still, some of them are not devoid of interest, as the following remarks will show. Infinitely the greater number are apparently referable to the fungous orders, Sphæronemei, Melanconei, Torulacei, Dematei, Mucedines, Pucciniai, and Ceomacei. Among these by far the most interesting was a specimen of the Torulacei, a species of the rare genus Tetroploa (see fig. 4, Plate CIX.). This was found only in two instances. The next curious specimen was one of a peculiar green algoid cell. This was obtained from Alipore. The specimen shows a number of green cells (see fig. 5), circular in outline, of various sizes, and showing a division of their contents into from two to four separate piles. The presence of these Dr. Cunningham apparently attributes to a tank in the neighbourhood, so that if this view be correct they have no real bearing on the case. However, we may suppose as at least possible that they were not due to the tank at all. Next in order is another peculiar specimen, which is supposed to be the pollen of some species of lily (see fig. 6). Of course its peculiarity in this instance is simply its rarity of occurrence.

Finally we come to the specimens which more than any other have an especial importance, and these are the ones represented in figs. 7 and 8 (Plate CIX.). They are of importance because they are peculiar, and because they are from two different localities, and from both of these places at the time when cholera was present largely. It is of interest to note the number of the peculiar curved spores that appear in both these cases, and to observe that these spores did not appear before when cholera was not present. They would, indeed, lead one to believe in them as the parents of cholera. But unhappily Dr. Cunningham's inquiries, which were conducted for a long time subsequently to this observation, show that these peculiar spore-like bodies did not appear in many other cases in which the mortality from cholera was even greater than it was during their presence; thus showing, as he thinks-with very good reason we imagine that it was only because their period of growth was at the time of the first experiment, that they were present, and that their absence was due not to the absence of cholera which raged violently at the time, but to the fact that their peculiar season had passed by. And, as he very properly observes, it shows the necessity for the carrying out of researches all the year round; for unquestionably if his inquiries had stopped at the period of finding these specimens, a very different conclusion would inevitably have been framed.

Although we have been unable to give even the briefest account of Dr. Cunningham's inquiries upon this subject in many of the channels he has adopted, we may nevertheless give the following conclusions which he has laid down, as they

enable the observer to perceive the different lines of research he has adopted:-1. The aeroscope affords a very convenient method for obtaining specimens really representing the nature of the true atmospheric dust. 2. Specimens of dust washed from exposed surfaces cannot be regarded as fair indices of the constituents of atmospheric dust, since they are liable to contain bodies which may have reached the surface otherwise than by means of the air, as well as others which are the result of local development. 3. Specimens collected by gravitation also fail to indicate the nature and amount of organic cells contained in the atmosphere, as the heavier amorphous and inorganic constituents of the dust are deposited in relative excess due to the method of collection. 4. Dew also fails to afford a good means of investigating the subject, as it is impossible to secure that all the bodies really present in a specimen of it should be collected into a sufficiently small space; and, moreover, because it is liable to accidental contaminations, and also affords a medium in which rapid growth and development are likely to take place. 5. Distinct infusorial animalcules, their germs or ova are almost entirely absent from atmospheric dust and even from many specimens of dust collected from exposed surfaces. 6. The cercomonads and ambæ appearing in certain specimens of pure rain-water appear to be zoospores developed from the mycelial filaments arising from common atmospheric spores. 7. Distinct bacteria can hardly ever be detected among the constituents of atmospheric dust, but fine molecules of uncertain nature are almost always present in abundance; they frequently appear in specimens of rain-water collected with all precautions to secure purity, and appear in many cases to arise from the mycelium developed from atmospheric spores. 8. Distinct bacteria are frequently to be found amongst the particles deposited from the moist air of sewers, though almost entirely absent as constituents of common atmospheric dust. 9. The addition of dry dust (which has been exposed to tropical heat) to putrescible fluids is followed by a rapid development of fungi and bacteria, although recognisable specimens of the latter are very rarely to be found in it while dry. 10. Spores and other vegetable cells are constantly present in atmospheric dust, and usually occur in considerable numbers; the majority of them are living and capable of growth and development: the amount of them present in the air appears to be independent of conditions of velocity and direction of wind; and their numbers are not diminished by moisture. 11. No connection can be traced between the numbers of bacteria, spores, &c., present in the air and the occurrence of diarrhoea, dysentery, cholera, ague or dengue; nor between the presence or abundance of any special form or forms of cells,

and the prevalence of any of these diseases. 12. The amount of inorganic and amorphous particles and other débris suspended in the atmosphere is directly dependent on conditions of moisture and of velocity of wind.

Dr. Cunningham concludes his observations with some valuable suggestions as to the reasons why his inquiries differ from those of Ehrenberg. For example, he shows that the methods employed by the two observers were totally distinct. The German worker obtained nearly all his specimens, not directly from the air, but from leaves, pieces of bark, bits of moss, &c. But of course this is a most unreliable mode of operating; for when surfaces have been wetted with rain, there is no difficulty for rotifers, tardigrades, and so forth, to make their way over them to a considerable extent; and it is this circumstance, doubtless, which gave the foreign observer such a wide zoological range to his collection. Dr. Cunningham's mode could alone gather what was actually in the air; hence his series of specimens have not been so wonderfully strange.

The author of the last work on the subject suggests in his final remarks that these collections might, if kept and then studied, have formed a different tale for the observing student. And here, doubtless, he has hit upon the right cord. We greatly regret that the idea did not occur to him before, as then it might have enabled him to give us something more worthy of a genuine microscopical student. It is, then, in absence of this and of another fact that, it appears to us, the author has, through omission, committed a serious error.

Two questions arise from the series of researches which he and others have conducted, and which have been described in the earlier pages; and these are-1. Are there always in the atmosphere a quantity of animal and vegetable germs ready, at any moment when offered the favourable conditions of development, to come into existence? 2. Do a certain number of these (animal or vegetable) promote those various epidemics and other diseases, so terrible in their manifestations, and which come and go in many instances just as a cloud of minute particles might be driven by the wind? The first of these questions is, it seems to us, partly answered in the affirmative; and it only requires the employment of the highest powers and the necessity of most careful watchmanship on the part of the student in order to have a complete and decisive reply. The Rev. Mr. Dallinger, F.R.M.S., has shown us by his recent inquiries ("Monthly Microscopical Journal," January, February, and March, 1874) how a whole host of organisms may completely escape observation by the use of powers even so high as those Dr. Cunningham has employed. It is only, then, by the employment of objectives of the inch and

-inch and the highest eye-pieces that we can hope for any useful result. And, furthermore, as Mr. Dallinger has pointed out, most careful and continuous observation must be carried on. Assuredly if he (Mr. Dallinger) had not adopted the continuous method of study, he would have described as three or more distinct individuals what were merely the successive stages of development of one and the same being. It is almost absurd to see, as we do, in the controversy which took place some few years ago between M. Pasteur and M. Pouchet, reference to the employment of a power actually of 150 diameters!!! Why, a power of 150 diameters would be altogether inadequate to such labours as those that are demanded; it would be as much, and even more, out of place than Malpighi's lenses would be if compared with the powers of the microscopic anatomist of to-day.

As to the second point of inquiry, that has been much less fully worked out. Although Mr. Blackley and a few others have attempted to show the immediate connection between disease and microscopic organisms, much less has been done than the subject demands. And there can be no doubt that it only awaits some one who, like Mr. Dallinger, will bring intense patience and the highest powers of the microscope to bear on it, to enable the subject to be completely cleared up.

At all events, we can congratulate the workers on the useful labour that they have given to the subject, and we must only hope that in a few years more, we shall be able to see more clearly that immense surface which is now as it were a barren plain with an immensity of exquisite mirages spread above it.

175

ON THE TRANSMISSION OF SOUND BY THE

ATMOSPHERE.

BY JOHN TYNDALL, D.C.L., LL.D., F.R.S.

TH

THIS notice embraces the scientific results of an inquiry on fog-signals, undertaken at the instance of the Elder Brethren of the Trinity House, and communicated, with their friendly concurrence, to the Royal Society.

The investigation was begun on May 19, 1873, and continued till July 4. It was resumed on October 8, and continued to the end of November. It also includes observations made during the dense fog which enveloped London on December 9 and the succeeding days.

Gongs and bells were excluded from this investigation, in consequence of their proved inferiority to other instruments of signalling. The experiments were made with trumpets blown by powerfully compressed air, with steam-whistles, guns, and a steam-syren, associated with a trumpet 16 feet long.

Daboll's horn, or trumpet, had been highly spoken of by writers on fog-signals. A third-order apparatus of the kind had been reported as sending its sound to a distance of from 7 to 9 miles against the wind, and to a distance of 12 to 14 miles with the wind. Holmes had improved upon Daboll; and with two instruments of Holmes-not of the third, but of the first order our experiments were made. On May 19, at 3 miles distance, they became useless as a fog-signal; at a distance of 4 miles, with paddles stopped and all on board quiet, they were wholly unheard. At a distance of 2 miles from the Foreland the whistles tested on May 19 became useless. The twelve o'clock gun, fired with a 1 lb. charge at Drop Fort in Dover, was well heard on May 19, when the horns and whistles were inaudible. On May 20 the permeability of the atmosphere had somewhat increased, but the steam-whistle failed to pierce it to a depth of 3 miles. At 4 miles the horns, though aided by quietness on board, were barely heard. By careful nursing, if I may use the expression, the horn-sounds were carried to a