The effects of compressed air on tunnel workers and the palliative measures to be adopted by those in charge, formed the subject of a lecture by Prof. Thos. Oliver, M. A., M. D., recently delivered before the British Royal Institue of Public Health. The author described the caisson used in such work as a strong box or casing which is allowed to sink below the surface of a body of water in order to carry on work. As the caisson has no bottom, its edges cut into the soil where it rests, sinking lower as the material excavated within is hauled to the surface and disposed of. To keep out the water, air has to be pumped in a at a pressure varying with the depth of water.

To allow the passage of material and men, what is known as an air-lock is provided at the top of the caisson, partly above water, so that access can be had to the working chamber while the pressure is maintained.

On descending into the lock, very little inconvenience is experienced by men accustomed to the work ; they have a way of swallowing air and diverting some to the eustachian tube, thus avoiding injury to the ear. Once inside the caisson, very little inconvenience is felt, but loud conversation is difficult. Although it has been found advisable to limit the working period to a few hours, a mau can do as much work in a given time as under ordinary conditions. It is during " decompression," on leaving the compressed atmosphere, that abnormal symptoms develop. In minor cases men suffer from pains all over the body, and bleeding from the nose, or even from the mouth and ears. In severe cases paralysis develops.

There are certain predisposing causes of this illness, among these being an insufficient supply of fresh air or the contamination of air by lamps or by gases emanating from the materials being excavated. Severe manual exertion is a predisposing cause, the men who guide buckets as they are hoisted not being subject to the disease.

When the decompressing process is carried on slowly, Nature allows the excess of gases to escape through the lungs. The red corpuscles of the blood carry carbonic acid gas, a waste product of the tissues, to the lungs. Interference with this process and the disturbance of the equilibrium between internal gases and the atmosphere are, in Prof. Oliver's opinion, the cause of caisson disease.

Since immunity from this malady among laborers will allow engineers to carry on public works requiring deeper foundations than hitherto attempted, preventive measures are of the utmost practical importance. Slow decompression, with recompression if any of the symptoms of the disease appear, is the chief preventive. Breathing an atmosphere of oxygen for five minutes before coming out of the caisson, by driving excess of nitrogen out of the blood, has been found to have a beneficial effect. In general, medical inspection and the use of every facility for the comfort and treatment of men on coming out of the caisson are advised.-" Municipal Journal. "