Now OK

WeatherActivity.kt

class WeatherActivity : AppCompatActivity() {

    private val thermometer = Thermometer.getInstance()
    private var disposable: Disposable? = null

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        disposable = thermometer // Save the result of subscribe() method
            .observeTemperature()
            .subscribe { celsius -> /* Display in a TextView */ }
    }

    override fun onDestroy() {
        disposable?.dispose() // Do not forget to dispose a resource!
        super.onDestroy()
    }
}