the camera lucida. The lower surface might, if necessary, be ground into the form of a lens, but this, on other accounts, is inconvenient. OLD BRATHAY, 13th November 1851.

*It is not anticipated that this mode of erecting the image will be generally used by astronomers; but the addition of such an appendage to the eye-piece of an astronomical telescope of low power forms an improved graphic telescope, which may be serviceable to draughtsmen.

Geological Notes. By Professor SILLIMAN, Junr.

Professor Silliman, junr., during his late visit to Europe, examined many interesting geological localities, of which he gives short notices in his Journal. Some of these we now lay before our readers.

1. Grotto del Cane and Lake Agnano.

The Grotto del Cane, or dog grotto, has been so much cited for its stratum of carbonic acid gas covering the floor, that all geological travellers who visit Naples feel an interest in seeing it. Unfortunately, like some other grottos, its enchantment disappears on a near view. It is a little hole dug artificially into the foot of a hill facing Lake Agnano. The aperture is closed by a door, and the space within is barely sufficient for one person to stand erect. Into this narrow cell a poor little dog is very unwillingly dragged and placed in a depression of the floor, where he is soon narcotized by the carbonic acid. The earth is warm to the hand, and the volume of gas given out is very constant.

Lake Agnano, as is well known, fills the bottom of an ancient crater whose walls are still to be distinctly traced. On the shore of the lake immediately opposite the Grotto del Cane, there is a constant and copious discharge of carbonic acid, and the water of the lake at that part is in great agitation from its escape. Nor is other evidence wanting of the present operation of the ancient causes which have characterised this region. At the foot of the hill, on the east, are numerous vents of steam and sulphur. Over some of these vapour baths (Sudatoria) have been established, which are held in some repute in cases of rheumatism. Digging a short distance into the hill, near the ruins of the villa of Lucullus, it is easy to perceive the presence of volcanic energy in the hot vapours, accompanied with sulphur, which issue copiously from the surface. The villa of Lucullus was entirely buried in lapilli and volcanic ashes, proceeding, as is supposed, from a vent in the immediate neighbourhood; but from which vent does not appear-evidently not from the one